Expose props for source rule

Neded for client side validation
and needed to know if a MR rule should be editable or not.

ee/app/models/approval_merge_request_rule.rb

@@ -14,6 +14,7 @@ class ApprovalMergeRequestRule < ApplicationRecord
   has_and_belongs_to_many :approved_approvers, class_name: 'User', join_table: :approval_merge_request_rules_approved_approvers
   has_one :approval_merge_request_rule_source
   has_one :approval_project_rule, through: :approval_merge_request_rule_source
+  alias_method :source_rule, :approval_project_rule
 
   validate :validate_approvals_required
 

ee/app/models/approval_project_rule.rb

@@ -18,4 +18,8 @@ class ApprovalProjectRule < ApplicationRecord
     false
   end
   alias_method :code_owner?, :code_owner
+
+  def source_rule
+    nil
+  end
 end

ee/app/models/approval_wrapped_rule.rb

@@ -8,7 +8,7 @@ class ApprovalWrappedRule
   attr_reader :merge_request
   attr_reader :approval_rule
 
-  def_delegators :@approval_rule, :id, :name, :users, :groups, :approvals_required, :code_owner
+  def_delegators :@approval_rule, :id, :name, :users, :groups, :approvals_required, :code_owner, :source_rule
 
   def initialize(merge_request, approval_rule)
     @merge_request = merge_request

ee/lib/ee/api/entities.rb

@@ -257,8 +257,13 @@ module EE
       end
 
       class MergeRequestApprovalRule < ApprovalRule
+        class SourceRule < Grape::Entity
+          expose :approvals_required
+        end
+
         expose :approved_approvers, as: :approved_by, using: ::API::Entities::UserBasic
         expose :code_owner
+        expose :source_rule, using: SourceRule
       end
 
       # Decorates ApprovalState

ee/spec/requests/api/merge_request_approvals_spec.rb

@@ -499,6 +499,28 @@ describe "API::MergeRequestApprovals with approval_rule enabled" do
       expect(rule_response['name']).to eq('foo')
       expect(rule_response['approvers'][0]['username']).to eq(approver.username)
       expect(rule_response['approved_by'][0]['username']).to eq(approver.username)
+      expect(rule_response['source_rule']).to eq(nil)
+    end
+
+    context 'when source rule is present' do
+      let!(:source_rule) { create(:approval_project_rule, project: project, approvals_required: 1, name: 'zoo') }
+      let!(:rule) { create(:approval_merge_request_rule, merge_request: merge_request, approvals_required: 2, name: 'foo') }
+
+      it 'returns source rule details' do
+        project.add_developer(approver)
+        merge_request.approvals.create(user: approver)
+        rule.users << approver
+        rule.create_approval_merge_request_rule_source!(approval_project_rule: source_rule)
+
+        get api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/approval_settings", user)
+
+        expect(response).to have_gitlab_http_status(200)
+        expect(json_response['rules'].size).to eq(1)
+
+        rule_response = json_response['rules'].first
+
+        expect(rule_response['source_rule']['approvals_required']).to eq(source_rule.approvals_required)
+      end
     end
 
     it 'excludes private groups' do