Add Different Ways To Add A Foreign Key Reference as a Rails til

Author: jbranchaud

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).
 
-_1096 TILs and counting..._
+_1097 TILs and counting..._
 
 ---
 
@@ -669,6 +669,7 @@ _1096 TILs and counting..._
 - [Customize The Path Of A Resource Route](rails/customize-the-path-of-a-resource-route.md)
 - [Delete Paranoid Records](rails/delete-paranoid-records.md)
 - [Demodulize A Class Name](rails/demodulize-a-class-name.md)
+- [Different Ways To Add A Foreign Key Reference](rails/different-ways-to-add-a-foreign-key-reference.md)
 - [Disambiguate Where In A Joined Relation](rails/disambiguate-where-in-a-joined-relation.md)
 - [Ensure Migrations Use The Latest Schema](rails/ensure-migrations-use-the-latest-schema.md)
 - [Force All Users To Sign Out](rails/force-all-users-to-sign-out.md)

rails/different-ways-to-add-a-foreign-key-reference.md

@@ -0,0 +1,47 @@
+# Different Ways To Add A Foreign Key Reference
+
+A foreign key reference creates a relationship between two tables that is
+guaranteed by a foreign key constraint.
+
+This is a minimal example.
+
+```ruby
+create_table :books
+  t.references :author, foreign_key: true
+end
+```
+
+The `foreign_key: true` is needed here, otherwise just the reference column is
+created without a backing constraint. When `foreign_key` is true, an index will
+be created for the column as well.
+
+This is a maximal example.
+
+```ruby
+create_table :books
+  t.references :author, index: true, foreign_key: true, type: :uuid, null: false
+end
+```
+
+It is explicit about the foreign key and index. It specifies a `not null`
+constraint. It declares the type as `uuid` assuming the `authors` table's
+primary key is of type `uuid`.
+
+Here is an example with a custom column name.
+
+```ruby
+create_table :books
+  t.references :written_by, foreign_key: { to_table: :authors }
+end
+```
+
+Here is adding a reference to an existing table.
+
+```ruby
+def up
+  add_reference :books, :author, index: true, foreign_key: true
+end
+```
+
+There are more combinations of these, but I hope there is enough here to be
+able to iterate to a solution that works for you.