Add guard clause to migration to check for index before attempting to add or remove it (#4644)

db/migrate/20191025185619_add_old_username_index_to_users.rb

@@ -1,8 +1,23 @@
 class AddOldUsernameIndexToUsers < ActiveRecord::Migration[5.2]
   disable_ddl_transaction!
 
-  def change
-    add_index :users, :old_username, algorithm: :concurrently
-    add_index :users, :old_old_username, algorithm: :concurrently
+  def up
+    if indexes(:users).none? { |idx| idx.columns.map(&:to_sym) == [:old_username] }
+      add_index :users, :old_username, algorithm: :concurrently
+    end
+
+    if indexes(:users).none? { |idx| idx.columns.map(&:to_sym) == [:old_old_username] }
+      add_index :users, :old_old_username, algorithm: :concurrently
+    end
+  end
+
+  def down
+    if indexes(:users).any? { |idx| idx.columns.map(&:to_sym) == [:old_username] }
+      remove_index :users, :old_username
+    end
+
+    if indexes(:users).any? { |idx| idx.columns.map(&:to_sym) == [:old_old_username] }
+      remove_index :users, :old_old_username
+    end
   end
 end