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, 1 insertions, 0 deletions

diff --git a/app/models/label.rb b/app/models/label.rb
index 8dc7ded53ad..9ef57a05b3e 100644
--- a/app/models/label.rb
+++ b/app/models/label.rb
@@ -7,6 +7,7 @@ class Label < ActiveRecord::Base
   include Gitlab::SQL::Pattern
   include OptionallySearch
   include Sortable
+  include FromUnion
 
   # Represents a "No Label" state used for filtering Issues and Merge
   # Requests that have no label assigned.