What is the behaviour of create_join_table of ActiveRecord::Migration?

Called without any block, create_join_table just creates a table with two foreign keys referring to the two joined tables.

However, you can actually pass a block when you call the method to do any additional operations (say, adding indexes for example). From the Rails doc:

create_join_table :products, :categories do |t|  t.index :product_id  t.index :category_idend

Have a look at create_join_table documentation.

You can check the create_join_table code at the bottom (click on Source: show).

SchemaStatements#create_join_table() only creates join table without any fancy indexes etc,... So if you wish to use uniqueness constraint on two fields you have to do something like this:

class CreateJoinTable < ActiveRecord::Migration  def change    create_join_table :posts, :users do |t|      t.integer :post_id, index: true      t.integer :user_id, index: true      t.index [:post_id, :user_id], name: 'post_user_un', unique: true    end  endend

Please also note that create_join_table by default does NOT create id field.

It turns out it doesn't do any more than the basics I described in the question. I found this out simply by running the migration and seeing what ends up in db/schema.rb

For those interested, to get the unique index do this:

class CreateJoinTable < ActiveRecord::Migration  def change    create_join_table :posts, :users    add_index :posts_users, [:post_id, :user_id], unique: true, name: 'index_posts_users'  endend