til/rails/create-a-join-table-with-the-migration-dsl.md

Create A Join Table With The Migration DSL

The Rails migration DSL comes with a helper for creating a join table between two other existing tables.

Call create_join_table with two arguments, symbols for the names of the two tables.

def change
  create_join_table :tags, :posts
end

This will create a table with id references columns to each of the tables. The db/schema.rb addition will look something like this:

  create_table "posts_tags", id: false, force: :cascade do |t|
    t.bigint "tag_id", null: false
    t.bigint "post_id", null: false
  end

A Rails/ActiveRecord convention that comes into play for the creation of this table.

1. The name should be the pluralized versions of the two joined table names.
2. The joined table names should show up in the table name in alphabetical order.

Notice that despite listing :tags before :posts it creates a table called posts_tags. The DSL handles that for us.

Some create_join_table defaults to be aware of:

• It doesn't generate foreign key constraints.
• It uses bigint (or int) for the keys (even if those tables use UUIDs).
• The references are named after their respective table names.