# ext/mutable.py # Copyright (C) 2005-2013 the SQLAlchemy authors and contributors # # This module is part of SQLAlchemy and is released under # the MIT License: http://www.opensource.org/licenses/mit-license.php """Provide support for tracking of in-place changes to scalar values, which are propagated into ORM change events on owning parent objects. The :mod:`sqlalchemy.ext.mutable` extension replaces SQLAlchemy's legacy approach to in-place mutations of scalar values, established by the :class:`.types.MutableType` class as well as the ``mutable=True`` type flag, with a system that allows change events to be propagated from the value to the owning parent, thereby removing the need for the ORM to maintain copies of values as well as the very expensive requirement of scanning through all "mutable" values on each flush call, looking for changes. .. _mutable_scalars: Establishing Mutability on Scalar Column Values =============================================== A typical example of a "mutable" structure is a Python dictionary. Following the example introduced in :ref:`types_toplevel`, we begin with a custom type that marshals Python dictionaries into JSON strings before being persisted:: from sqlalchemy.types import TypeDecorator, VARCHAR import json class JSONEncodedDict(TypeDecorator): "Represents an immutable structure as a json-encoded string." impl = VARCHAR def process_bind_param(self, value, dialect): if value is not None: value = json.dumps(value) return value def process_result_value(self, value, dialect): if value is not None: value = json.loads(value) return value The usage of ``json`` is only for the purposes of example. The :mod:`sqlalchemy.ext.mutable` extension can be used with any type whose target Python type may be mutable, including :class:`.PickleType`, :class:`.postgresql.ARRAY`, etc. When using the :mod:`sqlalchemy.ext.mutable` extension, the value itself tracks all parents which reference it. Below, we illustrate the a simple version of the :class:`.MutableDict` dictionary object, which applies the :class:`.Mutable` mixin to a plain Python dictionary:: import collections from sqlalchemy.ext.mutable import Mutable class MutableDict(Mutable, dict): @classmethod def coerce(cls, key, value): "Convert plain dictionaries to MutableDict." if not isinstance(value, MutableDict): if isinstance(value, dict): return MutableDict(value) # this call will raise ValueError return Mutable.coerce(key, value) else: return value def __setitem__(self, key, value): "Detect dictionary set events and emit change events." dict.__setitem__(self, key, value) self.changed() def __delitem__(self, key): "Detect dictionary del events and emit change events." dict.__delitem__(self, key) self.changed() The above dictionary class takes the approach of subclassing the Python built-in ``dict`` to produce a dict subclass which routes all mutation events through ``__setitem__``. There are variants on this approach, such as subclassing ``UserDict.UserDict`` or ``collections.MutableMapping``; the part that's important to this example is that the :meth:`.Mutable.changed` method is called whenever an in-place change to the datastructure takes place. We also redefine the :meth:`.Mutable.coerce` method which will be used to convert any values that are not instances of ``MutableDict``, such as the plain dictionaries returned by the ``json`` module, into the appropriate type. Defining this method is optional; we could just as well created our ``JSONEncodedDict`` such that it always returns an instance of ``MutableDict``, and additionally ensured that all calling code uses ``MutableDict`` explicitly. When :meth:`.Mutable.coerce` is not overridden, any values applied to a parent object which are not instances of the mutable type will raise a ``ValueError``. Our new ``MutableDict`` type offers a class method :meth:`~.Mutable.as_mutable` which we can use within column metadata to associate with types. This method grabs the given type object or class and associates a listener that will detect all future mappings of this type, applying event listening instrumentation to the mapped attribute. Such as, with classical table metadata:: from sqlalchemy import Table, Column, Integer my_data = Table('my_data', metadata, Column('id', Integer, primary_key=True), Column('data', MutableDict.as_mutable(JSONEncodedDict)) ) Above, :meth:`~.Mutable.as_mutable` returns an instance of ``JSONEncodedDict`` (if the type object was not an instance already), which will intercept any attributes which are mapped against this type. Below we establish a simple mapping against the ``my_data`` table:: from sqlalchemy import mapper class MyDataClass(object): pass # associates mutation listeners with MyDataClass.data mapper(MyDataClass, my_data) The ``MyDataClass.data`` member will now be notified of in place changes to its value. There's no difference in usage when using declarative:: from sqlalchemy.ext.declarative import declarative_base Base = declarative_base() class