Calculate average in VSA

This change adds a new calculation to Value Stream Analytics where
the average durations are calculated.

ee/app/controllers/groups/analytics/cycle_analytics/stages_controller.rb

@@ -9,7 +9,7 @@ module Groups
 
         before_action :load_group
         before_action :load_value_stream
-        before_action :validate_params, only: %i[median records duration_chart]
+        before_action :validate_params, only: %i[median average records duration_chart]
 
         def index
           return render_403 unless can?(current_user, :read_group_cycle_analytics, @group)
@@ -47,6 +47,12 @@ module Groups
           render json: { value: data_collector.median.seconds }
         end
 
+        def average
+          return render_403 unless can?(current_user, :read_group_stage, @group)
+
+          render json: { value: data_collector.average.seconds }
+        end
+
         def records
           return render_403 unless can?(current_user, :read_group_stage, @group)
 

ee/config/routes/group.rb

@@ -26,6 +26,7 @@ constraints(::Constraints::GroupUrlConstrainer.new) do
           member do
             get :duration_chart
             get :median
+            get :average
             get :records
           end
         end
@@ -34,6 +35,7 @@ constraints(::Constraints::GroupUrlConstrainer.new) do
             member do
               get :duration_chart
               get :median
+              get :average
               get :records
             end
           end

ee/spec/support/shared_examples/controllers/analytics/cycle_analytics/shared_stage_shared_examples.rb

@@ -197,7 +197,19 @@ RSpec.shared_examples 'Value Stream Analytics Stages controller' do
       it 'matches the response schema' do
         subject
 
-        expect(response).to match_response_schema('analytics/cycle_analytics/median', dir: 'ee')
+        expect(response).to match_response_schema('analytics/cycle_analytics/number_or_nil_value', dir: 'ee')
+      end
+
+      include_examples 'Value Stream Analytics data endpoint examples'
+    end
+
+    describe 'GET #average' do
+      subject { get :average, params: params }
+
+      it 'matches the response schema' do
+        subject
+
+        expect(response).to match_response_schema('analytics/cycle_analytics/number_or_nil_value', dir: 'ee')
       end
 
       include_examples 'Value Stream Analytics data endpoint examples'

lib/gitlab/analytics/cycle_analytics/average.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Analytics
+    module CycleAnalytics
+      class Average
+        include Gitlab::Utils::StrongMemoize
+        include StageQueryHelpers
+
+        def initialize(stage:, query:)
+          @stage = stage
+          @query = query
+        end
+
+        def seconds
+          select_average ? select_average['average'] : nil
+        end
+
+        def days
+          seconds ? seconds.fdiv(1.day) : nil
+        end
+
+        private
+
+        attr_reader :stage
+
+        # rubocop: disable CodeReuse/ActiveRecord
+        def select_average
+          strong_memoize(:select_average) do
+            execute_query(@query.select(average_in_seconds.as('average')).reorder(nil)).first
+          end
+        end
+        # rubocop: enable CodeReuse/ActiveRecord
+
+        def average
+          Arel::Nodes::NamedFunction.new(
+            'AVG',
+            [duration]
+          )
+        end
+
+        def average_in_seconds
+          Arel::Nodes::Extract.new(average, :epoch)
+        end
+      end
+    end
+  end
+end

lib/gitlab/analytics/cycle_analytics/data_collector.rb

@@ -31,6 +31,12 @@ module Gitlab
           end
         end
 
+        def average
+          strong_memoize(:average) do
+            Average.new(stage: stage, query: query)
+          end
+        end
+
         private
 
         attr_reader :stage, :params

spec/lib/gitlab/analytics/cycle_analytics/average_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Analytics::CycleAnalytics::Average do
+  let_it_be(:project) { create(:project) }
+
+  let_it_be(:issue_1) do
+    # Duration: 10 days
+    create(:issue, project: project, created_at: 20.days.ago).tap do |issue|
+      issue.metrics.update!(first_mentioned_in_commit_at: 10.days.ago)
+    end
+  end
+
+  let_it_be(:issue_2) do
+    # Duration: 5 days
+    create(:issue, project: project, created_at: 20.days.ago).tap do |issue|
+      issue.metrics.update!(first_mentioned_in_commit_at: 15.days.ago)
+    end
+  end
+
+  let(:stage) do
+    build(
+      :cycle_analytics_project_stage,
+      start_event_identifier: Gitlab::Analytics::CycleAnalytics::StageEvents::IssueCreated.identifier,
+      end_event_identifier: Gitlab::Analytics::CycleAnalytics::StageEvents::IssueFirstMentionedInCommit.identifier,
+      project: project
+    )
+  end
+
+  let(:query) { Issue.joins(:metrics).in_projects(project.id) }
+
+  around do |example|
+    freeze_time { example.run }
+  end
+
+  subject(:average) { described_class.new(stage: stage, query: query) }
+
+  describe '#seconds' do
+    subject(:average_duration_in_seconds) { average.seconds }
+
+    context 'when no results' do
+      let(:query) { Issue.none }
+
+      it { is_expected.to eq(nil) }
+    end
+
+    context 'returns the average duration in seconds' do
+      it { is_expected.to be_within(0.5).of(7.5.days.to_f) }
+    end
+  end
+
+  describe '#days' do
+    subject(:average_duration_in_days) { average.days }
+
+    context 'when no results' do
+      let(:query) { Issue.none }
+
+      it { is_expected.to eq(nil) }
+    end
+
+    context 'returns the average duration in days' do
+      it { is_expected.to be_within(0.01).of(7.5) }
+    end
+  end
+end