Add Replace An Index With A Unique Index as a Rails til

rails/replace-an-index-with-a-unique-index.md

@@ -0,0 +1,34 @@
+# Replace An Index With A Unique Index
+
+Indexes and uniqueness constraints often go together. In fact, in Postgres,
+when you create a unique constraint, an index is created under the hood to
+support that constraint.
+
+What if you already have an index, but you want to turn it into a unique index?
+There is no way to alter or update the index to be unique. Instead, what you'll
+want to do is drop the index and then recreate it as a unique index.
+
+Here's how you can do that with the Rails migration DSL:
+
+```ruby
+class ReplaceIndexWithUniqueIndex < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def up
+    remove_index :users_roles, [:user_id, :role_id]
+    add_index :users_roles, [:user_id, :role_id], unique: true, algorithm: :concurrently
+  end
+
+  def down
+    remove_index :users_roles, [:user_id, :role_id]
+    add_index :users_roles, [:user_id, :role_id], algorithm: :concurrently
+  end
+end
+```
+
+This removes the original multi-column index and then adds back in a unique
+index that covers the same columns. I added `disable_ddl_transactions!` so that
+the new index could be added concurrently.
+
+I've also included a `down` migration that reverses the process in case a
+rollback is needed.