Improve burndown chart algorithm

Take issues created in the middle of a milestone into account
for burndown charts

doc/user/project/milestones/burndown_charts.md

@@ -63,13 +63,6 @@ Since it only tracks when an issue was last closed (and not its full history), t
 assumes that issue was open on days prior to that date. Reopened issues are
 considered as open on one day after they were closed.
 
-Note that with this design, if you create a new issue in the middle of the milestone period
-(and assign the milestone to the issue), the Burndown Chart will appear as if the
-issue was already open at the beginning of the milestone. A workaround is to simply
-close the issue (so that a closed timestamp is stored in the system), and reopen
-it to get the desired effect, with a rise in the chart appearing on the day after.
-This is what appears in the example below.
-
 The Burndown Chart can also be toggled to display the cumulative open issue
 weight for a given day. When using this feature, make sure issue weights have
 been properly assigned, since an open issue with no weight adds zero to the

ee/app/models/burndown.rb

 class Burndown
+  include Gitlab::Utils::StrongMemoize
+
   class Issue
     attr_reader :closed_at, :weight, :state
 
@@ -13,7 +15,7 @@ class Burndown
     end
   end
 
-  attr_reader :start_date, :due_date, :end_date, :issues_count, :issues_weight, :accurate, :legacy_data
+  attr_reader :start_date, :due_date, :end_date, :accurate, :legacy_data, :milestone
   alias_method :accurate?, :accurate
   alias_method :empty?, :legacy_data
 
@@ -26,8 +28,6 @@ class Burndown
 
     @accurate = milestone_issues.all?(&:closed_at)
     @legacy_data = milestone_issues.any? && milestone_issues.none?(&:closed_at)
-
-    @issues_count, @issues_weight = milestone.issues.reorder(nil).pluck('COUNT(*), COALESCE(SUM(weight), 0)').first
   end
 
   # Returns the chart data in the following format:
@@ -35,8 +35,8 @@ class Burndown
   def as_json(opts = nil)
     return [] unless valid?
 
-    open_issues_count  = issues_count
-    open_issues_weight = issues_weight
+    open_issues_count = 0
+    open_issues_weight = 0
 
     start_date.upto(end_date).each_with_object([]) do |date, chart_data|
       closed, reopened = closed_and_reopened_issues_by(date)
@@ -44,6 +44,10 @@ class Burndown
       closed_issues_count = closed.count
       closed_issues_weight = sum_issues_weight(closed)
 
+      issues_created = opened_issues_on(date)
+      open_issues_count += issues_created.count
+      open_issues_weight += sum_issues_weight(issues_created)
+
       open_issues_count -= closed_issues_count
       open_issues_weight -= closed_issues_weight
 
@@ -63,6 +67,37 @@ class Burndown
 
   private
 
+  def opened_issues_on(date)
+    return {} if opened_issues_grouped_by_date.empty?
+
+    date = date.to_date
+
+    # If issues.created_at < milestone.start_date
+    # we consider all of them created at milestone.start_date
+    if date == start_date
+      first_issue_created_at = opened_issues_grouped_by_date.keys.first
+      days_before_start_date = start_date.downto(first_issue_created_at).to_a
+      opened_issues_grouped_by_date.values_at(*days_before_start_date).flatten.compact
+    else
+      opened_issues_grouped_by_date[date] || []
+    end
+  end
+
+  def opened_issues_grouped_by_date
+    strong_memoize(:opened_issues_grouped_by_date) do
+      issues =
+        @milestone
+          .issues
+          .where('created_at <= ?', end_date)
+          .reorder(nil)
+          .order(:created_at).to_a
+
+      issues.group_by do |issue|
+        issue.created_at.to_date
+      end
+    end
+  end
+
   def sum_issues_weight(issues)
     issues.map(&:weight).compact.sum
   end

ee/changelogs/unreleased/issue_6174.yml

+---
+title: Account for issues created in the middle of a milestone in burndown chart
+merge_request:
+author:
+type: changed

ee/spec/models/burndown_spec.rb

@@ -2,7 +2,6 @@ require 'spec_helper'
 
 describe Burndown do
   set(:user) { create(:user) }
-
   let(:start_date) { "2017-03-01" }
   let(:due_date) { "2017-03-03" }
 
@@ -82,6 +81,23 @@ describe Burndown do
         expect(burndown).not_to be_accurate
       end
     end
+
+    context 'when issues are created at the middle of the milestone' do
+      let(:creation_date) { "2017-03-02" }
+
+      it 'accounts for counts in issues created at the middle of the milestone' do
+        project = try(:group_project) || try(:project)
+
+        create(:issue, milestone: milestone, project: project, created_at: creation_date, weight: 2)
+        create(:issue, milestone: milestone, project: project, created_at: creation_date, weight: 3)
+
+        expect(subject).to eq([
+          ['2017-03-01', 3, 6],
+          ['2017-03-02', 6, 13],
+          ['2017-03-03', 4, 9]
+        ].to_json)
+      end
+    end
   end
 
   describe 'project milestone burndown' do
@@ -93,7 +109,8 @@ describe Burndown do
           milestone: milestone,
           weight: 2,
           project_id: project.id,
-          author: user
+          author: user,
+          created_at: milestone.start_date
         }
       end
       let(:scope) { project }
@@ -116,7 +133,8 @@ describe Burndown do
             milestone: milestone,
             weight: 2,
             project_id: nested_group_project.id,
-            author: user
+            author: user,
+            created_at: milestone.start_date
           }
         end
         let(:scope) { group }
@@ -131,7 +149,8 @@ describe Burndown do
             milestone: milestone,
             weight: 2,
             project_id: group_project.id,
-            author: user
+            author: user,
+            created_at: milestone.start_date
           }
         end
         let(:scope) { group }