# ext/mutable.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
"""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)