SQLAlchemy 1.4 Documentation
SQLAlchemy ORM
- ORM Quick Start
- Object Relational Tutorial (1.x API)
- ORM Mapped Class Configuration
- ORM Mapped Class Overview
- Mapping Classes with Declarative
- Declarative Mapping Styles
- Table Configuration with Declarative
- Mapper Configuration with Declarative
- Composing Mapped Hierarchies with Mixins¶
- Augmenting the Base
- Mixing in Columns
- Mixing in Relationships
- Mixing in deferred(), column_property(), and other MapperProperty classes
- Controlling table inheritance with mixins
- Mixing in Columns in Inheritance Scenarios
- Combining Table/Mapper Arguments from Multiple Mixins
- Creating Indexes with Mixins
- Integration with dataclasses and attrs
- Mapping Columns and Expressions
- Mapping Class Inheritance Hierarchies
- Non-Traditional Mappings
- Configuring a Version Counter
- Class Mapping API
- Relationship Configuration
- Querying Data, Loading Objects
- Using the Session
- Events and Internals
- ORM Extensions
- ORM Examples
Project Versions
- Previous: Mapper Configuration with Declarative
- Next: Integration with dataclasses and attrs
- Up: Home
- On this page:
- Composing Mapped Hierarchies with Mixins
- Augmenting the Base
- Mixing in Columns
- Mixing in Relationships
- Mixing in deferred(), column_property(), and other MapperProperty classes
- Controlling table inheritance with mixins
- Mixing in Columns in Inheritance Scenarios
- Combining Table/Mapper Arguments from Multiple Mixins
- Creating Indexes with Mixins
Composing Mapped Hierarchies with Mixins¶
A common need when mapping classes using the Declarative style is to share some functionality, such as a set of common columns, some common table options, or other mapped properties, across many classes. The standard Python idioms for this is to have the classes inherit from a superclass which includes these common features.
When using declarative mappings, this idiom is allowed via the
usage of mixin classes, as well as via augmenting the declarative base
produced by either the registry.generate_base()
method
or declarative_base()
functions.
When using mixins or abstract base classes with Declarative, a decorator
known as declared_attr()
is frequently used. This decorator
allows the creation of class methods that produce a parameter or ORM construct that will be
part of a declarative mapping. Generating constructs using a callable
allows for Declarative to get a new copy of a particular kind of object
each time it calls upon the mixin or abstract base on behalf of a new
class that’s being mapped.
An example of some commonly mixed-in idioms is below:
from sqlalchemy.orm import declarative_mixin, declared_attr
@declarative_mixin
class MyMixin:
@declared_attr
def __tablename__(cls):
return cls.__name__.lower()
__table_args__ = {"mysql_engine": "InnoDB"}
__mapper_args__ = {"always_refresh": True}
id = Column(Integer, primary_key=True)
class MyModel(MyMixin, Base):
name = Column(String(1000))
Where above, the class MyModel
will contain an “id” column
as the primary key, a __tablename__
attribute that derives
from the name of the class itself, as well as __table_args__
and __mapper_args__
defined by the MyMixin
mixin class. The
declared_attr()
decorator applied to a class method called
def __tablename__(cls):
has the effect of turning the method into a class
method while also indicating to Declarative that this attribute is significant
within the mapping.
Tip
The use of the declarative_mixin()
class decorator marks a
particular class as providing the service of providing SQLAlchemy declarative
assignments as a mixin for other classes. This decorator is currently only
necessary to provide a hint to the Mypy plugin that
this class should be handled as part of declarative mappings.
There’s no fixed convention over whether MyMixin
precedes
Base
or not. Normal Python method resolution rules apply, and
the above example would work just as well with:
class MyModel(Base, MyMixin):
name = Column(String(1000))
This works because Base
here doesn’t define any of the
variables that MyMixin
defines, i.e. __tablename__
,
__table_args__
, id
, etc. If the Base
did define
an attribute of the same name, the class placed first in the
inherits list would determine which attribute is used on the
newly defined class.
