Turn Group#owners into a has_many association

This allows the owners to be eager loaded where needed.

app/models/group.rb

@@ -9,6 +9,12 @@ class Group < Namespace
   has_many :group_members, dependent: :destroy, as: :source, class_name: 'GroupMember'
   alias_method :members, :group_members
   has_many :users, -> { where(members: { requested_at: nil }) }, through: :group_members
+
+  has_many :owners,
+    -> { where(members: { access_level: Gitlab::Access::OWNER }) },
+    through: :group_members,
+    source: :user
+
   has_many :project_group_links, dependent: :destroy
   has_many :shared_projects, through: :project_group_links, source: :project
   has_many :notification_settings, dependent: :destroy, as: :source
@@ -88,10 +94,6 @@ class Group < Namespace
     end
   end
 
-  def owners
-    @owners ||= group_members.owners.includes(:user).map(&:user)
-  end
-
   def add_users(user_ids, access_level, current_user = nil)
     user_ids.each do |user_id|
       Member.add_user(self.group_members, user_id, access_level, current_user)

spec/models/group_spec.rb

@@ -158,6 +158,18 @@ describe Group, models: true do
     it { expect(group.has_master?(@members[:requester])).to be_falsey }
   end
 
+  describe '#owners' do
+    let(:owner) { create(:user) }
+    let(:developer) { create(:user) }
+
+    it 'returns the owners of a Group' do
+      group.add_owner(owner)
+      group.add_developer(developer)
+
+      expect(group.owners).to eq([owner])
+    end
+  end
+
   def setup_group_members(group)
     members = {
       owner: create(:user),