Guard creation/removal with existence checks.

db/migrate/20180320182229_add_indexes_for_user_activity_queries.rb

@@ -6,25 +6,35 @@ class AddIndexesForUserActivityQueries < ActiveRecord::Migration
   disable_ddl_transaction!
 
   def up
-    add_concurrent_index :events, [:author_id, :project_id]
-    add_concurrent_index :user_interacted_projects, :user_id
+    add_concurrent_index :events, [:author_id, :project_id] unless index_exists?(:events, [:author_id, :project_id])
+    add_concurrent_index :user_interacted_projects, :user_id unless index_exists?(:user_interacted_projects, :user_id)
   end
 
   def down
-    remove_concurrent_index :events, [:author_id, :project_id]
+    remove_concurrent_index :events, [:author_id, :project_id] if index_exists?(:events, [:author_id, :project_id])
 
     patch_foreign_keys do
-      remove_concurrent_index :user_interacted_projects, :user_id
+      remove_concurrent_index :user_interacted_projects, :user_id if index_exists?(:user_interacted_projects, :user_id)
     end
   end
 
   private
 
   def patch_foreign_keys
+    return yield if Gitlab::Database.postgresql?
+
     # MySQL doesn't like to remove the index with a foreign key using it.
-    remove_foreign_key :user_interacted_projects, :users unless Gitlab::Database.postgresql?
+    remove_foreign_key :user_interacted_projects, :users if fk_exists?(:user_interacted_projects, :user_id)
+
     yield
+
     # Let's re-add the foreign key using the existing index on (user_id, project_id)
-    add_concurrent_foreign_key :user_interacted_projects, :users, column: :user_id unless Gitlab::Database.postgresql?
+    add_concurrent_foreign_key :user_interacted_projects, :users, column: :user_id unless fk_exists?(:user_interacted_projects, :user_id)
+  end
+
+  def fk_exists?(table, column)
+    foreign_keys(table).any? do |key|
+      key.options[:column] == column.to_s
+    end
   end
 end