You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
449 lines
15 KiB
Python
449 lines
15 KiB
Python
# ext/compiler.py
|
|
# Copyright (C) 2005-2013 the SQLAlchemy authors and contributors <see AUTHORS file>
|
|
#
|
|
# This module is part of SQLAlchemy and is released under
|
|
# the MIT License: http://www.opensource.org/licenses/mit-license.php
|
|
|
|
"""Provides an API for creation of custom ClauseElements and compilers.
|
|
|
|
Synopsis
|
|
========
|
|
|
|
Usage involves the creation of one or more
|
|
:class:`~sqlalchemy.sql.expression.ClauseElement` subclasses and one or
|
|
more callables defining its compilation::
|
|
|
|
from sqlalchemy.ext.compiler import compiles
|
|
from sqlalchemy.sql.expression import ColumnClause
|
|
|
|
class MyColumn(ColumnClause):
|
|
pass
|
|
|
|
@compiles(MyColumn)
|
|
def compile_mycolumn(element, compiler, **kw):
|
|
return "[%s]" % element.name
|
|
|
|
Above, ``MyColumn`` extends :class:`~sqlalchemy.sql.expression.ColumnClause`,
|
|
the base expression element for named column objects. The ``compiles``
|
|
decorator registers itself with the ``MyColumn`` class so that it is invoked
|
|
when the object is compiled to a string::
|
|
|
|
from sqlalchemy import select
|
|
|
|
s = select([MyColumn('x'), MyColumn('y')])
|
|
print str(s)
|
|
|
|
Produces::
|
|
|
|
SELECT [x], [y]
|
|
|
|
Dialect-specific compilation rules
|
|
==================================
|
|
|
|
Compilers can also be made dialect-specific. The appropriate compiler will be
|
|
invoked for the dialect in use::
|
|
|
|
from sqlalchemy.schema import DDLElement
|
|
|
|
class AlterColumn(DDLElement):
|
|
|
|
def __init__(self, column, cmd):
|
|
self.column = column
|
|
self.cmd = cmd
|
|
|
|
@compiles(AlterColumn)
|
|
def visit_alter_column(element, compiler, **kw):
|
|
return "ALTER COLUMN %s ..." % element.column.name
|
|
|
|
@compiles(AlterColumn, 'postgresql')
|
|
def visit_alter_column(element, compiler, **kw):
|
|
return "ALTER TABLE %s ALTER COLUMN %s ..." % (element.table.name, element.column.name)
|
|
|
|
The second ``visit_alter_table`` will be invoked when any ``postgresql``
|
|
dialect is used.
|
|
|
|
Compiling sub-elements of a custom expression construct
|
|
=======================================================
|
|
|
|
The ``compiler`` argument is the
|
|
:class:`~sqlalchemy.engine.interfaces.Compiled` object in use. This object
|
|
can be inspected for any information about the in-progress compilation,
|
|
including ``compiler.dialect``, ``compiler.statement`` etc. The
|
|
:class:`~sqlalchemy.sql.compiler.SQLCompiler` and
|
|
:class:`~sqlalchemy.sql.compiler.DDLCompiler` both include a ``process()``
|
|
method which can be used for compilation of embedded attributes::
|
|
|
|
from sqlalchemy.sql.expression import Executable, ClauseElement
|
|
|
|
class InsertFromSelect(Executable, ClauseElement):
|
|
def __init__(self, table, select):
|
|
self.table = table
|
|
self.select = select
|
|
|
|
@compiles(InsertFromSelect)
|
|
def visit_insert_from_select(element, compiler, **kw):
|
|
return "INSERT INTO %s (%s)" % (
|
|
compiler.process(element.table, asfrom=True),
|
|
compiler.process(element.select)
|
|
)
|
|
|
|
insert = InsertFromSelect(t1, select([t1]).where(t1.c.x>5))
|
|
print insert
|
|
|
|
Produces::
|
|
|
|
"INSERT INTO mytable (SELECT mytable.x, mytable.y, mytable.z FROM mytable WHERE mytable.x > :x_1)"
|
|
|
|
.. note::
|
|
|
|
The above ``InsertFromSelect`` construct is only an example, this actual
|
|
functionality is already available using the
|
|
:meth:`.Insert.from_select` method.
|
|
|
|
.. note::
|
|
|
|
The above ``InsertFromSelect`` construct probably wants to have "autocommit"
|
|
enabled. See :ref:`enabling_compiled_autocommit` for this step.
