Consider first_reassigned_at for code review analytics

Code review cycle should be started when
merge request was reassigned for the first time

ee/app/models/ee/merge_request/metrics.rb

@@ -7,7 +7,7 @@ module EE
 
       class_methods do
         def review_time_field
-          @review_time_field ||= Arel.sql("LEAST(merge_request_metrics.first_comment_at, merge_request_metrics.first_approved_at)")
+          @review_time_field ||= Arel.sql("LEAST(merge_request_metrics.first_comment_at, merge_request_metrics.first_approved_at, merge_request_metrics.first_reassigned_at)")
         end
       end
 
@@ -18,7 +18,7 @@ module EE
       end
 
       def review_start_at
-        [first_comment_at, first_approved_at].compact.min
+        [first_comment_at, first_approved_at, first_reassigned_at].compact.min
       end
 
       def review_end_at

ee/changelogs/unreleased/205597-start-code-review-on-assign-part-2.yml

+---
+title: Consider MR reassign as code review start
+merge_request: 26891
+author:
+type: changed

ee/spec/models/ee/merge_request/metrics_spec.rb

@@ -4,11 +4,12 @@ require 'spec_helper'
 
 describe MergeRequest::Metrics do
   describe '#review_start_at' do
-    it 'is the earliest date from first_comment_at or first_approved_at' do
-      subject.first_approved_at = 1.day.ago
-      subject.first_comment_at = 1.hour.ago
+    it 'is the earliest date from first_comment_at, first_approved_at or first_reassigned_at' do
+      subject.first_approved_at = 1.hour.ago
+      subject.first_comment_at = 1.day.ago
+      subject.first_reassigned_at = 1.week.ago
 
-      expect(subject.review_start_at).to be_like_time(1.day.ago)
+      expect(subject.review_start_at).to be_like_time(1.week.ago)
     end
 
     context 'when all review start candidates are nil' do
@@ -20,6 +21,7 @@ describe MergeRequest::Metrics do
     context 'when one of review start candidates is nil' do
       it 'is earliest date from non-nil values' do
         subject.first_approved_at = 1.day.ago
+        subject.first_reassigned_at = 1.hour.ago
 
         expect(subject.review_start_at).to be_like_time(1.day.ago)
       end