Using a method that was already there to simplify comparison

Also cleaning up the migration stubbed classes per review

ee/app/models/approval_merge_request_rule.rb

@@ -131,35 +131,10 @@ class ApprovalMergeRequestRule < ApplicationRecord
     refresh_license_scanning_approvals(project_approval_rule) if license_scanning?
   end
 
-  def different_from_project_rule?
-    return unless approval_project_rule
-
-    different_groups? || different_users? || different_name_or_approvals_required? ? true : false
-  end
-
   private
 
-  def different_groups?
-    group_ids = groups.collect(&:id)
-    apr_group_ids = approval_project_rule.groups.pluck(:id)
-
-    group_ids & apr_group_ids == group_ids
-  end
-
-  def different_users?
-    user_ids = users.collect(&:id)
-    apr_user_ids = approval_project_rule.users.pluck(:id)
-
-    user_ids & apr_user_ids == user_ids
-  end
-
-  def different_name_or_approvals_required?
-    true if approvals_required != approval_project_rule.approvals_required || name != approval_project_rule.name
-  end
-
   def compare_with_project_rule
-    self.modified_from_project_rule =
-      different_from_project_rule? ? true : false
+    self.modified_from_project_rule = overridden? ? true : false
   end
 
   def validate_approval_project_rule

ee/spec/lib/ee/gitlab/usage_data_spec.rb

@@ -321,7 +321,7 @@ RSpec.describe Gitlab::UsageData do
       end
 
       expect(described_class.usage_activity_by_stage_create({})).to include(
-        approval_project_rules: 8,
+        approval_project_rules: 10,
         approval_project_rules_with_target_branch: 2,
         approval_project_rules_with_more_approvers_than_required: 2,
         approval_project_rules_with_less_approvers_than_required: 2,
@@ -342,7 +342,7 @@ RSpec.describe Gitlab::UsageData do
         total_number_of_locked_files: 14
       )
       expect(described_class.usage_activity_by_stage_create(described_class.last_28_days_time_period)).to include(
-        approval_project_rules: 8,
+        approval_project_rules: 10,
         approval_project_rules_with_target_branch: 2,
         approval_project_rules_with_more_approvers_than_required: 2,
         approval_project_rules_with_less_approvers_than_required: 2,

ee/spec/models/approval_merge_request_rule_spec.rb

@@ -36,32 +36,6 @@ RSpec.describe ApprovalMergeRequestRule do
         it 'is valid' do
           expect(merge_request_rule).to be_valid
         end
-
-        context 'the MR rule shows that its modified from the original' do
-          it 'by having different groups' do
-            first_group = create(:group)
-            second_group = create(:group)
-            third_group = create(:group)
-
-            approval_project_rule.groups = [first_group, second_group]
-            approval_project_rule.save!
-            merge_request_rule.groups = [first_group, third_group]
-
-            expect(merge_request_rule.different_from_project_rule?).to be true
-          end
-
-          it 'by having different users' do
-            first_user = create(:user)
-            second_user = create(:user)
-            third_user = create(:user)
-
-            approval_project_rule.users = [first_user, second_user]
-            approval_project_rule.save!
-            merge_request_rule.users = [first_user, third_user]
-
-            expect(merge_request_rule.different_from_project_rule?).to be true
-          end
-        end
       end
 
       context 'when the project of approval_project_rule and merge request does not match' do

lib/gitlab/background_migration/add_modified_to_approval_merge_request_rule.rb

@@ -4,33 +4,37 @@ module Gitlab
   module BackgroundMigration
     # Compare all current rules to project rules
     class AddModifiedToApprovalMergeRequestRule
+      # Stubbed class to access the Group table
+      class Group < ActiveRecord::Base
+        self.table_name = 'namespaces'
+        self.inheritance_column = :_type_disabled
+      end
+
       # Stubbed class to access the ApprovalMergeRequestRule table
       class ApprovalMergeRequestRule < ActiveRecord::Base
         self.table_name = 'approval_merge_request_rules'
 
-        has_one :approval_merge_request_rule_source
+        has_one :approval_merge_request_rule_source, class_name: 'AddModifiedToApprovalMergeRequestRule::ApprovalMergeRequestRuleSource'
         has_one :approval_project_rule, through: :approval_merge_request_rule_source
-        has_and_belongs_to_many :users
         has_and_belongs_to_many :groups,
-          class_name: 'Group', join_table: "#{self.table_name}_groups"
+          class_name: 'AddModifiedToApprovalMergeRequestRule::Group', join_table: "#{self.table_name}_groups"
       end
 
       # Stubbed class to access the ApprovalProjectRule table
       class ApprovalProjectRule < ActiveRecord::Base
         self.table_name = 'approval_project_rules'
 
-        has_many :approval_merge_request_rule_sources
-        has_and_belongs_to_many :users
+        has_many :approval_merge_request_rule_sources, class_name: 'AddModifiedToApprovalMergeRequestRule::ApprovalMergeRequestRuleSource'
         has_and_belongs_to_many :groups,
-          class_name: 'Group', join_table: "#{self.table_name}_groups"
+          class_name: 'AddModifiedToApprovalMergeRequestRule::Group', join_table: "#{self.table_name}_groups"
       end
 
       # Stubbed class to access the ApprovalMergeRequestRuleSource table
       class ApprovalMergeRequestRuleSource < ActiveRecord::Base
         self.table_name = 'approval_merge_request_rule_sources'
 
-        belongs_to :approval_merge_request_rule
-        belongs_to :approval_project_rule
+        belongs_to :approval_merge_request_rule, class_name: 'AddModifiedToApprovalMergeRequestRule::ApprovalMergeRequestRule'
+        belongs_to :approval_project_rule, class_name: 'AddModifiedToApprovalMergeRequestRule::ApprovalProjectRule'
       end
 
       def perform(start_id, stop_id)