SQLAlchemy 2.1 Documentation
SQLAlchemy ORM
- ORM Quick Start
- ORM Mapped Class Configuration
- Relationship Configuration
- ORM Querying Guide
- Using the Session
- Events and Internals
- ORM Extensions
- Asynchronous I/O (asyncio)
- Association Proxy
- Automap
- Baked Queries
- Declarative Extensions
- Mypy / Pep-484 Support for ORM Mappings
- Mutation Tracking¶
- Ordering List
- Horizontal Sharding
- Hybrid Attributes
- Indexable
- Alternate Class Instrumentation
- ORM Examples
Project Versions
Mutation Tracking¶
Provide support for tracking of in-place changes to scalar values, which are propagated into ORM change events on owning parent objects.
Establishing Mutability on Scalar Column Values¶
A typical example of a “mutable” structure is a Python dictionary. Following the example introduced in SQL Datatype Objects, 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
, ARRAY
, etc.
When using the sqlalchemy.ext.mutable
extension, the value itself
tracks all parents which reference it. Below, we illustrate 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
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, 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
Mutable.associate_with()
. This is similar to
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 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 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 Mutable
mixin will re-establish the Mutable._parents
collection on each value
object as the owning parents themselves are unpickled.
Receiving Events¶
The 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 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)
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 the
special Python method __setattr__()
. In the example below, we expand upon the Point
class introduced in Composite Column Types to include
MutableComposite
in its bases and to route attribute set events via
__setattr__
to the 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 MutableComposite
class makes use of class mapping events to
automatically establish listeners for any usage of 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:
>>> 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)
>>> 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 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:
@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 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:
@dataclasses.dataclass
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¶
Object Name | Description |
---|---|
Mixin that defines transparent propagation of change events to a parent object. |
|
Common base class to |
|
Mixin that defines transparent propagation of change events on a SQLAlchemy “composite” object to its owning parent or parents. |
|
A dictionary type that implements |
|
A list type that implements |
|
A set type that implements |
- class sqlalchemy.ext.mutable.MutableBase¶
-
Common base class to
Mutable
andMutableComposite
.-
attribute
sqlalchemy.ext.mutable.MutableBase.
_parents¶ Dictionary of parent object’s
InstanceState
->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.Changed in version 1.4: the
InstanceState
is now used as the key in the weak dictionary rather than the instance itself.
-
classmethod
sqlalchemy.ext.mutable.MutableBase.
coerce(key: str, value: Any) → Any | None¶ 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 typeMutableComposite
. 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 thecomposite()
construct handle coercion during load operations.
-
attribute
- class sqlalchemy.ext.mutable.Mutable¶
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.
Members
_get_listen_keys(), _listen_on_attribute(), _parents, as_mutable(), associate_with(), associate_with_attribute(), changed(), coerce()
Class signature
class
sqlalchemy.ext.mutable.Mutable
(sqlalchemy.ext.mutable.MutableBase
)-
classmethod
sqlalchemy.ext.mutable.Mutable.
_get_listen_keys(attribute: QueryableAttribute[Any]) → Set[str]¶ inherited from the
sqlalchemy.ext.mutable.MutableBase._get_listen_keys
method ofMutableBase
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. aMutableComposite
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()
andInstanceEvents.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.
-
classmethod
sqlalchemy.ext.mutable.Mutable.
_listen_on_attribute(attribute: QueryableAttribute[Any], coerce: bool, parent_cls: _ExternalEntityType[Any]) → None¶ inherited from the
sqlalchemy.ext.mutable.MutableBase._listen_on_attribute
method ofMutableBase
Establish this type as a mutation listener for the given mapped descriptor.
-
attribute
sqlalchemy.ext.mutable.Mutable.
_parents¶ inherited from the
sqlalchemy.ext.mutable.MutableBase._parents
attribute ofMutableBase
Dictionary of parent object’s
InstanceState
->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.Changed in version 1.4: the
InstanceState
is now used as the key in the weak dictionary rather than the instance itself.
