Added FromUnion to easily select from a UNION

This commit adds the module `FromUnion`, which provides the class method
`from_union`. This simplifies the process of selecting data from the
result of a UNION, and reduces the likelihood of making mistakes. As a
result, instead of this:

    union = Gitlab::SQL::Union.new([foo, bar])

    Foo.from("(#{union.to_sql}) #{Foo.table_name}")

We can now write this instead:

    Foo.from_union([foo, bar])

This commit also includes some changes to make this new setup work
properly. For example, a bug in Rails 4
(https://github.com/rails/rails/issues/24193) would break the use of
`from("sub-query-here").includes(:relation)` in certain cases. There was
also a CI query which appeared to repeat a lot of conditions from an
outer query on an inner query, which isn't necessary.

Finally, we include a RuboCop cop to ensure developers use this new
module, instead of using Gitlab::SQL::Union directly.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/51307

1 files changed, 3 insertions, 2 deletions

diff --git a/lib/gitlab/contributions_calendar.rb b/lib/gitlab/contributions_calendar.rb
index 3236abfa43f..1ffc2639237 100644
--- a/lib/gitlab/contributions_calendar.rb
+++ b/lib/gitlab/contributions_calendar.rb
@@ -30,8 +30,9 @@ module Gitlab
       note_events = event_counts(date_from, :merge_requests)
         .having(action: [Event::COMMENTED])
 
-      union = Gitlab::SQL::Union.new([repo_events, issue_events, mr_events, note_events])
-      events = Event.find_by_sql(union.to_sql).map(&:attributes)
+      events = Event
+        .from_union([repo_events, issue_events, mr_events, note_events])
+        .map(&:attributes)
 
       @activity_dates = events.each_with_object(Hash.new {|h, k| h[k] = 0 }) do |event, activities|
         activities[event["date"]] += event["total_amount"]