Add warning when burndown is not accurate

app/models/burndown.rb

 class Burndown
-  attr_accessor :start_date, :due_date, :end_date, :issues_count, :issues_weight, :has_data
+  attr_reader :start_date, :due_date, :end_date, :issues_count, :issues_weight, :accurate
+  alias_method :accurate?, :accurate
 
   def initialize(milestone)
     @milestone = milestone
@@ -7,7 +8,7 @@ class Burndown
     @due_date = @milestone.due_date
     @end_date = @milestone.due_date
     @end_date = Date.today if @end_date.present? && @end_date > Date.today
-    @has_data = issues_with_closed_at.pluck(:closed_at).compact.any?
+    @accurate = issues_with_closed_at.where(closed_at: nil).empty?
 
     @issues_count, @issues_weight = milestone.issues.reorder(nil).pluck('COUNT(*), COALESCE(SUM(weight), 0)').first
   end

app/views/shared/milestones/_burndown.html.haml

@@ -7,6 +7,11 @@
   = page_specific_javascript_bundle_tag('common_d3')
   = page_specific_javascript_bundle_tag('burndown_chart')
 
+- unless burndown.accurate?
+  #no-data-warning.settings-message.prepend-top-20
+    Some issues can't be shown in the burndown chart, as they were closed before GitLab 9.1 update.
+    = link_to "More information.", help_page_path('workflow/milestones')
+
 - if can_generate_chart
   .burndown-header
     %h3

spec/features/milestones/show_spec.rb

@@ -3,12 +3,12 @@ require 'rails_helper'
 describe 'Milestone show', feature: true do
   let(:user) { create(:user) }
   let(:project) { create(:empty_project) }
-  let(:milestone) { create(:milestone, project: project) }
+  let(:milestone) { create(:milestone, project: project, start_date: Date.today, due_date: 7.days.from_now) }
   let(:labels) { create_list(:label, 2, project: project) }
   let(:issue_params) { { project: project, assignee: user, author: user, milestone: milestone, labels: labels } }
 
   before do
-    project.add_user(user, :developer) 
+    project.add_user(user, :developer)
     login_as(user)
   end
 
@@ -23,4 +23,33 @@ describe 'Milestone show', feature: true do
 
     expect { visit_milestone }.not_to exceed_query_limit(control_count)
   end
+
+  context 'burndown' do
+    before { issue_params.delete(:labels) }
+
+    context 'when closed issues does not have closed_at value' do
+      it 'shows warning' do
+        create(:issue, issue_params)
+        issue = create(:issue, issue_params)
+        issue.update(state: 'closed')
+        issue.update(closed_at: nil)
+
+        visit_milestone
+
+        expect(page).to have_selector('#no-data-warning')
+      end
+    end
+
+    context 'data is accurate' do
+      it 'does not show warning' do
+        create(:issue, issue_params)
+        issue = create(:issue, issue_params)
+        issue.close
+
+        visit_milestone
+
+        expect(page).not_to have_selector('#no-data-warning')
+      end
+    end
+  end
 end

spec/models/burndown_spec.rb

@@ -60,7 +60,7 @@ describe Burndown, models: true do
   it "it sets attribute has_data to true" do
     burndown = described_class.new(milestone)
 
-    expect(burndown.has_data).to be_truthy
+    expect(burndown.accurate?).to be_truthy
   end
 
   context "when closed and reopened issues does not have closed_at" do
@@ -79,7 +79,7 @@ describe Burndown, models: true do
     it "it sets attribute has_data to false" do
       burndown = described_class.new(milestone)
 
-      expect(burndown.has_data).to be_falsy
+      expect(burndown.accurate?).to be_falsy
     end
   end