-
classmethod
sqlalchemy.ext.mutable.Mutable.
as_mutable(sqltype: _TypeEngineArgument[_T]) → TypeEngine[_T]¶ 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 particularMutable
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
sqlalchemy.ext.mutable.Mutable.
associate_with(sqltype: type) → None¶ 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
sqlalchemy.ext.mutable.Mutable.
associate_with_attribute(attribute: InstrumentedAttribute[_O]) → None¶ Establish this type as a mutation listener for the given mapped descriptor.
-
method
sqlalchemy.ext.mutable.Mutable.
changed() → None¶ Subclasses should call this method whenever change events occur.
-
classmethod
sqlalchemy.ext.mutable.Mutable.
coerce(key: str, value: Any) → Any | None¶ inherited from the
MutableBase.coerce()
method ofMutableBase
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 typeMutableComposite
. 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 thecomposite()
construct handle coercion during load operations.
-
classmethod
- class sqlalchemy.ext.mutable.MutableComposite¶
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.
Members
Class signature
class
sqlalchemy.ext.mutable.MutableComposite
(sqlalchemy.ext.mutable.MutableBase
)-
method
sqlalchemy.ext.mutable.MutableComposite.
changed() → None¶ Subclasses should call this method whenever change events occur.
-
method
- class sqlalchemy.ext.mutable.MutableDict¶
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 ofMutableDict
that provides appropriate coercion to the values placed in the dictionary so that they too are “mutable”, and emit events up to their parent structure.Class signature
class
sqlalchemy.ext.mutable.MutableDict
(sqlalchemy.ext.mutable.Mutable
,builtins.dict
,typing.Generic
)-
method
sqlalchemy.ext.mutable.MutableDict.
clear() → None. Remove all items from D.¶
-
classmethod
sqlalchemy.ext.mutable.MutableDict.
coerce(key: str, value: Any) → MutableDict[_KT, _VT] | None¶ Convert plain dictionary to instance of this class.
-
method
sqlalchemy.ext.mutable.MutableDict.
pop(k[, d]) → v, remove specified key and return the corresponding value.¶ If the key is not found, return the default if given; otherwise, raise a KeyError.
-
method
sqlalchemy.ext.mutable.MutableDict.
popitem() → Tuple[_KT, _VT]¶ Remove and return a (key, value) pair as a 2-tuple.
Pairs are returned in LIFO (last-in, first-out) order. Raises KeyError if the dict is empty.
-
method
sqlalchemy.ext.mutable.MutableDict.
setdefault(*arg)¶ Insert key with a value of default if key is not in the dictionary.
Return the value for key if key is in the dictionary, else default.
-
method
sqlalchemy.ext.mutable.MutableDict.
update([E, ]**F) → None. Update D from dict/iterable E and F.¶ If E is present and has a .keys() method, then does: for k in E: D[k] = E[k] If E is present and lacks a .keys() method, then does: for k, v in E: D[k] = v In either case, this is followed by: for k in F: D[k] = F[k]
-
method
- class sqlalchemy.ext.mutable.MutableList¶
A list type that implements
Mutable
.The
MutableList
object implements a list that will emit change events to the underlying mapping when the contents of the list are altered, including when values are added or removed.Note that
MutableList
does not apply mutable tracking to the values themselves inside the list. Therefore it is not a sufficient solution for the use case of tracking deep changes to a recursive mutable structure, such as a JSON structure. To support this use case, build a subclass ofMutableList
that provides appropriate coercion to the values placed in the dictionary so that they too are “mutable”, and emit events up to their parent structure.Members
append(), clear(), coerce(), extend(), insert(), is_iterable(), is_scalar(), pop(), remove(), reverse(), sort()
Class signature
class
sqlalchemy.ext.mutable.MutableList
(sqlalchemy.ext.mutable.Mutable
,builtins.list
,typing.Generic
)-
method
sqlalchemy.ext.mutable.MutableList.
append(x: _T) → None¶ Append object to the end of the list.
-
method
sqlalchemy.ext.mutable.MutableList.
clear() → None¶ Remove all items from list.
