Add source members url to billable memberships api

Link to group or project memberships page

ee/lib/ee/api/entities/billable_membership.rb

@@ -4,11 +4,21 @@ module EE
   module API
     module Entities
       class BillableMembership < Grape::Entity
+        include ::Gitlab::Routing
+
         expose :id
         expose :source_id
         expose :source_full_name do |member|
           member.source.full_name
         end
+        expose :source_members_url do |member|
+          case member.source_type
+          when 'Namespace'
+            group_group_members_url(member.source)
+          when 'Project'
+            project_project_members_url(member.source)
+          end
+        end
         expose :created_at
         expose :expires_at
         expose :access_level do

ee/spec/lib/ee/api/entities/billable_membership_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe ::EE::API::Entities::BillableMembership do
+  describe '#as_json' do
+    it 'returns source_members_url for a group' do
+      membership = create(:group_member)
+      group_members_url = Gitlab::Routing.url_helpers.group_group_members_url(membership.source)
+
+      expect(described_class.new(membership).as_json[:source_members_url]).to eq(group_members_url)
+    end
+
+    it 'returns source_members_url for a project' do
+      membership = create(:project_member)
+      project_members_url = Gitlab::Routing.url_helpers.project_project_members_url(membership.source)
+
+      expect(described_class.new(membership).as_json[:source_members_url]).to eq(project_members_url)
+    end
+  end
+end

ee/spec/requests/api/members_spec.rb

@@ -522,6 +522,7 @@ RSpec.describe API::Members do
           'id' => membership.id,
           'source_id' => group.id,
           'source_full_name' => group.full_name,
+          'source_members_url' => group_group_members_url(group),
           'created_at' => membership.created_at.as_json,
           'expires_at' => nil,
           'access_level' => {