sqlalchemy.ext.declarative.AbstractConcreteBase

A helper class for 'concrete' declarative mappings.

.AbstractConcreteBase will use the .polymorphic_union function automatically, against all tables mapped as a subclass to this class. The function is called via the __declare_last__() function, which is essentially a hook for the .after_configured event.

.AbstractConcreteBase does produce a mapped class for the base class, however it is not persisted to any table; it is instead mapped directly to the "polymorphic" selectable directly and is only used for selecting. Compare to .ConcreteBase, which does create a persisted table for the base class.

from sqlalchemy.orm import configure_mappers

configure_mappers()

Example:

from sqlalchemy.ext.declarative import AbstractConcreteBase

class Employee(AbstractConcreteBase, Base):
    pass

class Manager(Employee):
    __tablename__ = 'manager'
    employee_id = Column(Integer, primary_key=True)
    name = Column(String(50))
    manager_data = Column(String(40))

    __mapper_args__ = {
        'polymorphic_identity':'manager',
        'concrete':True}

configure_mappers()

The abstract base class is handled by declarative in a special way; at class configuration time, it behaves like a declarative mixin or an __abstract__ base class. Once classes are configured and mappings are produced, it then gets mapped itself, but after all of its descendants. This is a very unique system of mapping not found in any other SQLAlchemy system.

Using this approach, we can specify columns and properties that will take place on mapped subclasses, in the way that we normally do as in :ref:`declarative_mixins`:

class Company(Base):
    __tablename__ = 'company'
    id = Column(Integer, primary_key=True)

class Employee(AbstractConcreteBase, Base):
    employee_id = Column(Integer, primary_key=True)

    @declared_attr
    def company_id(cls):
        return Column(ForeignKey('company.id'))

    @declared_attr
    def company(cls):
        return relationship("Company")

class Manager(Employee):
    __tablename__ = 'manager'

    name = Column(String(50))
    manager_data = Column(String(40))

    __mapper_args__ = {
        'polymorphic_identity':'manager',
        'concrete':True}

configure_mappers()

When we make use of our mappings however, both Manager and Employee will have an independently usable .company attribute:

session.query(Employee).filter(Employee.company.has(id=5))