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

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
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-_1125 TILs and counting..._
+_1126 TILs and counting..._
 
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@@ -721,6 +721,7 @@ _1125 TILs and counting..._
 - [Remove The Default Value On A Column](rails/remove-the-default-value-on-a-column.md)
 - [Render An Alternative ActionMailer Template](rails/render-an-alternative-action-mailer-template.md)
 - [Render The Response Body In Controller Specs](rails/render-the-response-body-in-controller-specs.md)
+- [Replace An Index With A Unique Index](rails/replace-an-index-with-a-unique-index.md)
 - [Rescue From](rails/rescue-from.md)
 - [Retrieve An Object If It Exists](rails/retrieve-an-object-if-it-exists.md)
 - [Rollback A Specific Migration Out Of Order](rails/rollback-a-specific-migration-out-of-order.md)

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

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+# 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.