|
|
|
|
Cross Compiling between SQL and DDL compilers
|
|
---------------------------------------------
|
|
|
|
SQL and DDL constructs are each compiled using different base compilers -
|
|
``SQLCompiler`` and ``DDLCompiler``. A common need is to access the
|
|
compilation rules of SQL expressions from within a DDL expression. The
|
|
``DDLCompiler`` includes an accessor ``sql_compiler`` for this reason, such as
|
|
below where we generate a CHECK constraint that embeds a SQL expression::
|
|
|
|
@compiles(MyConstraint)
|
|
def compile_my_constraint(constraint, ddlcompiler, **kw):
|
|
return "CONSTRAINT %s CHECK (%s)" % (
|
|
constraint.name,
|
|
ddlcompiler.sql_compiler.process(constraint.expression)
|
|
)
|
|
|
|
.. _enabling_compiled_autocommit:
|
|
|
|
Enabling Autocommit on a Construct
|
|
==================================
|
|
|
|
Recall from the section :ref:`autocommit` that the :class:`.Engine`, when
|
|
asked to execute a construct in the absence of a user-defined transaction,
|
|
detects if the given construct represents DML or DDL, that is, a data
|
|
modification or data definition statement, which requires (or may require,
|
|
in the case of DDL) that the transaction generated by the DBAPI be committed
|
|
(recall that DBAPI always has a transaction going on regardless of what
|
|
SQLAlchemy does). Checking for this is actually accomplished by checking for
|
|
the "autocommit" execution option on the construct. When building a
|
|
construct like an INSERT derivation, a new DDL type, or perhaps a stored
|
|
procedure that alters data, the "autocommit" option needs to be set in order
|
|
for the statement to function with "connectionless" execution
|
|
(as described in :ref:`dbengine_implicit`).
|
|
|
|
Currently a quick way to do this is to subclass :class:`.Executable`, then
|
|
add the "autocommit" flag to the ``_execution_options`` dictionary (note this
|
|
is a "frozen" dictionary which supplies a generative ``union()`` method)::
|
|
|
|
from sqlalchemy.sql.expression import Executable, ClauseElement
|
|
|
|
class MyInsertThing(Executable, ClauseElement):
|
|
_execution_options = \\
|
|
Executable._execution_options.union({'autocommit': True})
|
|
|
|
More succinctly, if the construct is truly similar to an INSERT, UPDATE, or
|
|
DELETE, :class:`.UpdateBase` can be used, which already is a subclass
|
|
of :class:`.Executable`, :class:`.ClauseElement` and includes the
|
|
``autocommit`` flag::
|
|
|
|
from sqlalchemy.sql.expression import UpdateBase
|
|
|
|
class MyInsertThing(UpdateBase):
|
|
def __init__(self, ...):
|
|
...
|
|
|
|
|
|
|
|
|
|
DDL elements that subclass :class:`.DDLElement` already have the
|
|
"autocommit" flag turned on.
|
|
|
|
|
|
|
|
|
|
Changing the default compilation of existing constructs
|
|
=======================================================
|
|
|
|
The compiler extension applies just as well to the existing constructs. When
|
|
overriding the compilation of a built in SQL construct, the @compiles
|
|
decorator is invoked upon the appropriate class (be sure to use the class,
|
|
i.e. ``Insert`` or ``Select``, instead of the creation function such
|
|
as ``insert()`` or ``select()``).
|
|
|
|
Within the new compilation function, to get at the "original" compilation
|
|
routine, use the appropriate visit_XXX method - this
|
|
because compiler.process() will call upon the overriding routine and cause
|
|
an endless loop. Such as, to add "prefix" to all insert statements::
|
|
|
|
from sqlalchemy.sql.expression import Insert
|
|
|
|
@compiles(Insert)
|
|
def prefix_inserts(insert, compiler, **kw):
|
|
return compiler.visit_insert(insert.prefix_with("some prefix"), **kw)
|
|
|
|
The above compiler will prefix all INSERT statements with "some prefix" when
|
|
compiled.
|
|
|
|
.. _type_compilation_extension:
|
|
|
|
Changing Compilation of Types
|
|
=============================
|
|
|
|
``compiler`` works for types, too, such as below where we implement the
|
|
MS-SQL specific 'max' keyword for ``String``/``VARCHAR``::
|
|
|
|
@compiles(String, 'mssql')
|
|
@compiles(VARCHAR, 'mssql')
|
|
def compile_varchar(element, compiler, **kw):
|
|
if element.length == 'max':
|
|
return "VARCHAR('max')"
|
|
else:
|
|
return compiler.visit_VARCHAR(element, **kw)
|
|
|
|
foo = Table('foo', metadata,
|
|
Column('data', VARCHAR('max'))
|
|
)
|
|
|
|
Subclassing Guidelines
|
|
======================
|
|
|
|
A big part of using the compiler extension is subclassing SQLAlchemy
|
|
expression constructs. To make this easier, the expression and
|
|
schema packages feature a set of "bases" intended for common tasks.
