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809 lines
27 KiB
Python
809 lines
27 KiB
Python
# ext/hybrid.py
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# Copyright (C) 2005-2013 the SQLAlchemy authors and contributors <see AUTHORS file>
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#
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# This module is part of SQLAlchemy and is released under
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# the MIT License: http://www.opensource.org/licenses/mit-license.php
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"""Define attributes on ORM-mapped classes that have "hybrid" behavior.
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"hybrid" means the attribute has distinct behaviors defined at the
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class level and at the instance level.
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The :mod:`~sqlalchemy.ext.hybrid` extension provides a special form of
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method decorator, is around 50 lines of code and has almost no
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dependencies on the rest of SQLAlchemy. It can, in theory, work with
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any descriptor-based expression system.
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Consider a mapping ``Interval``, representing integer ``start`` and ``end``
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values. We can define higher level functions on mapped classes that produce
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SQL expressions at the class level, and Python expression evaluation at the
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instance level. Below, each function decorated with :class:`.hybrid_method` or
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:class:`.hybrid_property` may receive ``self`` as an instance of the class, or
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as the class itself::
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from sqlalchemy import Column, Integer
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from sqlalchemy.ext.declarative import declarative_base
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from sqlalchemy.orm import Session, aliased
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from sqlalchemy.ext.hybrid import hybrid_property, hybrid_method
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Base = declarative_base()
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class Interval(Base):
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__tablename__ = 'interval'
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id = Column(Integer, primary_key=True)
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start = Column(Integer, nullable=False)
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end = Column(Integer, nullable=False)
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def __init__(self, start, end):
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self.start = start
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self.end = end
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@hybrid_property
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def length(self):
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return self.end - self.start
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@hybrid_method
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def contains(self,point):
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return (self.start <= point) & (point < self.end)
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@hybrid_method
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def intersects(self, other):
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return self.contains(other.start) | self.contains(other.end)
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Above, the ``length`` property returns the difference between the
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``end`` and ``start`` attributes. With an instance of ``Interval``,
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this subtraction occurs in Python, using normal Python descriptor
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mechanics::
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>>> i1 = Interval(5, 10)
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>>> i1.length
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5
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When dealing with the ``Interval`` class itself, the :class:`.hybrid_property`
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descriptor evaluates the function body given the ``Interval`` class as
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the argument, which when evaluated with SQLAlchemy expression mechanics
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returns a new SQL expression::
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>>> print Interval.length
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interval."end" - interval.start
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>>> print Session().query(Interval).filter(Interval.length > 10)
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SELECT interval.id AS interval_id, interval.start AS interval_start,
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interval."end" AS interval_end
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FROM interval
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WHERE interval."end" - interval.start > :param_1
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ORM methods such as :meth:`~.Query.filter_by` generally use ``getattr()`` to
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locate attributes, so can also be used with hybrid attributes::
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>>> print Session().query(Interval).filter_by(length=5)
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SELECT interval.id AS interval_id, interval.start AS interval_start,
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interval."end" AS interval_end
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FROM interval
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WHERE interval."end" - interval.start = :param_1
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The ``Interval`` class example also illustrates two methods,
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``contains()`` and ``intersects()``, decorated with
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:class:`.hybrid_method`. This decorator applies the same idea to
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methods that :class:`.hybrid_property` applies to attributes. The
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methods return boolean values, and take advantage of the Python ``|``
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and ``&`` bitwise operators to produce equivalent instance-level and
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SQL expression-level boolean behavior::
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>>> i1.contains(6)
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True
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>>> i1.contains(15)
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False
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>>> i1.intersects(Interval(7, 18))
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True
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>>> i1.intersects(Interval(25, 29))
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False
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>>> print Session().query(Interval).filter(Interval.contains(15))
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SELECT interval.id AS interval_id, interval.start AS interval_start,
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interval."end" AS interval_end
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FROM interval
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WHERE interval.start <= :start_1 AND interval."end" > :end_1
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>>> ia = aliased(Interval)
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>>> print Session().query(Interval, ia).filter(Interval.intersects(ia))
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SELECT interval.id AS interval_id, interval.start AS interval_start,
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interval."end" AS interval_end, interval_1.id AS interval_1_id,
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interval_1.start AS interval_1_start, interval_1."end" AS interval_1_end
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FROM interval, interval AS interval_1
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WHERE interval.start <= interval_1.start
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AND interval."end" > interval_1.start
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OR interval.start <= interval_1."end"
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AND interval."end" > interval_1."end"
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Defining Expression Behavior Distinct from Attribute Behavior
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--------------------------------------------------------------
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Our usage of the ``&`` and ``|`` bitwise operators above was
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fortunate, considering our functions operated on two boolean values to
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return a new one. In many cases, the construction of an in-Python
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function and a SQLAlchemy SQL expression have enough differences that
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two separate Python expressions should be defined. The
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:mod:`~sqlalchemy.ext.hybrid` decorators define the
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:meth:`.hybrid_property.expression` modifier for this purpose. As an
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example we'll define the radius of the interval, which requires the
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usage of the absolute value function::
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from sqlalchemy import func
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class Interval(object):
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# ...