MyDataClass(Base): __tablename__ = 'my_data' id = Column(Integer, primary_key=True) data = Column(MutableDict.as_mutable(JSONEncodedDict)) Any in-place changes to the ``MyDataClass.data`` member will flag the attribute as "dirty" on the parent object:: >>> from sqlalchemy.orm import Session >>> sess = Session() >>> m1 = MyDataClass(data={'value1':'foo'}) >>> sess.add(m1) >>> sess.commit() >>> m1.data['value1'] = 'bar' >>> assert m1 in sess.dirty True The ``MutableDict`` can be associated with all future instances of ``JSONEncodedDict`` in one step, using :meth:`~.Mutable.associate_with`. This is similar to :meth:`~.Mutable.as_mutable` except it will intercept all occurrences of ``MutableDict`` in all mappings unconditionally, without the need to declare it individually:: MutableDict.associate_with(JSONEncodedDict) class MyDataClass(Base): __tablename__ = 'my_data' id = Column(Integer, primary_key=True) data = Column(JSONEncodedDict) Supporting Pickling -------------------- The key to the :mod:`sqlalchemy.ext.mutable` extension relies upon the placement of a ``weakref.WeakKeyDictionary`` upon the value object, which stores a mapping of parent mapped objects keyed to the attribute name under which they are associated with this value. ``WeakKeyDictionary`` objects are not picklable, due to the fact that they contain weakrefs and function callbacks. In our case, this is a good thing, since if this dictionary were picklable, it could lead to an excessively large pickle size for our value objects that are pickled by themselves outside of the context of the parent. The developer responsibility here is only to provide a ``__getstate__`` method that excludes the :meth:`~.MutableBase._parents` collection from the pickle stream:: class MyMutableType(Mutable): def __getstate__(self): d = self.__dict__.copy() d.pop('_parents', None) return d With our dictionary example, we need to return the contents of the dict itself (and also restore them on __setstate__):: class MutableDict(Mutable, dict): # .... def __getstate__(self): return dict(self) def __setstate__(self, state): self.update(state) In the case that our mutable value object is pickled as it is attached to one or more parent objects that are also part of the pickle, the :class:`.Mutable` mixin will re-establish the :attr:`.Mutable._parents` collection on each value object as the owning parents themselves are unpickled. .. _mutable_composites: Establishing Mutability on Composites ===================================== Composites are a special ORM feature which allow a single scalar attribute to be assigned an object value which represents information "composed" from one or more columns from the underlying mapped table. The usual example is that of a geometric "point", and is introduced in :ref:`mapper_composite`. .. versionchanged:: 0.7 The internals of :func:`.orm.composite` have been greatly simplified and in-place mutation detection is no longer enabled by default; instead, the user-defined value must detect changes on its own and propagate them to all owning parents. The :mod:`sqlalchemy.ext.mutable` extension provides the helper class :class:`.MutableComposite`, which is a slight variant on the :class:`.Mutable` class. As is the case with :class:`.Mutable`, the user-defined composite class subclasses :class:`.MutableComposite` as a mixin, and detects and delivers change events to its parents via the :meth:`.MutableComposite.changed` method. In the case of a composite class, the detection is usually via the usage of Python descriptors (i.e. ``@property``), or alternatively via the special Python method ``__setattr__()``. Below we expand upon the ``Point`` class introduced in :ref:`mapper_composite` to subclass :class:`.MutableComposite` and to also route attribute set events via ``__setattr__`` to the :meth:`.MutableComposite.changed` method:: from sqlalchemy.ext.mutable import MutableComposite class Point(MutableComposite): def __init__(self, x, y): self.x = x self.y = y def __setattr__(self, key, value): "Intercept set events" # set the attribute object.__setattr__(self, key, value) # alert all parents to the change self.changed() def __composite_values__(self): return self.x, self.y def __eq__(self, other): return isinstance(other, Point) and \\ other.x == self.x and \\ other.y == self.y def __ne__(self, other): return not self.