|
|
A synopsis is as follows:
|
|
|
|
* :class:`~sqlalchemy.sql.expression.ClauseElement` - This is the root
|
|
expression class. Any SQL expression can be derived from this base, and is
|
|
probably the best choice for longer constructs such as specialized INSERT
|
|
statements.
|
|
|
|
* :class:`~sqlalchemy.sql.expression.ColumnElement` - The root of all
|
|
"column-like" elements. Anything that you'd place in the "columns" clause of
|
|
a SELECT statement (as well as order by and group by) can derive from this -
|
|
the object will automatically have Python "comparison" behavior.
|
|
|
|
:class:`~sqlalchemy.sql.expression.ColumnElement` classes want to have a
|
|
``type`` member which is expression's return type. This can be established
|
|
at the instance level in the constructor, or at the class level if its
|
|
generally constant::
|
|
|
|
class timestamp(ColumnElement):
|
|
type = TIMESTAMP()
|
|
|
|
* :class:`~sqlalchemy.sql.expression.FunctionElement` - This is a hybrid of a
|
|
``ColumnElement`` and a "from clause" like object, and represents a SQL
|
|
function or stored procedure type of call. Since most databases support
|
|
statements along the line of "SELECT FROM <some function>"
|
|
``FunctionElement`` adds in the ability to be used in the FROM clause of a
|
|
``select()`` construct::
|
|
|
|
from sqlalchemy.sql.expression import FunctionElement
|
|
|
|
class coalesce(FunctionElement):
|
|
name = 'coalesce'
|
|
|
|
@compiles(coalesce)
|
|
def compile(element, compiler, **kw):
|
|
return "coalesce(%s)" % compiler.process(element.clauses)
|
|
|
|
@compiles(coalesce, 'oracle')
|
|
def compile(element, compiler, **kw):
|
|
if len(element.clauses) > 2:
|
|
raise TypeError("coalesce only supports two arguments on Oracle")
|
|
return "nvl(%s)" % compiler.process(element.clauses)
|
|
|
|
* :class:`~sqlalchemy.schema.DDLElement` - The root of all DDL expressions,
|
|
like CREATE TABLE, ALTER TABLE, etc. Compilation of ``DDLElement``
|
|
subclasses is issued by a ``DDLCompiler`` instead of a ``SQLCompiler``.
|
|
``DDLElement`` also features ``Table`` and ``MetaData`` event hooks via the
|
|
``execute_at()`` method, allowing the construct to be invoked during CREATE
|
|
TABLE and DROP TABLE sequences.
|
|
|
|
* :class:`~sqlalchemy.sql.expression.Executable` - This is a mixin which
|
|
should be used with any expression class that represents a "standalone"
|
|
SQL statement that can be passed directly to an ``execute()`` method. It
|
|
is already implicit within ``DDLElement`` and ``FunctionElement``.
|
|
|
|
Further Examples
|
|
================
|
|
|
|
"UTC timestamp" function
|
|
-------------------------
|
|
|
|
A function that works like "CURRENT_TIMESTAMP" except applies the
|
|
appropriate conversions so that the time is in UTC time. Timestamps are best
|
|
stored in relational databases as UTC, without time zones. UTC so that your
|
|
database doesn't think time has gone backwards in the hour when daylight
|
|
savings ends, without timezones because timezones are like character
|
|
encodings - they're best applied only at the endpoints of an application
|
|
(i.e. convert to UTC upon user input, re-apply desired timezone upon display).
