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

Question

I've discovered a nice way to generate join tables for my HABTM relationships in my Rails app.

rails g migration CreateJoinTable table1 table2

This generates an ActiveRecord::Migration that employs the method create_join_table

I'm wondering what this wonderful mysterious method does. I guess it makes a table (probably without an id field) that has a column for table1 foreign key and a column for table2 foreign key, but does the table have any other features?. My habit for join tables has always been to add a unique index across both those columns so that a relationship between a record in table1 and a record in table2 cannot be entered twice.

My question boils down to: If I use create_join_table do I need to keep adding that unique index, or does this method do that for me (I think it should)?

The documentation I usually look at doesn't go into this sort of detail.

Answer 1

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_id
end

Have a look at create_join_table documentation.

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

Answer 2

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
  end
end

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