Add Add A Reference Column With An Index as a rails til

rails/add-a-reference-column-with-an-index.md

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+# Add A Reference Column With An Index
+
+Though I prefer to always back my reference columns with a [foreign
+key](add-a-foreign-key-reference-to-a-table.md), sometimes you may just want to
+add the reference column on its own. Though this could be done manually with
+the `add_column` directive, you can be more explicit with `add_reference` --
+which allows you to specify whether or not an index is to be added.
+
+```ruby
+def up
+  add_reference :books, :authors, index: true
+end
+```
+
+This will add `authors_id` and an index to the `books` table.
+
+You can additionally specify the type of the column. This is handy if you are
+using `uuid`s for all your primary keys.
+
+```ruby
+def up
+  add_reference :books, :authors, type: :uuid, index: true
+end
+```
+
+[source](https://nandovieira.com/using-uuid-with-postgresql-and-activerecord)