drnic/composite_primary_keys — pull request #103 by TylerRick (2012-04-27 16:42:49+00:00)
Title: Fixed: When you call association.build for a has_many association that uses a composite foreign key, the newly built child record should have the values in its belongs_to foreign_key populated from its owner's values.

For example:
Department: has_many :employees, :foreign_key => [:department_id, :location_id]

employee = department.employees.build
assert_equal department.department_id, employee.department_id
assert_equal department.location_id, employee.location_id
assert_equal department, employee.department