Add Add A Foreign Key Reference To A Table as a rails til

rails/add-a-foreign-key-reference-to-a-table.md

@@ -0,0 +1,33 @@
+# Add A Foreign Key Reference To A Table
+
+Foreign keys are a great way to maintain referential integrity within our data.
+We can add reference columns with foreign key constraints using the Rails
+migration DSL.
+
+Here is how we include one as part of creating a table:
+
+```ruby
+def up
+  create_table :books do |t|
+    # ... other columns
+
+    t.references :author, index: true, foreign_key: true
+  end
+end
+```
+
+This will add a column, `author_id`, to the `books` table that references the
+`authors` table. It will have both a foreign key constraint and an index
+applied to it.
+
+Here is how we do the same for an existing table:
+
+```ruby
+def up
+  add_reference :books, :author, index: true, foreign_key: true
+end
+```
+
+As of Rails 5, this is a bit verbose as `index: true` happens by default.
+Though I'm always in favor of explicitness. If for whatever reason you don't
+want an index, you will have to specify `index: false`.