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@hybrid_property
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def radius(self):
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return abs(self.length) / 2
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@radius.expression
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def radius(cls):
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return func.abs(cls.length) / 2
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Above the Python function ``abs()`` is used for instance-level
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operations, the SQL function ``ABS()`` is used via the :attr:`.func`
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object for class-level expressions::
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>>> i1.radius
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2
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>>> print Session().query(Interval).filter(Interval.radius > 5)
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SELECT interval.id AS interval_id, interval.start AS interval_start,
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interval."end" AS interval_end
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FROM interval
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WHERE abs(interval."end" - interval.start) / :abs_1 > :param_1
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Defining Setters
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----------------
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Hybrid properties can also define setter methods. If we wanted
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``length`` above, when set, to modify the endpoint value::
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class Interval(object):
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# ...
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@hybrid_property
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def length(self):
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return self.end - self.start
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@length.setter
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def length(self, value):
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self.end = self.start + value
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The ``length(self, value)`` method is now called upon set::
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>>> i1 = Interval(5, 10)
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>>> i1.length
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5
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>>> i1.length = 12
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>>> i1.end
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17
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Working with Relationships
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--------------------------
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There's no essential difference when creating hybrids that work with
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related objects as opposed to column-based data. The need for distinct
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expressions tends to be greater. Two variants of we'll illustrate
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are the "join-dependent" hybrid, and the "correlated subquery" hybrid.
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Join-Dependent Relationship Hybrid
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Consider the following declarative
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mapping which relates a ``User`` to a ``SavingsAccount``::
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from sqlalchemy import Column, Integer, ForeignKey, Numeric, String
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from sqlalchemy.orm import relationship
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from sqlalchemy.ext.declarative import declarative_base
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from sqlalchemy.ext.hybrid import hybrid_property
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Base = declarative_base()
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class SavingsAccount(Base):
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__tablename__ = 'account'
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id = Column(Integer, primary_key=True)
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user_id = Column(Integer, ForeignKey('user.id'), nullable=False)
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balance = Column(Numeric(15, 5))
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class User(Base):
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__tablename__ = 'user'
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id = Column(Integer, primary_key=True)
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name = Column(String(100), nullable=False)
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accounts = relationship("SavingsAccount", backref="owner")
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@hybrid_property
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def balance(self):
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if self.accounts:
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return self.accounts[0].balance
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else:
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return None
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@balance.setter
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def balance(self, value):
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if not self.accounts:
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account = Account(owner=self)
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else:
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account = self.accounts[0]
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account.balance = value
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@balance.expression
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def balance(cls):
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return SavingsAccount.balance
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The above hybrid property ``balance`` works with the first
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``SavingsAccount`` entry in the list of accounts for this user. The
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in-Python getter/setter methods can treat ``accounts`` as a Python
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list available on ``self``.
