Mapping a Class against Multiple Tables¶

Mappers can be constructed against arbitrary relational units (called selectables) in addition to plain tables. For example, the join() function creates a selectable unit comprised of multiple tables, complete with its own composite primary key, which can be mapped in the same way as a Table:

from sqlalchemy import Table, Column, Integer, \
        String, MetaData, join, ForeignKey
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import column_property

metadata = MetaData()

# define two Table objects
user_table = Table('user', metadata,
            Column('id', Integer, primary_key=True),
            Column('name', String),
        )

address_table = Table('address', metadata,
            Column('id', Integer, primary_key=True),
            Column('user_id', Integer, ForeignKey('user.id')),
            Column('email_address', String)
            )

# define a join between them.  This
# takes place across the user.id and address.user_id
# columns.
user_address_join = join(user_table, address_table)

Base = declarative_base()

# map to it
class AddressUser(Base):
    __table__ = user_address_join

    id = column_property(user_table.c.id, address_table.c.user_id)
    address_id = address_table.c.id

In the example above, the join expresses columns for both the user and the address table. The user.id and address.user_id columns are equated by foreign key, so in the mapping they are defined as one attribute, AddressUser.id, using column_property() to indicate a specialized column mapping. Based on this part of the configuration, the mapping will copy new primary key values from user.id into the address.user_id column when a flush occurs.

Additionally, the address.id column is mapped explicitly to an attribute named address_id. This is to disambiguate the mapping of the address.id column from the same-named AddressUser.id attribute, which here has been assigned to refer to the user table combined with the address.user_id foreign key.

The natural primary key of the above mapping is the composite of (user.id, address.id), as these are the primary key columns of the user and address table combined together. The identity of an AddressUser object will be in terms of these two values, and is represented from an AddressUser object as (AddressUser.id, AddressUser.address_id).

Mapping a Class against Arbitrary Selects¶

Similar to mapping against a join, a plain select() object can be used with a mapper as well. The example fragment below illustrates mapping a class called Customer to a select() which includes a join to a subquery:

from sqlalchemy import select, func

subq = select([
            func.count(orders.c.id).label('order_count'),
            func.max(orders.c.price).label('highest_order'),
            orders.c.customer_id
            ]).group_by(orders.c.customer_id).alias()

customer_select = select([customers, subq]).\
            select_from(
                join(customers, subq,
                        customers.c.id == subq.c.customer_id)
            ).alias()

class Customer(Base):
    __table__ = customer_select

Above, the full row represented by customer_select will be all the columns of the customers table, in addition to those columns exposed by the subq subquery, which are order_count, highest_order, and customer_id. Mapping the Customer class to this selectable then creates a class which will contain those attributes.

When the ORM persists new instances of Customer, only the customers table will actually receive an INSERT. This is because the primary key of the orders table is not represented in the mapping; the ORM will only emit an INSERT into a table for which it has mapped the primary key.

Multiple Mappers for One Class¶

In modern SQLAlchemy, a particular class is mapped by only one so-called primary mapper at a time. This mapper is involved in three main areas of functionality: querying, persistence, and instrumentation of the mapped class. The rationale of the primary mapper relates to the fact that the mapper() modifies the class itself, not only persisting it towards a particular Table, but also instrumenting attributes upon the class which are structured specifically according to the table metadata. It’s not possible for more than one mapper to be associated with a class in equal measure, since only one mapper can actually instrument the class.

However, there is a class of mapper known as the non primary mapper with allows additional mappers to be associated with a class, but with a limited scope of use. This scope typically applies to being able to load rows from an alternate table or selectable unit, but still producing classes which are ultimately persisted using the primary mapping. The non-primary mapper is created using the classical style of mapping against a class that is already mapped with a primary mapper, and involves the use of the non_primary flag.

The non primary mapper is of very limited use in modern SQLAlchemy, as the task of being able to load classes from subqueries or other compound statements can be now accomplished using the Query object directly.

There is really only one use case for the non-primary mapper, which is that we wish to build a relationship() to such a mapper; this is useful in the rare and advanced case that our relationship is attempting to join two classes together using many tables and/or joins in between. An example of this pattern is at Relationship to Non Primary Mapper.

As far as the use case of a class that can actually be fully persisted to different tables under different scenarios, very early versions of SQLAlchemy offered a feature for this adapted from Hibernate, known as the “entity name” feature. However, this use case became infeasable within SQLAlchemy once the mapped class itself became the source of SQL expression construction; that is, the class’ attributes themselves link directly to mapped table columns. The feature was removed and replaced with a simple recipe-oriented approach to accomplishing this task without any ambiguity of instrumentation - to create new subclasses, each mapped individually. This pattern is now available as a recipe at Entity Name.