A foreign key is an object defined by several properties. It
represents a relationship from a column or columns in one table to a
column or columns in another table.
This relationship is defined by its cardinality (one to one, one to
many, or many to one) and its dependencies (whether or not table X is
dependent on table Y, and vice versa).
Many to many relationships are not allowed. However, you may indicate
such a relationship when using the
method method, and it will create
the necessary intermediate linking table for you.