Join tables without IDs

gistfile1.md

These need to be backed up by a migration to create the assemblies_parts table. This table should be created without a primary key:

class CreateAssembliesPartsJoinTable < ActiveRecord::Migration
  def change
    create_table :assemblies_parts, id: false do |t|
      t.integer :assembly_id
      t.integer :part_id
    end
  end
end

We pass id: false to create_table because that table does not represent a model. That's required for the association to work properly. If you observe any strange behavior in a has_and_belongs_to_many association like mangled models IDs, or exceptions about conflicting IDs, chances are you forgot that bit.