From 9428a47604d544d2b523df812b4db970683546a5 Mon Sep 17 00:00:00 2001
From: xilefff <42689203+xilefff@users.noreply.github.com>
Date: Tue, 20 Nov 2018 04:16:30 +0100
Subject: [PATCH 1/2] Create unique index migration

Adds concurrently unique index between friendable_id and friend_id to prevent duplicate friendships.
---
 db/migrate/4_add_unique_index_to_friendships.rb | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)
 create mode 100644 db/migrate/4_add_unique_index_to_friendships.rb

diff --git a/db/migrate/4_add_unique_index_to_friendships.rb b/db/migrate/4_add_unique_index_to_friendships.rb
new file mode 100644
index 0000000..d40c85d
--- /dev/null
+++ b/db/migrate/4_add_unique_index_to_friendships.rb
@@ -0,0 +1,17 @@
+if ActiveRecord.gem_version >= Gem::Version.new('5.0')
+  class AddUniqueIndexToFriendships < ActiveRecord::Migration[4.2]; end
+else
+  class AddUniqueIndexToFriendships < ActiveRecord::Migration; end
+end
+
+AddUniqueIndexToFriendships.class_eval do
+  disable_ddl_transaction!
+
+  def self.up
+    add_index :friendships, [:friendable_id, :friend_id], unique: true, algorithm: :concurrently
+  end
+
+  def self.down
+    remove_index :friendships, [:friendable_id, :friend_id]
+  end
+end

From 980d1eddcdaf2fb5a82f172f24f8d4b22d95d9fd Mon Sep 17 00:00:00 2001
From: xilefff <42689203+xilefff@users.noreply.github.com>
Date: Tue, 20 Nov 2018 14:21:30 +0100
Subject: [PATCH 2/2] Check if unique index already exists

Add the unique index unless it already exists between friendable_id and friend_id.
---
 db/migrate/4_add_unique_index_to_friendships.rb | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/db/migrate/4_add_unique_index_to_friendships.rb b/db/migrate/4_add_unique_index_to_friendships.rb
index d40c85d..7341340 100644
--- a/db/migrate/4_add_unique_index_to_friendships.rb
+++ b/db/migrate/4_add_unique_index_to_friendships.rb
@@ -8,7 +8,7 @@ class AddUniqueIndexToFriendships < ActiveRecord::Migration; end
   disable_ddl_transaction!
 
   def self.up
-    add_index :friendships, [:friendable_id, :friend_id], unique: true, algorithm: :concurrently
+    add_index :friendships, [:friendable_id, :friend_id], unique: true, algorithm: :concurrently unless index_exists? (:friendable_id, :friend_id)
   end
 
   def self.down