Augmenting the Base¶
In addition to using a pure mixin, most of the techniques in this
section can also be applied to the base class itself, for patterns that
should apply to all classes derived from a particular base. This is achieved
using the cls
argument of the declarative_base()
function:
from sqlalchemy.orm import declarative_base, declared_attr
class Base:
@declared_attr
def __tablename__(cls):
return cls.__name__.lower()
__table_args__ = {"mysql_engine": "InnoDB"}
id = Column(Integer, primary_key=True)
Base = declarative_base(cls=Base)
class MyModel(Base):
name = Column(String(1000))
Where above, MyModel
and all other classes that derive from Base
will
have a table name derived from the class name, an id
primary key column,
as well as the “InnoDB” engine for MySQL.
Mixing in Columns¶
The most basic way to specify a column on a mixin is by simple declaration:
@declarative_mixin
class TimestampMixin:
created_at = Column(DateTime, default=func.now())
class MyModel(TimestampMixin, Base):
__tablename__ = "test"
id = Column(Integer, primary_key=True)
name = Column(String(1000))
Where above, all declarative classes that include TimestampMixin
will also have a column created_at
that applies a timestamp to
all row insertions.
Those familiar with the SQLAlchemy expression language know that
the object identity of clause elements defines their role in a schema.
Two Table
objects a
and b
may both have a column called
id
, but the way these are differentiated is that a.c.id
and b.c.id
are two distinct Python objects, referencing their
parent tables a
and b
respectively.
In the case of the mixin column, it seems that only one
Column
object is explicitly created, yet the ultimate
created_at
column above must exist as a distinct Python object
for each separate destination class. To accomplish this, the declarative
extension creates a copy of each Column
object encountered on
a class that is detected as a mixin.
This copy mechanism is limited to simple columns that have no foreign
keys, as a ForeignKey
itself contains references to columns
which can’t be properly recreated at this level. For columns that
have foreign keys, as well as for the variety of mapper-level constructs
that require destination-explicit context, the
declared_attr
decorator is provided so that
patterns common to many classes can be defined as callables:
from sqlalchemy.orm import declared_attr
@declarative_mixin
class ReferenceAddressMixin:
@declared_attr
def address_id(cls):
return Column(Integer, ForeignKey("address.id"))
class User(ReferenceAddressMixin, Base):
__tablename__ = "user"
id = Column(Integer, primary_key=True)
Where above, the address_id
class-level callable is executed at the
point at which the User
class is constructed, and the declarative
extension can use the resulting Column
object as returned by
the method without the need to copy it.
Columns generated by declared_attr
can also be
referenced by __mapper_args__
to a limited degree, currently
by polymorphic_on
and version_id_col
; the declarative extension
will resolve them at class construction time:
@declarative_mixin
class MyMixin:
@declared_attr
def type_(cls):
return Column(String(50))
__mapper_args__ = {"polymorphic_on": type_}
class MyModel(MyMixin, Base):
__tablename__ = "test"
id = Column(Integer, primary_key=True)
Mixing in Relationships¶
Relationships created by relationship()
are provided
with declarative mixin classes exclusively using the
declared_attr
approach, eliminating any ambiguity
which could arise when copying a relationship and its possibly column-bound
contents. Below is an example which combines a foreign key column and a
relationship so that two classes Foo
and Bar
can both be configured to
reference a common target class via many-to-one:
@declarative_mixin
class RefTargetMixin:
@declared_attr
def target_id(cls):
return Column("target_id", ForeignKey("target.id"))
@declared_attr
def target(cls):
return relationship("Target")
class Foo(RefTargetMixin, Base):
__tablename__ = "foo"
id = Column(Integer, primary_key=True)
class Bar(RefTargetMixin, Base):
__tablename__ = "bar"
id = Column(Integer, primary_key=True)
class Target(Base):
__tablename__ = "target"
id = Column(Integer, primary_key=True)
Using Advanced Relationship Arguments (e.g. primaryjoin
, etc.)¶
relationship()
definitions which require explicit
primaryjoin, order_by etc. expressions should in all but the most
simplistic cases use late bound forms
for these arguments, meaning, using either the string form or a function/lambda.
