Release: 1.0.0 | Release Date: Not released

SQLAlchemy 1.0 Documentation

Mutation Tracking

Provide support for tracking of in-place changes to scalar values, which are propagated into ORM change events on owning parent objects.

New in version 0.7: sqlalchemy.ext.mutable replaces SQLAlchemy’s legacy approach to in-place mutations of scalar values; see Mutation event extension, supersedes “mutable=True”.

Establishing Mutability on Scalar Column Values

A typical example of a “mutable” structure is a Python dictionary. Following the example introduced in Column and Data Types, 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 sqlalchemy.ext.mutable extension can be used with any type whose target Python type may be mutable, including PickleType, postgresql.ARRAY, etc.

When using the sqlalchemy.ext.mutable extension, the value itself tracks all parents which reference it. Below, we illustrate the a simple version of the MutableDict dictionary object, which applies the Mutable mixin to a plain Python dictionary:

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 Mutable.changed() method is called whenever an in-place change to the datastructure takes place.

We also redefine the 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 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 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, 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 associate_with(). This is similar to 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 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 _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 Mutable mixin will re-establish the Mutable._parents collection on each value object as the owning parents themselves are unpickled.

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 Composite Column Types.

Changed in version 0.7: The internals of 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 sqlalchemy.ext.mutable extension provides the helper class MutableComposite, which is a slight variant on the Mutable class.

As is the case with Mutable, the user-defined composite class subclasses MutableComposite as a mixin, and detects and delivers change events to its parents via the 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 Composite Column Types to subclass MutableComposite and to also route attribute set events via __setattr__ to the 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 MutableComposite class uses a Python metaclass to automatically establish listeners for any usage of 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 MutableBase.coerce() method is also supported on composite types. In the case of MutableComposite, the MutableBase.coerce() method is only called for attribute set operations, not load operations. Overriding the MutableBase.coerce() method is essentially equivalent to using a 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

New in version 0.7.10,0.8.0b2: Support for the MutableBase.coerce() method in conjunction with objects of type MutableComposite.

Supporting Pickling

As is the case with Mutable, the MutableComposite helper class uses a weakref.WeakKeyDictionary available via the 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 Mutable, the MutableComposite augments the pickling process of the parent’s object-relational state so that the MutableBase._parents() collection is restored to all Point objects.

API Reference

class sqlalchemy.ext.mutable.MutableBase

Common base class to Mutable and MutableComposite.

_parents

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.

classmethod coerce(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 Mutable or of type 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 composite() construct handle coercion during load operations.

Parameters:
  • key – string name of the ORM-mapped attribute being set.
  • value – the incoming value.
Returns:

the method should return the coerced value, or raise ValueError if the coercion cannot be completed.

class sqlalchemy.ext.mutable.Mutable

Bases: sqlalchemy.ext.mutable.MutableBase

Mixin that defines transparent propagation of change events to a parent object.

See the example in Establishing Mutability on Scalar Column Values for usage information.

__init__
inherited from the __init__ attribute of object

x.__init__(...) initializes x; see help(type(x)) for signature

classmethod _listen_on_attribute(attribute, coerce, parent_cls)
inherited from the _listen_on_attribute() method of MutableBase

Establish this type as a mutation listener for the given mapped descriptor.

_parents
inherited from the _parents attribute of MutableBase

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.

classmethod as_mutable(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 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 Mutable.associate_with() classmethod of the particular 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 as_mutable() for types that are permanent to an application, not with ad-hoc types else this will cause unbounded growth in memory usage.

classmethod associate_with(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 associate_with() for types that are permanent to an application, not with ad-hoc types else this will cause unbounded growth in memory usage.

classmethod associate_with_attribute(attribute)

Establish this type as a mutation listener for the given mapped descriptor.

changed()

Subclasses should call this method whenever change events occur.

classmethod coerce(key, value)
inherited from the coerce() method of MutableBase

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 Mutable or of type 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 composite() construct handle coercion during load operations.

Parameters:
  • key – string name of the ORM-mapped attribute being set.
  • value – the incoming value.
Returns:

the method should return the coerced value, or raise ValueError if the coercion cannot be completed.

class sqlalchemy.ext.mutable.MutableComposite

Bases: sqlalchemy.ext.mutable.MutableBase

Mixin that defines transparent propagation of change events on a SQLAlchemy “composite” object to its owning parent or parents.

See the example in Establishing Mutability on Composites for usage information.

changed()

Subclasses should call this method whenever change events occur.

class sqlalchemy.ext.mutable.MutableDict

Bases: sqlalchemy.ext.mutable.Mutable, __builtin__.dict

A dictionary type that implements Mutable.

New in version 0.8.

classmethod coerce(key, value)

Convert plain dictionary to instance of this class.