Add specs for issuable states sync

app/models/concerns/issuable_states.rb

@@ -4,6 +4,7 @@ module IssuableStates
   # 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.
+  # Check https://gitlab.com/gitlab-org/gitlab-ce/issues/51789 for more information
   included do
     before_save :set_state_id
   end

spec/models/concerns/issuable_states_spec.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+# This spec checks if state_id column of issues and merge requests
+# are being synced on every save.
+# It can be removed in the next release. Check https://gitlab.com/gitlab-org/gitlab-ce/issues/51789  for more information.
+describe IssuableStates do
+  [Issue, MergeRequest].each do |klass|
+    it "syncs state_id column when #{klass.model_name.human} gets created" do
+      klass.available_states.each do |state, state_id|
+        issuable = build(klass.model_name.param_key, state: state.to_s)
+
+        issuable.save!
+
+        expect(issuable.state_id).to eq(state_id)
+      end
+    end
+
+    it "syncs state_id column when #{klass.model_name.human} gets updated" do
+      klass.available_states.each do |state, state_id|
+        issuable = create(klass.model_name.param_key, state: state.to_s)
+
+        issuable.update(state: state)
+
+        expect(issuable.state_id).to eq(state_id)
+      end
+    end
+  end
+end