__eq__(other) The :class:`.MutableComposite` class uses a Python metaclass to automatically establish listeners for any usage of :func:`.orm.composite` that specifies our ``Point`` type. Below, when ``Point`` is mapped to the ``Vertex`` class, listeners are established which will route change events from ``Point`` objects to each of the ``Vertex.start`` and ``Vertex.end`` attributes:: from sqlalchemy.orm import composite, mapper from sqlalchemy import Table, Column vertices = Table('vertices', metadata, Column('id', Integer, primary_key=True), Column('x1', Integer), Column('y1', Integer), Column('x2', Integer), Column('y2', Integer), ) class Vertex(object): pass mapper(Vertex, vertices, properties={ 'start': composite(Point, vertices.c.x1, vertices.c.y1), 'end': composite(Point, vertices.c.x2, vertices.c.y2) }) Any in-place changes to the ``Vertex.start`` or ``Vertex.end`` members will flag the attribute as "dirty" on the parent object:: >>> from sqlalchemy.orm import Session >>> sess = Session() >>> v1 = Vertex(start=Point(3, 4), end=Point(12, 15)) >>> sess.add(v1) >>> sess.commit() >>> v1.end.x = 8 >>> assert v1 in sess.dirty True Coercing Mutable Composites --------------------------- The :meth:`.MutableBase.coerce` method is also supported on composite types. In the case of :class:`.MutableComposite`, the :meth:`.MutableBase.coerce` method is only called for attribute set operations, not load operations. Overriding the :meth:`.MutableBase.coerce` method is essentially equivalent to using a :func:`.validates` validation routine for all attributes which make use of the custom composite type:: class Point(MutableComposite): # other Point methods # ... def coerce(cls, key, value): if isinstance(value, tuple): value = Point(*value) elif not isinstance(value, Point): raise ValueError("tuple or Point expected") return value .. versionadded:: 0.7.10,0.8.0b2 Support for the :meth:`.MutableBase.coerce` method in conjunction with objects of type :class:`.MutableComposite`. Supporting Pickling -------------------- As is the case with :class:`.Mutable`, the :class:`.MutableComposite` helper class uses a ``weakref.WeakKeyDictionary`` available via the :meth:`.MutableBase._parents` attribute which isn't picklable. If we need to pickle instances of ``Point`` or its owning class ``Vertex``, we at least need to define a ``__getstate__`` that doesn't include the ``_parents`` dictionary. Below we define both a ``__getstate__`` and a ``__setstate__`` that package up the minimal form of our ``Point`` class:: class Point(MutableComposite): # ... def __getstate__(self): return self.x, self.y def __setstate__(self, state): self.x, self.y = state As with :class:`.Mutable`, the :class:`.MutableComposite` augments the pickling process of the parent's object-relational state so that the :meth:`.MutableBase._parents` collection is restored to all ``Point`` objects. """ from ..orm.attributes import flag_modified from .. import event, types from ..orm import mapper, object_mapper, Mapper from ..util import memoized_property import weakref class MutableBase(object): """Common base class to :class:`.Mutable` and :class:`.MutableComposite`. """ @memoized_property def _parents(self): """Dictionary of parent object->attribute name on the parent. This attribute is a so-called "memoized" property. It initializes itself with a new ``weakref.WeakKeyDictionary`` the first time it is accessed, returning the same object upon subsequent access. """ return weakref.WeakKeyDictionary() @classmethod def coerce(cls, key, value): """Given a value, coerce it into the target type. Can be overridden by custom subclasses to coerce incoming data into a particular type. By default, raises ``ValueError``. This method is called in different scenarios depending on if the parent class is of type :class:`.Mutable` or of type :class:`.MutableComposite`. In the case of the former, it is called for both attribute-set operations as well as during ORM loading operations. For the latter, it is only called during attribute-set operations; the mechanics of the :func:`.composite` construct handle coercion during load operations. :param key: string name of the ORM-mapped attribute being set. :param value: the incoming value. :return: the method should return the coerced value, or raise ``ValueError`` if the coercion cannot be completed. """ if value is None: return None msg = "Attribute '%s' does not accept objects of type %s" raise ValueError(msg % (key, type(value))) @classmethod def _listen_on_attribute(cls, attribute, coerce, parent_cls): """Establish this type as a mutation listener for the given mapped descriptor. """ key = attribute.key if parent_cls is not attribute.class_: return # rely on "propagate" here parent_cls = attribute.class_ def load(state, *args): """Listen for objects loaded or refreshed. Wrap the target data member's value with ``Mutable``. """ val = state.dict.get(key, None) if val is not None: if coerce: val = cls.coerce(key, val) state.dict[key] = val val._parents[state.obj()] = key def set(target, value, oldvalue, initiator): """Listen for set/replace events on the target data member. Establish a weak reference to the parent object on the incoming value, remove it for the one outgoing. """ if not isinstance(value, cls): value = cls.coerce(key, value) if value is not None: value._parents[target.obj()] = key if isinstance(oldvalue, cls): oldvalue._parents.pop(target.obj(), None) return value def pickle(state, state_dict): val = state.dict.get(key, None) if val is not None: if 'ext.mutable.values' not in state_dict: state_dict['ext.mutable.values'] = [] state_dict['ext.mutable.values'].append(val) def