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However, at the expression level, it's expected that the ``User`` class will
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be used in an appropriate context such that an appropriate join to
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``SavingsAccount`` will be present::
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>>> print Session().query(User, User.balance).\\
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... join(User.accounts).filter(User.balance > 5000)
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SELECT "user".id AS user_id, "user".name AS user_name,
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account.balance AS account_balance
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FROM "user" JOIN account ON "user".id = account.user_id
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WHERE account.balance > :balance_1
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Note however, that while the instance level accessors need to worry
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about whether ``self.accounts`` is even present, this issue expresses
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itself differently at the SQL expression level, where we basically
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would use an outer join::
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>>> from sqlalchemy import or_
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>>> print (Session().query(User, User.balance).outerjoin(User.accounts).
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... filter(or_(User.balance < 5000, User.balance == None)))
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SELECT "user".id AS user_id, "user".name AS user_name,
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account.balance AS account_balance
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FROM "user" LEFT OUTER JOIN account ON "user".id = account.user_id
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WHERE account.balance < :balance_1 OR account.balance IS NULL
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Correlated Subquery Relationship Hybrid
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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We can, of course, forego being dependent on the enclosing query's usage
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of joins in favor of the correlated subquery, which can portably be packed
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into a single column expression. A correlated subquery is more portable, but
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often performs more poorly at the SQL level. Using the same technique
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illustrated at :ref:`mapper_column_property_sql_expressions`,
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we can adjust our ``SavingsAccount`` example to aggregate the balances for
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*all* accounts, and use a correlated subquery for the column expression::
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from sqlalchemy import Column, Integer, ForeignKey, Numeric, String
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from sqlalchemy.orm import relationship
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from sqlalchemy.ext.declarative import declarative_base
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from sqlalchemy.ext.hybrid import hybrid_property
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from sqlalchemy import select, func
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Base = declarative_base()
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class SavingsAccount(Base):
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__tablename__ = 'account'
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id = Column(Integer, primary_key=True)
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user_id = Column(Integer, ForeignKey('user.id'), nullable=False)
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balance = Column(Numeric(15, 5))
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class User(Base):
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__tablename__ = 'user'
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id = Column(Integer, primary_key=True)
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name = Column(String(100), nullable=False)
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accounts = relationship("SavingsAccount", backref="owner")
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@hybrid_property
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def balance(self):
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return sum(acc.balance for acc in self.accounts)
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@balance.expression
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def balance(cls):
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return select([func.sum(SavingsAccount.balance)]).\\
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where(SavingsAccount.user_id==cls.id).\\
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label('total_balance')
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The above recipe will give us the ``balance`` column which renders
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a correlated SELECT::
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>>> print s.query(User).filter(User.balance > 400)
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SELECT "user".id AS user_id, "user".name AS user_name
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FROM "user"
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WHERE (SELECT sum(account.balance) AS sum_1
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FROM account
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WHERE account.user_id = "user".id) > :param_1
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.. _hybrid_custom_comparators:
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Building Custom Comparators
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---------------------------
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The hybrid property also includes a helper that allows construction of
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custom comparators. A comparator object allows one to customize the
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behavior of each SQLAlchemy expression operator individually. They
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are useful when creating custom types that have some highly
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idiosyncratic behavior on the SQL side.