The reason for this is that the related Column
objects which are to
be configured using @declared_attr
are not available to another
@declared_attr
attribute; while the methods will work and return new
Column
objects, those are not the Column
objects that
Declarative will be using as it calls the methods on its own, thus using
different Column
objects.
The canonical example is the primaryjoin condition that depends upon another mixed-in column:
@declarative_mixin
class RefTargetMixin:
@declared_attr
def target_id(cls):
return Column("target_id", ForeignKey("target.id"))
@declared_attr
def target(cls):
return relationship(
Target,
primaryjoin=Target.id == cls.target_id, # this is *incorrect*
)
Mapping a class using the above mixin, we will get an error like:
sqlalchemy.exc.InvalidRequestError: this ForeignKey's parent column is not
yet associated with a Table.
This is because the target_id
Column
we’ve called upon in our
target()
method is not the same Column
that declarative is
actually going to map to our table.
The condition above is resolved using a lambda:
@declarative_mixin
class RefTargetMixin:
@declared_attr
def target_id(cls):
return Column("target_id", ForeignKey("target.id"))
@declared_attr
def target(cls):
return relationship(Target, primaryjoin=lambda: Target.id == cls.target_id)
or alternatively, the string form (which ultimately generates a lambda):
@declarative_mixin
class RefTargetMixin:
@declared_attr
def target_id(cls):
return Column("target_id", ForeignKey("target.id"))
@declared_attr
def target(cls):
return relationship(Target, primaryjoin=f"Target.id=={cls.__name__}.target_id")
Mixing in deferred(), column_property(), and other MapperProperty classes¶
Like relationship()
, all
MapperProperty
subclasses such as
deferred()
, column_property()
,
etc. ultimately involve references to columns, and therefore, when
used with declarative mixins, have the declared_attr
requirement so that no reliance on copying is needed:
@declarative_mixin
class SomethingMixin:
@declared_attr
def dprop(cls):
return deferred(Column(Integer))
class Something(SomethingMixin, Base):
__tablename__ = "something"
The column_property()
or other construct may refer
to other columns from the mixin. These are copied ahead of time before
the declared_attr
is invoked:
@declarative_mixin
class SomethingMixin:
x = Column(Integer)
y = Column(Integer)
@declared_attr
def x_plus_y(cls):
return column_property(cls.x + cls.y)
Changed in version 1.0.0: mixin columns are copied to the final mapped class
so that declared_attr
methods can access the actual column
that will be mapped.
Controlling table inheritance with mixins¶
The __tablename__
attribute may be used to provide a function that
will determine the name of the table used for each class in an inheritance
hierarchy, as well as whether a class has its own distinct table.
This is achieved using the declared_attr
indicator in conjunction
with a method named __tablename__()
. Declarative will always
invoke declared_attr
for the special names
__tablename__
, __mapper_args__
and __table_args__
function for each mapped class in the hierarchy, except if overridden
in a subclass. The function therefore
needs to expect to receive each class individually and to provide the
correct answer for each.
For example, to create a mixin that gives every class a simple table name based on class name:
from sqlalchemy.orm import declarative_mixin, declared_attr
@declarative_mixin
class Tablename:
@declared_attr
def __tablename__(cls):
return cls.__name__.lower()
class Person(Tablename, Base):
id = Column(Integer, primary_key=True)
discriminator = Column("type", String(50))
__mapper_args__ = {"polymorphic_on": discriminator}
class Engineer(Person):
__tablename__ = None
__mapper_args__ = {"polymorphic_identity": "engineer"}
primary_language = Column(String(50))
Alternatively, we can modify our __tablename__
function to return
None
for subclasses, using has_inherited_table()
. This has
the effect of those subclasses being mapped with single table inheritance
against the parent:
from sqlalchemy.orm import (
declarative_mixin,
declared_attr,
has_inherited_table,
)
@declarative_mixin
class Tablename:
@declared_attr
def __tablename__(cls):
if has_inherited_table(cls):
return None
return cls.__name__.lower()
class Person(Tablename, Base):
id = Column(Integer, primary_key=True)
discriminator = Column("type", String(50))
__mapper_args__ = {"polymorphic_on": discriminator}
class Engineer(Person):
primary_language = Column(String(50))
__mapper_args__ = {"polymorphic_identity": "engineer"}
Mixing in Columns in Inheritance Scenarios¶
In contrast to how __tablename__
and other special names are handled when
used with declared_attr
, when we mix in columns and properties (e.g.
