Prevent query of calculating milestone dates when fixed dates are not used

ee/app/models/ee/epic.rb

@@ -123,12 +123,12 @@ module EE
 
     # Earliest start date from issues' milestones
     def start_date_from_milestones
-      epic_issues.joins(issue: :milestone).minimum('milestones.start_date')
+      start_date_is_fixed? ? epic_issues.joins(issue: :milestone).minimum('milestones.start_date') : start_date
     end
 
     # Latest end date from issues' milestones
     def due_date_from_milestones
-      epic_issues.joins(issue: :milestone).maximum('milestones.due_date')
+      due_date_is_fixed? ? epic_issues.joins(issue: :milestone).maximum('milestones.due_date') : due_date
     end
 
     def to_reference(from = nil, full: false)

ee/spec/models/epic_spec.rb

@@ -98,7 +98,8 @@ describe Epic do
   end
 
   describe '#start_date_from_milestones' do
-    subject { create(:epic) }
+    context 'fixed date' do
+      subject { create(:epic, :use_fixed_dates) }
       let(:date) { Date.new(2017, 3, 4) }
 
       before do
@@ -124,8 +125,19 @@ describe Epic do
       end
     end
 
+    context 'milestone date' do
+      subject { create(:epic, start_date: date) }
+      let(:date) { Date.new(2017, 3, 4) }
+
+      it 'returns start_date' do
+        expect(subject.start_date_from_milestones).to eq(date)
+      end
+    end
+  end
+
   describe '#due_date_from_milestones' do
-    subject { create(:epic) }
+    context 'fixed date' do
+      subject { create(:epic, :use_fixed_dates) }
       let(:date) { Date.new(2017, 3, 4) }
 
       before do
@@ -151,6 +163,16 @@ describe Epic do
       end
     end
 
+    context 'milestone date' do
+      subject { create(:epic, due_date: date) }
+      let(:date) { Date.new(2017, 3, 4) }
+
+      it 'returns due_date' do
+        expect(subject.due_date_from_milestones).to eq(date)
+      end
+    end
+  end
+
   describe '#update_dates' do
     context 'fixed date is set' do
       subject { create(:epic, :use_fixed_dates, start_date: nil, end_date: nil) }