Allow custom relations in Limitable

app/models/concerns/limitable.rb

@@ -6,6 +6,7 @@ module Limitable
 
   included do
     class_attribute :limit_scope
+    class_attribute :limit_relation
     class_attribute :limit_name
     class_attribute :limit_feature_flag
     self.limit_name = self.name.demodulize.tableize
@@ -28,7 +29,7 @@ module Limitable
     return unless scope_relation
     return if limit_feature_flag && ::Feature.disabled?(limit_feature_flag, scope_relation, default_enabled: :yaml)
 
-    relation = self.class.where(limit_scope => scope_relation)
+    relation = limit_relation ? self.public_send(limit_relation) : self.class.where(limit_scope => scope_relation) # rubocop:disable GitlabSecurity/PublicSend
     limits = scope_relation.actual_limits
 
     check_plan_limit_not_exceeded(limits, relation)

spec/models/concerns/limitable_spec.rb

 # frozen_string_literal: true
 
-require 'spec_helper'
+require 'fast_spec_helper'
+require 'active_model'
 
 RSpec.describe Limitable do
   let(:minimal_test_class) do
@@ -35,6 +36,28 @@ RSpec.describe Limitable do
 
       instance.valid?(:create)
     end
+
+    context 'with custom relation' do
+      before do
+        MinimalTestClass.limit_relation = :custom_relation
+      end
+
+      it 'triggers custom limit_relation' do
+        instance = MinimalTestClass.new
+
+        def instance.project
+          @project ||= Object.new
+        end
+
+        limits = Object.new
+        custom_relation = Object.new
+        expect(instance).to receive(:custom_relation).and_return(custom_relation)
+        expect(instance.project).to receive(:actual_limits).and_return(limits)
+        expect(limits).to receive(:exceeded?).with(instance.class.name.demodulize.tableize, custom_relation).and_return(false)
+
+        instance.valid?(:create)
+      end
+    end
   end
 
   context 'with global limit' do