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)

class MyMutableType(Mutable):
    def __getstate__(self):
        d = self.__dict__.copy()
        d.pop('_parents', None)
        return d

class MutableDict(Mutable, dict):
    # ....

    def __getstate__(self):
        return dict(self)

    def __setstate__(self, state):
        self.update(state)

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.

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

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

class Point(MutableComposite):
    # ...

    def __getstate__(self):
        return self.x, self.y

    def __setstate__(self, state):
        self.x, self.y = state

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

_get_listen_keys(attribute)¶

inherited from the _get_listen_keys() method of MutableBase

Given a descriptor attribute, return a set() of the attribute keys which indicate a change in the state of this attribute.

This is normally just set([attribute.key]), but can be overridden to provide for additional keys. E.g. a MutableComposite augments this set with the attribute keys associated with the columns that comprise the composite value.

This collection is consulted in the case of intercepting the InstanceEvents.refresh() and InstanceEvents.refresh_flush() events, which pass along a list of attribute names that have been refreshed; the list is compared against this set to determine if action needs to be taken.

New in version 1.0.5.

_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.

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.

The MutableDict object implements a dictionary that will emit change events to the underlying mapping when the contents of the dictionary are altered, including when values are added or removed.

Note that MutableDict does not apply mutable tracking to the values themselves inside the dictionary. Therefore it is not a sufficient solution for the use case of tracking deep changes to a recursive dictionary structure, such as a JSON structure. To support this use case, build a subclass of MutableDict that provides appropriate coersion to the values placed in the dictionary so that they too are “mutable”, and emit events up to their parent structure.

New in version 0.8.

clear()¶

classmethod coerce(key, value)¶

Convert plain dictionary to instance of this class.

pop(*arg)¶

popitem()¶

setdefault(key, value)¶

update(*a, **kw)¶