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The example class below allows case-insensitive comparisons on the attribute
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named ``word_insensitive``::
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from sqlalchemy.ext.hybrid import Comparator, hybrid_property
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from sqlalchemy import func, Column, Integer, String
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from sqlalchemy.orm import Session
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from sqlalchemy.ext.declarative import declarative_base
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Base = declarative_base()
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class CaseInsensitiveComparator(Comparator):
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def __eq__(self, other):
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return func.lower(self.__clause_element__()) == func.lower(other)
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class SearchWord(Base):
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__tablename__ = 'searchword'
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id = Column(Integer, primary_key=True)
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word = Column(String(255), nullable=False)
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@hybrid_property
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def word_insensitive(self):
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return self.word.lower()
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@word_insensitive.comparator
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def word_insensitive(cls):
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return CaseInsensitiveComparator(cls.word)
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Above, SQL expressions against ``word_insensitive`` will apply the ``LOWER()``
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SQL function to both sides::
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>>> print Session().query(SearchWord).filter_by(word_insensitive="Trucks")
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SELECT searchword.id AS searchword_id, searchword.word AS searchword_word
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FROM searchword
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WHERE lower(searchword.word) = lower(:lower_1)
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The ``CaseInsensitiveComparator`` above implements part of the
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:class:`.ColumnOperators` interface. A "coercion" operation like
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lowercasing can be applied to all comparison operations (i.e. ``eq``,
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``lt``, ``gt``, etc.) using :meth:`.Operators.operate`::
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class CaseInsensitiveComparator(Comparator):
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def operate(self, op, other):
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return op(func.lower(self.__clause_element__()), func.lower(other))
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Hybrid Value Objects
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--------------------
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Note in our previous example, if we were to compare the
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``word_insensitive`` attribute of a ``SearchWord`` instance to a plain
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Python string, the plain Python string would not be coerced to lower
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case - the ``CaseInsensitiveComparator`` we built, being returned by
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``@word_insensitive.comparator``, only applies to the SQL side.
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A more comprehensive form of the custom comparator is to construct a
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*Hybrid Value Object*. This technique applies the target value or
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expression to a value object which is then returned by the accessor in
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all cases. The value object allows control of all operations upon
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the value as well as how compared values are treated, both on the SQL
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expression side as well as the Python value side. Replacing the
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previous ``CaseInsensitiveComparator`` class with a new
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``CaseInsensitiveWord`` class::
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class CaseInsensitiveWord(Comparator):
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"Hybrid value representing a lower case representation of a word."
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def __init__(self, word):
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if isinstance(word, basestring):
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self.word = word.lower()
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elif isinstance(word, CaseInsensitiveWord):
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self.word = word.word
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else:
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self.word = func.lower(word)
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def operate(self, op, other):
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if not isinstance(other, CaseInsensitiveWord):
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other = CaseInsensitiveWord(other)
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return op(self.word, other.word)
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def __clause_element__(self):
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return self.word
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def __str__(self):
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return self.word
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key = 'word'
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"Label to apply to Query tuple results"
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Above, the ``CaseInsensitiveWord`` object represents ``self.word``,
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which may be a SQL function, or may be a Python native. By
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overriding ``operate()`` and ``__clause_element__()`` to work in terms
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of ``self.word``, all comparison operations will work against the
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"converted" form of ``word``, whether it be SQL side or Python side.
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Our ``SearchWord`` class can now deliver the ``CaseInsensitiveWord``
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object unconditionally from a single hybrid call::
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class SearchWord(Base):
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__tablename__ = 'searchword'
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id = Column(Integer, primary_key=True)
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word = Column(String(255), nullable=False)
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@hybrid_property
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def word_insensitive(self):
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return CaseInsensitiveWord(self.word)
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The ``word_insensitive`` attribute now has case-insensitive comparison
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behavior universally, including SQL expression vs. Python expression
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(note the Python value is converted to lower case on the Python side
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here)::
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>>> print Session().query(SearchWord).filter_by(word_insensitive="Trucks")
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SELECT searchword.id AS searchword_id, searchword.word AS searchword_word
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FROM searchword
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WHERE lower(searchword.word) = :lower_1
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SQL expression versus SQL expression::
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>>> sw1 = aliased(SearchWord)
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>>> sw2 = aliased(SearchWord)
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>>> print Session().query(
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... sw1.word_insensitive,
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... sw2.word_insensitive).\\
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... filter(
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... sw1.word_insensitive > sw2.word_insensitive
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... )
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SELECT lower(searchword_1.word) AS lower_1,
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lower(searchword_2.word) AS lower_2
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FROM searchword AS searchword_1, searchword AS searchword_2
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WHERE lower(searchword_1.word) > lower(searchword_2.word)
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Python only expression::
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>>> ws1 = SearchWord(word="SomeWord")
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>>> ws1.word_insensitive == "sOmEwOrD"
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True
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>>> ws1.word_insensitive == "XOmEwOrX"
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False
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>>> print ws1.word_insensitive
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someword
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The Hybrid Value pattern is very useful for any kind of value that may
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have multiple representations, such as timestamps, time deltas, units
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of measurement, currencies and encrypted passwords.
