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

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
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-_816 TILs and counting..._
+_817 TILs and counting..._
 
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@@ -474,6 +474,7 @@ _816 TILs and counting..._
 
 ### Rails
 
+- [Add A Foreign Key Reference To A Table](rails/add-a-foreign-key-reference-to-a-table.md)
 - [Add React With Webpacker To A New Rails App](rails/add-react-with-webpacker-to-a-new-rails-app.md)
 - [Access Secrets In A Rails 5.2 App](rails/access-secrets-in-a-rails-5-2-app.md)
 - [ActiveRecord Query For This Or That](rails/active-record-query-for-this-or-that.md)

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

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+# 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`.