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

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://tinyletter.com/jbranchaud).
 
-_1093 TILs and counting..._
+_1094 TILs and counting..._
 
 ---
 
@@ -661,6 +661,7 @@ _1093 TILs and counting..._
 - [Convert A Symbol To A Constant](rails/convert-a-symbol-to-a-constant.md)
 - [Count The Number Of Records By Attribute](rails/count-the-number-of-records-by-attribute.md)
 - [Create A Custom Named References Column](rails/create-a-custom-named-references-column.md)
+- [Create A Join Table With The Migration DSL](rails/create-a-join-table-with-the-migration-dsl.md)
 - [Creating Records of Has_One Associations](rails/creating-records-of-has-one-associations.md)
 - [Custom Validation Message](rails/custom-validation-message.md)
 - [Customize Paths And Helpers For Devise Routes](rails/customize-paths-and-helpers-for-devise-routes.md)

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.