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")

See also

Late-Evaluation of Relationship Arguments

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)