unpickle(state, state_dict): if 'ext.mutable.values' in state_dict: for val in state_dict['ext.mutable.values']: val._parents[state.obj()] = key event.listen(parent_cls, 'load', load, raw=True, propagate=True) event.listen(parent_cls, 'refresh', load, raw=True, propagate=True) event.listen(attribute, 'set', set, raw=True, retval=True, propagate=True) event.listen(parent_cls, 'pickle', pickle, raw=True, propagate=True) event.listen(parent_cls, 'unpickle', unpickle, raw=True, propagate=True) class Mutable(MutableBase): """Mixin that defines transparent propagation of change events to a parent object. See the example in :ref:`mutable_scalars` for usage information. """ def changed(self): """Subclasses should call this method whenever change events occur.""" for parent, key in self._parents.items(): flag_modified(parent, key) @classmethod def associate_with_attribute(cls, attribute): """Establish this type as a mutation listener for the given mapped descriptor. """ cls._listen_on_attribute(attribute, True, attribute.class_) @classmethod def associate_with(cls, sqltype): """Associate this wrapper with all future mapped columns of the given type. This is a convenience method that calls ``associate_with_attribute`` automatically. .. warning:: The listeners established by this method are *global* to all mappers, and are *not* garbage collected. Only use :meth:`.associate_with` for types that are permanent to an application, not with ad-hoc types else this will cause unbounded growth in memory usage. """ def listen_for_type(mapper, class_): for prop in mapper.column_attrs: if isinstance(prop.columns[0].type, sqltype): cls.associate_with_attribute(getattr(class_, prop.key)) event.listen(mapper, 'mapper_configured', listen_for_type) @classmethod def as_mutable(cls, sqltype): """Associate a SQL type with this mutable Python type. This establishes listeners that will detect ORM mappings against the given type, adding mutation event trackers to those mappings. The type is returned, unconditionally as an instance, so that :meth:`.as_mutable` can be used inline:: Table('mytable', metadata, Column('id', Integer, primary_key=True), Column('data', MyMutableType.as_mutable(PickleType)) ) Note that the returned type is always an instance, even if a class is given, and that only columns which are declared specifically with that type instance receive additional instrumentation. To associate a particular mutable type with all occurrences of a particular type, use the :meth:`.Mutable.associate_with` classmethod of the particular :meth:`.Mutable` subclass to establish a global association. .. warning:: The listeners established by this method are *global* to all mappers, and are *not* garbage collected. Only use :meth:`.as_mutable` for types that are permanent to an application, not with ad-hoc types else this will cause unbounded growth in memory usage. """ sqltype = types.to_instance(sqltype) def listen_for_type(mapper, class_): for prop in mapper.column_attrs: if prop.columns[0].type is sqltype: cls.associate_with_attribute(getattr(class_, prop.key)) event.listen(mapper, 'mapper_configured', listen_for_type) return sqltype class MutableComposite(MutableBase): """Mixin that defines transparent propagation of change events on a SQLAlchemy "composite" object to its owning parent or parents. See the example in :ref:`mutable_composites` for usage information. """ def changed(self): """Subclasses should call this method whenever change events occur.""" for parent, key in self._parents.items(): prop = object_mapper(parent).get_property(key) for value, attr_name in zip( self.__composite_values__(), prop._attribute_keys): setattr(parent, attr_name, value) def _setup_composite_listener(): import types def _listen_for_type(mapper, class_): for prop in mapper.iterate_properties: if (hasattr(prop, 'composite_class') and (type(prop.composite_class) in (types.ClassType, types.TypeType)) and issubclass(prop.composite_class, MutableComposite)): prop.composite_class._listen_on_attribute( getattr(class_, prop.key), False, class_) if not Mapper.dispatch.mapper_configured._contains(Mapper, _listen_for_type): event.listen(Mapper, 'mapper_configured', _listen_for_type) _setup_composite_listener() class MutableDict(Mutable, dict): """A dictionary type that implements :class:`.Mutable`. .. versionadded:: 0.8 """ def __setitem__(self, key, value): """Detect dictionary set events and emit change events.""" dict.__setitem__(self, key, value) self.changed() def __delitem__(self, key): """Detect dictionary del events and emit change events.""" dict.__delitem__(self, key) self.changed() def clear(self): dict.clear(self) self.changed() @classmethod def coerce(cls, key, value): """Convert plain dictionary to MutableDict.""" if not isinstance(value, MutableDict): if isinstance(value, dict): return MutableDict(value) return Mutable.coerce(key, value) else: return value def __getstate__(self): return dict(self) def __setstate__(self, state): self.update(state)