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.. seealso::
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`Hybrids and Value Agnostic Types
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<http://techspot.zzzeek.org/2011/10/21/hybrids-and-value-agnostic-types/>`_ -
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on the techspot.zzzeek.org blog
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`Value Agnostic Types, Part II
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<http://techspot.zzzeek.org/2011/10/29/value-agnostic-types-part-ii/>`_ -
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on the techspot.zzzeek.org blog
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.. _hybrid_transformers:
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Building Transformers
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----------------------
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A *transformer* is an object which can receive a :class:`.Query`
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object and return a new one. The :class:`.Query` object includes a
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method :meth:`.with_transformation` that returns a new :class:`.Query`
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transformed by the given function.
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We can combine this with the :class:`.Comparator` class to produce one type
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of recipe which can both set up the FROM clause of a query as well as assign
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filtering criterion.
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Consider a mapped class ``Node``, which assembles using adjacency list
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into a hierarchical tree pattern::
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from sqlalchemy import Column, Integer, ForeignKey
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from sqlalchemy.orm import relationship
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from sqlalchemy.ext.declarative import declarative_base
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Base = declarative_base()
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class Node(Base):
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__tablename__ = 'node'
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id =Column(Integer, primary_key=True)
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parent_id = Column(Integer, ForeignKey('node.id'))
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parent = relationship("Node", remote_side=id)
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Suppose we wanted to add an accessor ``grandparent``. This would
|
|
return the ``parent`` of ``Node.parent``. When we have an instance of
|
|
``Node``, this is simple::
|
|
|
|
from sqlalchemy.ext.hybrid import hybrid_property
|
|
|
|
class Node(Base):
|
|
# ...
|
|
|
|
@hybrid_property
|
|
def grandparent(self):
|
|
return self.parent.parent
|
|
|
|
For the expression, things are not so clear. We'd need to construct
|
|
a :class:`.Query` where we :meth:`~.Query.join` twice along
|
|
``Node.parent`` to get to the ``grandparent``. We can instead return
|
|
a transforming callable that we'll combine with the
|
|
:class:`.Comparator` class to receive any :class:`.Query` object, and
|
|
return a new one that's joined to the ``Node.parent`` attribute and
|
|
filtered based on the given criterion::
|
|
|
|
from sqlalchemy.ext.hybrid import Comparator
|
|
|
|
class GrandparentTransformer(Comparator):
|
|
def operate(self, op, other):
|
|
def transform(q):
|
|
cls = self.__clause_element__()
|
|
parent_alias = aliased(cls)
|
|
return q.join(parent_alias, cls.parent).\\
|
|
filter(op(parent_alias.parent, other))
|
|
return transform
|
|
|
|
Base = declarative_base()
|
|
|
|
class Node(Base):
|
|
__tablename__ = 'node'
|
|
id =Column(Integer, primary_key=True)
|
|
parent_id = Column(Integer, ForeignKey('node.id'))
|
|
parent = relationship("Node", remote_side=id)
|
|
|
|
@hybrid_property
|
|
def grandparent(self):
|
|
return self.parent.parent
|
|
|
|
@grandparent.comparator
|
|
def grandparent(cls):
|
|
return GrandparentTransformer(cls)
|
|
|
|
The ``GrandparentTransformer`` overrides the core
|
|
:meth:`.Operators.operate` method at the base of the
|
|
:class:`.Comparator` hierarchy to return a query-transforming
|
|
callable, which then runs the given comparison operation in a
|
|
particular context. Such as, in the example above, the ``operate``
|
|
method is called, given the :attr:`.Operators.eq` callable as well as
|
|
the right side of the comparison ``Node(id=5)``. A function
|
|
``transform`` is then returned which will transform a :class:`.Query`
|
|
first to join to ``Node.parent``, then to compare ``parent_alias``
|
|
using :attr:`.Operators.eq` against the left and right sides, passing
|
|
into :class:`.Query.filter`:
|
|
|
|
.. sourcecode:: pycon+sql
|
|
|
|
>>> from sqlalchemy.orm import Session
|
|
>>> session = Session()
|
|
{sql}>>> session.query(Node).\\
|
|
... with_transformation(Node.grandparent==Node(id=5)).\\
|
|
... all()
|
|
SELECT node.id AS node_id, node.parent_id AS node_parent_id
|
|
FROM node JOIN node AS node_1 ON node_1.id = node.parent_id
|
|
WHERE :param_1 = node_1.parent_id
|
|
{stop}
|
|
|
|
We can modify the pattern to be more verbose but flexible by separating
|
|
the "join" step from the "filter" step. The tricky part here is ensuring
|
|
that successive instances of ``GrandparentTransformer`` use the same
|
|
:class:`.AliasedClass` object against ``Node``. Below we use a simple
|
|
memoizing approach that associates a ``GrandparentTransformer``
|
|
with each class::
|
|
|
|
class Node(Base):
