Add a spec for the `CycleAnalytics#issue` method.

Gemfile

@@ -319,6 +319,7 @@ group :test do
   gem 'webmock', '~> 1.21.0'
   gem 'test_after_commit', '~> 0.4.2'
   gem 'sham_rack', '~> 1.3.6'
+  gem 'timecop', '~> 0.8.0'
 end
 
 group :production do

Gemfile.lock

@@ -971,6 +971,7 @@ DEPENDENCIES
   teaspoon-jasmine (~> 2.2.0)
   test_after_commit (~> 0.4.2)
   thin (~> 1.7.0)
+  timecop (~> 0.8.0)
   turbolinks (~> 2.5.0)
   u2f (~> 0.2.1)
   uglifier (~> 2.7.2)

spec/models/cycle_analytics/issue_spec.rb

+require 'spec_helper'
+
+describe 'CycleAnalytics#issue', models: true do
+  let(:project) { create(:project) }
+  subject { CycleAnalytics.new }
+
+  context "when calculating the median of times between:
+               start: issue created_at
+                 end: milestone first added to issue
+                      OR
+                      list-label first added to issue
+           " do
+    context "when a milestone is added to the issue" do
+      it "calculates the median of available durations" do
+        start_and_end_times = Array.new(5) do
+          start_time = Time.now
+          end_time = rand(1..10).days.from_now
+
+          milestone = create(:milestone, project: project)
+          issue = Timecop.freeze(start_time) { create(:issue, project: project) }
+          Timecop.freeze(end_time) { issue.update(milestone: milestone) }
+
+          [start_time, end_time]
+        end
+
+        median_start_time, median_end_time = start_and_end_times[2]
+        expect(subject.issue).to eq(median_end_time - median_start_time)
+      end
+    end
+
+    context "when a label is added to the issue" do
+      it "calculates the median of available durations when the label is a list-label" do
+        start_and_end_times = Array.new(5) do
+          start_time = Time.now
+          end_time = rand(1..10).days.from_now
+
+          list_label = create(:label, lists: [create(:list)])
+          issue = Timecop.freeze(start_time) { create(:issue, project: project) }
+          Timecop.freeze(end_time) { issue.update(label_ids: [list_label.id]) }
+
+          [start_time, end_time]
+        end
+
+        median_start_time, median_end_time = start_and_end_times[2]
+        expect(subject.issue).to eq(median_end_time - median_start_time)
+      end
+
+      it "does not make a calculation for regular labels" do
+        5.times do
+          start_time = Time.now
+          end_time = rand(1..10).days.from_now
+
+          regular_label = create(:label)
+          issue = Timecop.freeze(start_time) { create(:issue, project: project) }
+          Timecop.freeze(end_time) { issue.update(label_ids: [regular_label.id]) }
+
+          [start_time, end_time]
+        end
+
+        expect(subject.issue).to be_nil
+      end
+    end
+
+    context "when a milestone and list-label are both added to the issue" do
+      it "uses the time the milestone was added as the 'end time'" do
+        start_time = Time.now
+        milestone_add_time = rand(1..10).days.from_now
+        list_label_add_time = rand(1..10).days.from_now
+
+        milestone = create(:milestone, project: project)
+        list_label = create(:label, lists: [create(:list)])
+        issue = Timecop.freeze(start_time) { create(:issue, project: project) }
+        Timecop.freeze(milestone_add_time) { issue.update(milestone: milestone) }
+        Timecop.freeze(list_label_add_time) { issue.update(label_ids: [list_label.id]) }
+
+        expect(subject.issue).to eq(milestone_add_time - start_time)
+      end
+    end
+  end
+end