Fix broken specs

app/models/concerns/issuable_states.rb

+# frozen_string_literal: true
+
 module IssuableStates
   extend ActiveSupport::Concern
 
   # The state:string column is being migrated to state_id:integer column
   # This is a temporary hook to populate state_id column with new values
-  # and can be removed after the state column is removed.
+  # and should be removed after the state column is removed.
   # Check https://gitlab.com/gitlab-org/gitlab-ce/issues/51789 for more information
   included do
     before_save :set_state_id
@@ -12,6 +14,12 @@ module IssuableStates
   def set_state_id
     return if state.nil? || state.empty?
 
+    # Needed to prevent breaking some migration specs that
+    # rollback database to a point where state_id does not exist.
+    # We can use this guard clause for now since this file will should
+    # be removed in the next release.
+    return unless self.respond_to?(:state_id)
+
     states_hash = self.class.available_states
 
     self.state_id = states_hash[state]

app/models/merge_request.rb

@@ -194,7 +194,7 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def self.available_states
-    @states ||= super.merge(merged: 3, locked: 4)
+    @available_states ||= super.merge(merged: 3, locked: 4)
   end
 
   def rebase_in_progress?

db/migrate/20190211131150_add_state_id_to_issuables.rb

+# frozen_string_literal: true
+
 class AddStateIdToIssuables < ActiveRecord::Migration[5.0]
   include Gitlab::Database::MigrationHelpers