-
classmethod
sqlalchemy.ext.mutable.MutableList.
coerce(key: str, value: MutableList[_T] | _T) → MutableList[_T] | None¶ Convert plain list to instance of this class.
-
method
sqlalchemy.ext.mutable.MutableList.
extend(x: Iterable[_T]) → None¶ Extend list by appending elements from the iterable.
-
method
sqlalchemy.ext.mutable.MutableList.
insert(i: SupportsIndex, x: _T) → None¶ Insert object before index.
-
method
sqlalchemy.ext.mutable.MutableList.
is_iterable(value: _T | Iterable[_T]) → TypeGuard[Iterable[_T]]¶
-
method
sqlalchemy.ext.mutable.MutableList.
is_scalar(value: _T | Iterable[_T]) → TypeGuard[_T]¶
-
method
sqlalchemy.ext.mutable.MutableList.
pop(*arg: SupportsIndex) → _T¶ Remove and return item at index (default last).
Raises IndexError if list is empty or index is out of range.
-
method
sqlalchemy.ext.mutable.MutableList.
remove(i: _T) → None¶ Remove first occurrence of value.
Raises ValueError if the value is not present.
-
method
sqlalchemy.ext.mutable.MutableList.
reverse() → None¶ Reverse IN PLACE.
-
method
sqlalchemy.ext.mutable.MutableList.
sort(**kw: Any) → None¶ Sort the list in ascending order and return None.
The sort is in-place (i.e. the list itself is modified) and stable (i.e. the order of two equal elements is maintained).
If a key function is given, apply it once to each list item and sort them, ascending or descending, according to their function values.
The reverse flag can be set to sort in descending order.
-
method
- class sqlalchemy.ext.mutable.MutableSet¶
A set type that implements
Mutable
.The
MutableSet
object implements a set that will emit change events to the underlying mapping when the contents of the set are altered, including when values are added or removed.Note that
MutableSet
does not apply mutable tracking to the values themselves inside the set. Therefore it is not a sufficient solution for the use case of tracking deep changes to a recursive mutable structure. To support this use case, build a subclass ofMutableSet
that provides appropriate coercion to the values placed in the dictionary so that they too are “mutable”, and emit events up to their parent structure.Members
add(), clear(), coerce(), difference_update(), discard(), intersection_update(), pop(), remove(), symmetric_difference_update(), update()
Class signature
class
sqlalchemy.ext.mutable.MutableSet
(sqlalchemy.ext.mutable.Mutable
,builtins.set
,typing.Generic
)-
method
sqlalchemy.ext.mutable.MutableSet.
add(elem: _T) → None¶ Add an element to a set.
This has no effect if the element is already present.
-
method
sqlalchemy.ext.mutable.MutableSet.
clear() → None¶ Remove all elements from this set.
-
classmethod
sqlalchemy.ext.mutable.MutableSet.
coerce(index: str, value: Any) → MutableSet[_T] | None¶ Convert plain set to instance of this class.
-
method
sqlalchemy.ext.mutable.MutableSet.
difference_update(*arg: Iterable[Any]) → None¶ Remove all elements of another set from this set.
-
method
sqlalchemy.ext.mutable.MutableSet.
discard(elem: _T) → None¶ Remove an element from a set if it is a member.
If the element is not a member, do nothing.
-
method
sqlalchemy.ext.mutable.MutableSet.
intersection_update(*arg: Iterable[Any]) → None¶ Update a set with the intersection of itself and another.
-
method
sqlalchemy.ext.mutable.MutableSet.
pop(*arg: Any) → _T¶ Remove and return an arbitrary set element. Raises KeyError if the set is empty.
-
method
sqlalchemy.ext.mutable.MutableSet.
remove(elem: _T) → None¶ Remove an element from a set; it must be a member.
If the element is not a member, raise a KeyError.
-
method
sqlalchemy.ext.mutable.MutableSet.
symmetric_difference_update(*arg: Iterable[_T]) → None¶ Update a set with the symmetric difference of itself and another.
-
method
sqlalchemy.ext.mutable.MutableSet.
update(*arg: Iterable[_T]) → None¶ Update a set with the union of itself and others.
-
method
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