|
|
|
|
# ...
|
|
|
|
@grandparent.comparator
|
|
def grandparent(cls):
|
|
# memoize a GrandparentTransformer
|
|
# per class
|
|
if '_gp' not in cls.__dict__:
|
|
cls._gp = GrandparentTransformer(cls)
|
|
return cls._gp
|
|
|
|
class GrandparentTransformer(Comparator):
|
|
|
|
def __init__(self, cls):
|
|
self.parent_alias = aliased(cls)
|
|
|
|
@property
|
|
def join(self):
|
|
def go(q):
|
|
return q.join(self.parent_alias, Node.parent)
|
|
return go
|
|
|
|
def operate(self, op, other):
|
|
return op(self.parent_alias.parent, other)
|
|
|
|
.. sourcecode:: pycon+sql
|
|
|
|
{sql}>>> session.query(Node).\\
|
|
... with_transformation(Node.grandparent.join).\\
|
|
... filter(Node.grandparent==Node(id=5))
|
|
SELECT node.id AS node_id, node.parent_id AS node_parent_id
|
|
FROM node JOIN node AS node_1 ON node_1.id = node.parent_id
|
|
WHERE :param_1 = node_1.parent_id
|
|
{stop}
|
|
|
|
The "transformer" pattern is an experimental pattern that starts
|
|
to make usage of some functional programming paradigms.
|
|
While it's only recommended for advanced and/or patient developers,
|
|
there's probably a whole lot of amazing things it can be used for.
|
|
|
|
"""
|
|
from .. import util
|
|
from ..orm import attributes, interfaces
|
|
|
|
HYBRID_METHOD = util.symbol('HYBRID_METHOD')
|
|
"""Symbol indicating an :class:`_InspectionAttr` that's
|
|
of type :class:`.hybrid_method`.
|
|
|
|
Is assigned to the :attr:`._InspectionAttr.extension_type`
|
|
attibute.
|
|
|
|
.. seealso::
|
|
|
|
:attr:`.Mapper.all_orm_attributes`
|
|
|
|
"""
|
|
|
|
HYBRID_PROPERTY = util.symbol('HYBRID_PROPERTY')
|
|
"""Symbol indicating an :class:`_InspectionAttr` that's
|
|
of type :class:`.hybrid_method`.
|
|
|
|
Is assigned to the :attr:`._InspectionAttr.extension_type`
|
|
attibute.
|
|
|
|
.. seealso::
|
|
|
|
:attr:`.Mapper.all_orm_attributes`
|
|
|
|
"""
|
|
|
|
class hybrid_method(interfaces._InspectionAttr):
|
|
"""A decorator which allows definition of a Python object method with both
|
|
instance-level and class-level behavior.
|
|
|
|
"""
|
|
|
|
is_attribute = True
|
|
extension_type = HYBRID_METHOD
|
|
|
|
def __init__(self, func, expr=None):
|
|
"""Create a new :class:`.hybrid_method`.
