Fallback to `created_at` when `merged_at` is missing in Insights charts

Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/user/project/insights/index.md

@@ -287,6 +287,11 @@ The `period_field` is automatically set to:
 - `merged_at` if `query.issuable_state` is `merged`
 - `created_at` otherwise
 
+NOTE: **Note:**
+Until [this bug](https://gitlab.com/gitlab-org/gitlab/-/issues/26911), is resolved, you may see `created_at`
+in place of `merged_at`.
+`created_at` will be used instead.
+
 ### `projects`
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/10904) in [GitLab Ultimate](https://about.gitlab.com/pricing/) 12.4.

ee/changelogs/unreleased/229118-charts-with-project-filter-seem-to-timeout.yml

+---
+title: Fallback to `created_at` when `merged_at` is missing in Insights charts
+merge_request: 36930
+author:
+type: fixed

ee/lib/gitlab/insights/reducers/count_per_period_reducer.rb

@@ -62,7 +62,8 @@ module Gitlab
         #   }
         def issuables_grouped_by_normalized_period
           @issuables_grouped_by_normalized_period ||= issuables.group_by do |issuable|
-            normalized_time(issuable.public_send(period_field)) # rubocop:disable GitlabSecurity/PublicSend
+            time_field = issuable.public_send(period_field) || issuable.created_at # rubocop:disable GitlabSecurity/PublicSend
+            normalized_time(time_field)
           end
         end
 

ee/spec/lib/gitlab/insights/reducers/count_per_period_reducers_spec.rb → ee/spec/lib/gitlab/insights/reducers/count_per_period_reducer_spec.rb

@@ -133,9 +133,9 @@ RSpec.describe Gitlab::Insights::Reducers::CountPerPeriodReducer do
   context 'with merged merge requests' do
     include_context 'Insights merge requests reducer context', :merged
 
-    # Populate the MR metrics' merged_at
+    # Populate the MR metrics' merged_at, except for issuable3 to reproduce a real use-case where merged_at is null.
     before do
-      (0..3).each do |i|
+      (0..2).each do |i|
         merge_request = public_send("issuable#{i}")
         merge_request_metrics_service = MergeRequestMetricsService.new(merge_request.metrics)
         Event.transaction do