Add Create A Join Table With The Migration DSL as a rails til

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

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+# 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.
+
+```ruby
+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:
+
+```ruby
+  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.