|
|
|
|
Usage is typically via decorator::
|
|
|
|
from sqlalchemy.ext.hybrid import hybrid_method
|
|
|
|
class SomeClass(object):
|
|
@hybrid_method
|
|
def value(self, x, y):
|
|
return self._value + x + y
|
|
|
|
@value.expression
|
|
def value(self, x, y):
|
|
return func.some_function(self._value, x, y)
|
|
|
|
"""
|
|
self.func = func
|
|
self.expr = expr or func
|
|
|
|
def __get__(self, instance, owner):
|
|
if instance is None:
|
|
return self.expr.__get__(owner, owner.__class__)
|
|
else:
|
|
return self.func.__get__(instance, owner)
|
|
|
|
def expression(self, expr):
|
|
"""Provide a modifying decorator that defines a
|
|
SQL-expression producing method."""
|
|
|
|
self.expr = expr
|
|
return self
|
|
|
|
|
|
class hybrid_property(interfaces._InspectionAttr):
|
|
"""A decorator which allows definition of a Python descriptor with both
|
|
instance-level and class-level behavior.
|
|
|
|
"""
|
|
|
|
is_attribute = True
|
|
extension_type = HYBRID_PROPERTY
|
|
|
|
def __init__(self, fget, fset=None, fdel=None, expr=None):
|
|
"""Create a new :class:`.hybrid_property`.
|
|
|
|
Usage is typically via decorator::
|
|
|
|
from sqlalchemy.ext.hybrid import hybrid_property
|
|
|
|
class SomeClass(object):
|
|
@hybrid_property
|
|
def value(self):
|
|
return self._value
|
|
|
|
@value.setter
|
|
def value(self, value):
|
|
self._value = value
|
|
|
|
"""
|
|
self.fget = fget
|
|
self.fset = fset
|
|
self.fdel = fdel
|
|
self.expr = expr or fget
|
|
util.update_wrapper(self, fget)
|
|
|
|
def __get__(self, instance, owner):
|
|
if instance is None:
|
|
return self.expr(owner)
|
|
else:
|
|
return self.fget(instance)
|
|
|
|
def __set__(self, instance, value):
|
|
if self.fset is None:
|
|
raise AttributeError("can't set attribute")
|
|
self.fset(instance, value)
|
|
|
|
def __delete__(self, instance):
|
|
if self.fdel is None:
|
|
raise AttributeError("can't delete attribute")
|
|
self.fdel(instance)
|
|
|
|
def setter(self, fset):
|
|
"""Provide a modifying decorator that defines a value-setter method."""
|
|
|
|
self.fset = fset
|
|
return self
|
|
|
|
def deleter(self, fdel):
|
|
"""Provide a modifying decorator that defines a
|
|
value-deletion method."""
|
|
|
|
self.fdel = fdel
|
|
return self
|
|
|
|
def expression(self, expr):
|
|
"""Provide a modifying decorator that defines a SQL-expression
|
|
producing method."""
|
|
|
|
self.expr = expr
|
|
return self
|
|
|
|
def comparator(self, comparator):
|
|
"""Provide a modifying decorator that defines a custom
|
|
comparator producing method.
|
|
|
|
The return value of the decorated method should be an instance of
|
|
:class:`~.hybrid.Comparator`.
|
|
|
|
"""
|
|
|
|
proxy_attr = attributes.\
|
|
create_proxied_attribute(self)
|
|
|
|
def expr(owner):
|
|
return proxy_attr(owner, self.__name__, self, comparator(owner))
|
|
self.expr = expr
|
|
return self
|
|
|
|
|
|
class Comparator(interfaces.PropComparator):
|
|
"""A helper class that allows easy construction of custom
|
|
:class:`~.orm.interfaces.PropComparator`
|
|
classes for usage with hybrids."""
|
|
|
|
property = None
|
|
|
|
def __init__(self, expression):
|
|
self.expression = expression
|
|
|
|
def __clause_element__(self):
|
|
expr = self.expression
|
|
while hasattr(expr, '__clause_element__'):
|
|
expr = expr.__clause_element__()
|
|
return expr
|
|
|
|
def adapted(self, adapter):
|
|
# interesting....
|
|
return self
|