Non-Traditional Mappings — SQLAlchemy 1.2 Documentation

Release: 1.2.19 | Release Date: April 15, 2019

# SQLAlchemy 1.2 Documentation

# Non-Traditional Mappings

## 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.

Note

The practice of mapping to arbitrary SELECT statements, especially complex ones as above, is almost never needed; it necessarily tends to produce complex queries which are often less efficient than that which would be produced by direct query construction. The practice is to some degree based on the very early history of SQLAlchemy where the `mapper()` construct was meant to represent the primary querying interface; in modern usage, the `Query` object can be used to construct virtually any SELECT statement, including complex composites, and should be favored over the “map-to-selectable” approach.

## 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 which 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.