Validate uniqueness of milestone title per project

Fixes #3903

CHANGELOG

@@ -35,6 +35,7 @@ v 8.5.0 (unreleased)
   - Fixed logo animation on Safari (Roman Rott)
   - Hide remove source branch button when the MR is merged but new commits are pushed (Zeger-Jan van de Weg)
   - In seach autocomplete show only groups and projects you are member of
+  - Title for milestones should be unique (Zeger-Jan van de Weg)
 
 v 8.4.4
   - Update omniauth-saml gem to 1.4.2

app/models/milestone.rb

@@ -34,7 +34,7 @@ class Milestone < ActiveRecord::Base
   scope :closed, -> { with_state(:closed) }
   scope :of_projects, ->(ids) { where(project_id: ids) }
 
-  validates :title, presence: true
+  validates :title, presence: true, uniqueness: { scope: :project_id }
   validates :project, presence: true
 
   strip_attributes :title

spec/models/milestone_spec.rb

@@ -33,6 +33,20 @@
   let(:milestone) { create(:milestone) }
   let(:issue) { create(:issue) }
 
+  describe "unique milestone title per project" do
+    it "shouldn't accept the same title in a project twice" do
+      new_milestone = Milestone.new(project: milestone.project, title: milestone.title)
+      expect(new_milestone).not_to be_valid
+    end
+
+    it "should accept the same title in another project" do
+      project = build(:project)
+      new_milestone = Milestone.new(project: project, title: milestone.title)
+
+      expect(new_milestone).to be_valid
+    end
+  end
+
   describe "#percent_complete" do
     it "should not count open issues" do
       milestone.issues << issue