|
|
|
|
For Postgresql and Microsoft SQL Server::
|
|
|
|
from sqlalchemy.sql import expression
|
|
from sqlalchemy.ext.compiler import compiles
|
|
from sqlalchemy.types import DateTime
|
|
|
|
class utcnow(expression.FunctionElement):
|
|
type = DateTime()
|
|
|
|
@compiles(utcnow, 'postgresql')
|
|
def pg_utcnow(element, compiler, **kw):
|
|
return "TIMEZONE('utc', CURRENT_TIMESTAMP)"
|
|
|
|
@compiles(utcnow, 'mssql')
|
|
def ms_utcnow(element, compiler, **kw):
|
|
return "GETUTCDATE()"
|
|
|
|
Example usage::
|
|
|
|
from sqlalchemy import (
|
|
Table, Column, Integer, String, DateTime, MetaData
|
|
)
|
|
metadata = MetaData()
|
|
event = Table("event", metadata,
|
|
Column("id", Integer, primary_key=True),
|
|
Column("description", String(50), nullable=False),
|
|
Column("timestamp", DateTime, server_default=utcnow())
|
|
)
|
|
|
|
"GREATEST" function
|
|
-------------------
|
|
|
|
The "GREATEST" function is given any number of arguments and returns the one
|
|
that is of the highest value - it's equivalent to Python's ``max``
|
|
function. A SQL standard version versus a CASE based version which only
|
|
accommodates two arguments::
|
|
|
|
from sqlalchemy.sql import expression
|
|
from sqlalchemy.ext.compiler import compiles
|
|
from sqlalchemy.types import Numeric
|
|
|
|
class greatest(expression.FunctionElement):
|
|
type = Numeric()
|
|
name = 'greatest'
|
|
|
|
@compiles(greatest)
|
|
def default_greatest(element, compiler, **kw):
|
|
return compiler.visit_function(element)
|
|
|
|
@compiles(greatest, 'sqlite')
|
|
@compiles(greatest, 'mssql')
|
|
@compiles(greatest, 'oracle')
|
|
def case_greatest(element, compiler, **kw):
|
|
arg1, arg2 = list(element.clauses)
|
|
return "CASE WHEN %s > %s THEN %s ELSE %s END" % (
|
|
compiler.process(arg1),
|
|
compiler.process(arg2),
|
|
compiler.process(arg1),
|
|
compiler.process(arg2),
|
|
)
|
|
|
|
Example usage::
|
|
|
|
Session.query(Account).\\
|
|
filter(
|
|
greatest(
|
|
Account.checking_balance,
|
|
Account.savings_balance) > 10000
|
|
)
|
|
|
|
"false" expression
|
|
------------------
|
|
|
|
Render a "false" constant expression, rendering as "0" on platforms that
|
|
don't have a "false" constant::
|
|
|
|
from sqlalchemy.sql import expression
|
|
from sqlalchemy.ext.compiler import compiles
|
|
|
|
class sql_false(expression.ColumnElement):
|
|
pass
|
|
|
|
@compiles(sql_false)
|
|
def default_false(element, compiler, **kw):
|
|
return "false"
|
|
|
|
@compiles(sql_false, 'mssql')
|
|
@compiles(sql_false, 'mysql')
|
|
@compiles(sql_false, 'oracle')
|
|
def int_false(element, compiler, **kw):
|
|
return "0"
|
|
|
|
Example usage::
|
|
|
|
from sqlalchemy import select, union_all
|
|
|
|
exp = union_all(
|
|
select([users.c.name, sql_false().label("enrolled")]),
|
|
select([customers.c.name, customers.c.enrolled])
|
|
)
|
|
|
|
"""
|
|
from .. import exc
|
|
from ..sql import visitors
|
|
|
|
|
|
def compiles(class_, *specs):
|
|
"""Register a function as a compiler for a
|
|
given :class:`.ClauseElement` type."""
|
|
|
|
def decorate(fn):
|
|
existing = class_.__dict__.get('_compiler_dispatcher', None)
|
|
existing_dispatch = class_.__dict__.get('_compiler_dispatch')
|
|
if not existing:
|
|
existing = _dispatcher()
|
|
|
|
if existing_dispatch:
|
|
existing.specs['default'] = existing_dispatch
|
|
|
|
# TODO: why is the lambda needed ?
|
|
setattr(class_, '_compiler_dispatch',
|
|
lambda *arg, **kw: existing(*arg, **kw))
|
|
setattr(class_, '_compiler_dispatcher', existing)
|
|
|
|
if specs:
|
|
for s in specs:
|
|
existing.specs[s] = fn
|
|
|
|
else:
|
|
existing.specs['default'] = fn
|
|
return fn
|
|
return decorate
|
|
|
|
|
|
def deregister(class_):
|
|
"""Remove all custom compilers associated with a given
|
|
:class:`.ClauseElement` type."""
|
|
|
|
if hasattr(class_, '_compiler_dispatcher'):
|
|
# regenerate default _compiler_dispatch
|
|
visitors._generate_dispatch(class_)
|
|
# remove custom directive
|
|
del class_._compiler_dispatcher
|
|
|
|
|
|
class _dispatcher(object):
|
|
def __init__(self):
|
|
self.specs = {}
|
|
|
|
def __call__(self, element, compiler, **kw):
|
|
# TODO: yes, this could also switch off of DBAPI in use.
|
|
fn = self.specs.get(compiler.dialect.name, None)
|
|
if not fn:
|
|
try:
|
|
fn = self.specs['default']
|
|
except KeyError:
|
|
raise exc.CompileError(
|
|
"%s construct has no default "
|
|
"compilation handler." % type(element))
|
|
return fn(element, compiler, **kw)
|