relationships, column properties, etc.), the function is
invoked for the base class only in the hierarchy. Below, only the
Person
class will receive a column
called id
; the mapping will fail on Engineer
, which is not given
a primary key:
@declarative_mixin
class HasId:
@declared_attr
def id(cls):
return Column("id", Integer, primary_key=True)
class Person(HasId, Base):
__tablename__ = "person"
discriminator = Column("type", String(50))
__mapper_args__ = {"polymorphic_on": discriminator}
class Engineer(Person):
__tablename__ = "engineer"
primary_language = Column(String(50))
__mapper_args__ = {"polymorphic_identity": "engineer"}
It is usually the case in joined-table inheritance that we want distinctly
named columns on each subclass. However in this case, we may want to have
an id
column on every table, and have them refer to each other via
foreign key. We can achieve this as a mixin by using the
declared_attr.cascading
modifier, which indicates that the
function should be invoked for each class in the hierarchy, in almost
(see warning below) the same way as it does for __tablename__
:
@declarative_mixin
class HasIdMixin:
@declared_attr.cascading
def id(cls):
if has_inherited_table(cls):
return Column(ForeignKey("person.id"), primary_key=True)
else:
return Column(Integer, primary_key=True)
class Person(HasIdMixin, Base):
__tablename__ = "person"
discriminator = Column("type", String(50))
__mapper_args__ = {"polymorphic_on": discriminator}
class Engineer(Person):
__tablename__ = "engineer"
primary_language = Column(String(50))
__mapper_args__ = {"polymorphic_identity": "engineer"}
Warning
The declared_attr.cascading
feature currently does
not allow for a subclass to override the attribute with a different
function or value. This is a current limitation in the mechanics of
how @declared_attr
is resolved, and a warning is emitted if
this condition is detected. This limitation does not
exist for the special attribute names such as __tablename__
, which
resolve in a different way internally than that of
declared_attr.cascading
.
New in version 1.0.0: added declared_attr.cascading
.
Combining Table/Mapper Arguments from Multiple Mixins¶
In the case of __table_args__
or __mapper_args__
specified with declarative mixins, you may want to combine
some parameters from several mixins with those you wish to
define on the class itself. The
declared_attr
decorator can be used
here to create user-defined collation routines that pull
from multiple collections:
from sqlalchemy.orm import declarative_mixin, declared_attr
@declarative_mixin
class MySQLSettings:
__table_args__ = {"mysql_engine": "InnoDB"}
@declarative_mixin
class MyOtherMixin:
__table_args__ = {"info": "foo"}
class MyModel(MySQLSettings, MyOtherMixin, Base):
__tablename__ = "my_model"
@declared_attr
def __table_args__(cls):
args = dict()
args.update(MySQLSettings.__table_args__)
args.update(MyOtherMixin.__table_args__)
return args
id = Column(Integer, primary_key=True)
Creating Indexes with Mixins¶
To define a named, potentially multicolumn Index
that applies to all
tables derived from a mixin, use the “inline” form of Index
and
establish it as part of __table_args__
:
@declarative_mixin
class MyMixin:
a = Column(Integer)
b = Column(Integer)
@declared_attr
def __table_args__(cls):
return (Index(f"test_idx_{cls.__tablename__}", "a", "b"),)
class MyModel(MyMixin, Base):
__tablename__ = "atable"
c = Column(Integer, primary_key=True)
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