From a979dc4253ad029d03f3e52cce34be1c751d21d9 Mon Sep 17 00:00:00 2001
From: jbranchaud <jbranchaud@gmail.com>
Date: Tue, 18 May 2021 11:57:54 -0500
Subject: [PATCH] Add Replace An Index With A Unique Index as a Rails til

---
 README.md                                     |  3 +-
 rails/replace-an-index-with-a-unique-index.md | 34 +++++++++++++++++++
 2 files changed, 36 insertions(+), 1 deletion(-)
 create mode 100644 rails/replace-an-index-with-a-unique-index.md

diff --git a/README.md b/README.md
index a4627cb..6c9af5b 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://tinyletter.com/jbranchaud).
 
-_1125 TILs and counting..._
+_1126 TILs and counting..._
 
 ---
 
@@ -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)
diff --git a/rails/replace-an-index-with-a-unique-index.md b/rails/replace-an-index-with-a-unique-index.md
new file mode 100644
index 0000000..33adc5b
--- /dev/null
+++ b/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.