6.3. Declaring Relationships Between Mappers

Although the features that SQLAlchemy provides for mapping tables and other selectables to classes are powerful in their own right, SQLAlchemy also allows you to model relationships between tables as simple Python lists and properties using the relation() function in the properties parameter of the mapper() function.

6.3.1. Basic Relationships

The three main relationships modeled by SQLAlchemy are 1:N, M:N, and 1:1 (which is actually a special case of 1:N). In a 1:N relationship, one table (the “N” side) generally has a foreign key to another table (the “1” side). In M:N, two tables (the “primary” tables) are related via a scondary, “join” table that has foreign keys into both primary tables. A 1:1 relationship is simply a 1:N relationship where there is only one “N”-side row with a foreign key to any particular “1”-side row.