HABTM and Migrations Gotcha

If you are using migrations in your Rails apps (and you should) and you are also using has-and-belongs-to-many relationships then there is a gotcha to be aware of. The join tables in a HABTM relationship should *not* include an ID column, however migrations, by default, creates ID columns from all of your tables. Therefore, to avoid this problem, all join tables in HABTM relationships should include :id => false in the create_table invocation.

Example:

create_table :people_pets, :id => false do |t|
 t.column :person_id, :integer, :null => false
 t.column :pet_id, :integer, :null => false
end

Good stuff to know.