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, 5 insertions, 5 deletions

diff --git a/app/finders/union_finder.rb b/app/finders/union_finder.rb
index 798c3eba0f3..c3b02f7e52f 100644
--- a/app/finders/union_finder.rb
+++ b/app/finders/union_finder.rb
@@ -1,15 +1,15 @@
 # frozen_string_literal: true
 
 class UnionFinder
-  # rubocop: disable CodeReuse/ActiveRecord
   def find_union(segments, klass)
-    if segments.length > 1
-      union = Gitlab::SQL::Union.new(segments.map { |s| s.select(:id) })
+    unless klass < FromUnion
+      raise TypeError, "#{klass.inspect} must include the FromUnion module"
+    end
 
-      klass.where("#{klass.table_name}.id IN (#{union.to_sql})")
+    if segments.length > 1
+      klass.from_union(segments)
     else
       segments.first
     end
   end
-  # rubocop: enable CodeReuse/ActiveRecord
 end