Add method for updating Epic date fields

ee/app/models/ee/epic.rb

@@ -109,6 +109,12 @@ module EE
     # Needed to use EntityDateHelper#remaining_days_in_words
     alias_attribute(:due_date, :end_date)
 
+    def update_dates
+      self.start_date = compute_start_date
+      self.due_date = compute_due_date
+      save if changed?
+    end
+
     # Earliest start date from issues' milestones
     def start_date_from_milestones
       epic_issues.joins(issue: :milestone).minimum('milestones.start_date')
@@ -155,5 +161,15 @@ module EE
     def banzai_render_context(field)
       super.merge(label_url_method: :group_epics_url)
     end
+
+    private
+
+    def compute_start_date
+      start_date_is_fixed? ? start_date_fixed : start_date_from_milestones
+    end
+
+    def compute_due_date
+      due_date_is_fixed? ? due_date_fixed : due_date_from_milestones
+    end
   end
 end

ee/spec/factories/epics.rb

@@ -4,6 +4,15 @@ FactoryBot.define do
     group
     author
 
+    trait :use_fixed_dates do
+      start_date { Date.new(2010, 1, 1) }
+      start_date_fixed { Date.new(2010, 1, 1) }
+      start_date_is_fixed true
+      end_date { Date.new(2010, 1, 3) }
+      due_date_fixed { Date.new(2010, 1, 3) }
+      due_date_is_fixed true
+    end
+
     factory :labeled_epic do
       transient do
         labels []

ee/spec/models/epic_spec.rb

@@ -85,6 +85,18 @@ describe Epic do
     end
   end
 
+  describe '#start_date' do
+    let(:date) { Date.new(2000, 1, 1) }
+
+    context 'is set' do
+      subject { build(:epic, :use_fixed_dates, start_date: date) }
+
+      it 'returns as is' do
+        expect(subject.start_date).to eq(date)
+      end
+    end
+  end
+
   describe '#start_date_from_milestones' do
     subject { create(:epic) }
     let(:date) { Date.new(2017, 3, 4) }
@@ -139,6 +151,35 @@ describe Epic do
     end
   end
 
+  describe '#update_dates' do
+    context 'fixed date is set' do
+      subject { create(:epic, :use_fixed_dates, start_date: nil, end_date: nil) }
+
+      it 'updates to fixed date' do
+        subject.update_dates
+
+        expect(subject.start_date).to eq(subject.start_date_fixed)
+        expect(subject.due_date).to eq(subject.due_date_fixed)
+      end
+    end
+
+    context 'fixed date is not set' do
+      subject { create(:epic, start_date: nil, end_date: nil) }
+      let(:start_date) { Date.new(2001, 1, 1) }
+      let(:due_date) { Date.new(2002, 1, 1) }
+
+      it 'updates to milestone dates' do
+        expect(subject).to receive(:start_date_from_milestones).and_return(start_date)
+        expect(subject).to receive(:due_date_from_milestones).and_return(due_date)
+
+        subject.update_dates
+
+        expect(subject.start_date).to eq(start_date)
+        expect(subject.due_date).to eq(due_date)
+      end
+    end
+  end
+
   describe '#issues' do
     let(:user) { create(:user) }
     let(:group) { create(:group, :private) }