sqlalchemy.ext.mutable

Provide support for tracking of in-place changes to scalar values, which are propagated into ORM change events on owning parent objects. .. _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 a simple version of the :class:`.MutableDict` dictionary object, which applies the :class:`.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 :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.orm import DeclarativeBase from sqlalchemy.orm import Mapped from sqlalchemy.orm import mapped_column class Base(DeclarativeBase): pass class MyDataClass(Base): __tablename__ = 'my_data' id: Mapped[int] = mapped_column(primary_key=True) data: Mapped[dict[str, str]] = mapped_column(MutableDict.as_mutable(JSONEncodedDict)) The ``MyDataClass.data`` member will now be notified of in place changes to its value. 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(some_engine) >>> 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:: from sqlalchemy.orm import DeclarativeBase from sqlalchemy.orm import Mapped from sqlalchemy.orm import mapped_column MutableDict.associate_with(JSONEncodedDict) class Base(DeclarativeBase): pass class MyDataClass(Base): __tablename__ = 'my_data' id: Mapped[int] = mapped_column(primary_key=True) data: Mapped[dict[str, str]] = mapped_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. Receiving Events ---------------- The :meth:`.AttributeEvents.modified` event handler may be used to receive an event when a mutable scalar emits a change event. This event handler is called when the :func:`.attributes.flag_modified` function is called from within the mutable extension:: from sqlalchemy.orm import DeclarativeBase from sqlalchemy.orm import Mapped from sqlalchemy.orm import mapped_column from sqlalchemy import event class Base(DeclarativeBase): pass class MyDataClass(Base): __tablename__ = 'my_data' id: Mapped[int] = mapped_column(primary_key=True) data: Mapped[dict[str, str]] = mapped_column(MutableDict.as_mutable(JSONEncodedDict)) @event.listens_for(MyDataClass.data, "modified") def modified_json(instance, initiator): print("json value modified:", instance.data) .. _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`. 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 the special Python method ``__setattr__()``. In the example below, we expand upon the ``Point`` class introduced in :ref:`mapper_composite` to include :class:`.MutableComposite` in its bases and to route attribute set events via ``__setattr__`` to the :meth:`.MutableComposite.changed` method:: import dataclasses from sqlalchemy.ext.mutable import MutableComposite @dataclasses.dataclass class Point(MutableComposite): x: int y: int def __setattr__(self, key, value): "Intercept set events" # set the attribute object.__setattr__(self, key, value) # alert all parents to the change self.changed() The :class:`.MutableComposite` class makes use of class mapping events 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 DeclarativeBase, Mapped from sqlalchemy.orm import composite, mapped_column class Base(DeclarativeBase): pass class Vertex(Base): __tablename__ = "vertices" id: Mapped[int] = mapped_column(primary_key=True) start: Mapped[Point] = composite(mapped_column("x1"), mapped_column("y1")) end: Mapped[Point] = composite(mapped_column("x2"), mapped_column("y2")) def __repr__(self): return f"Vertex(start={self.start}, end={self.end})" Any in-place changes to the ``Vertex.start`` or ``Vertex.end`` members will flag the attribute as "dirty" on the parent object: .. sourcecode:: python+sql >>> from sqlalchemy.orm import Session >>> sess = Session(engine) >>> v1 = Vertex(start=Point(3, 4), end=Point(12, 15)) >>> sess.add(v1) {sql}>>> sess.flush() BEGIN (implicit) INSERT INTO vertices (x1, y1, x2, y2) VALUES (?, ?, ?, ?) [...] (3, 4, 12, 15) {stop}>>> v1.end.x = 8 >>> assert v1 in sess.dirty True {sql}>>> sess.commit() UPDATE vertices SET x2=? WHERE vertices.id = ? [...] (8, 1) COMMIT 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:: @dataclasses.dataclass 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 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:: @dataclasses.dataclass 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.