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

2 files changed, 28 insertions, 0 deletions

diff --git a/rubocop/cop/gitlab/union.rb b/rubocop/cop/gitlab/union.rb
new file mode 100644
index 00000000000..09541d8af3b
--- /dev/null
+++ b/rubocop/cop/gitlab/union.rb
@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+require_relative '../../spec_helpers'
+
+module RuboCop
+  module Cop
+    module Gitlab
+      # Cop that disallows the use of `Gitlab::SQL::Union`, in favour of using
+      # the `FromUnion` module.
+      class Union < RuboCop::Cop::Cop
+        include SpecHelpers
+
+        MSG = 'Use the `FromUnion` concern, instead of using `Gitlab::SQL::Union` directly'
+
+        def_node_matcher :raw_union?, <<~PATTERN
+          (send (const (const (const nil? :Gitlab) :SQL) :Union) :new ...)
+        PATTERN
+
+        def on_send(node)
+          return unless raw_union?(node)
+          return if in_spec?(node)
+
+          add_offense(node, location: :expression)
+        end
+      end
+    end
+  end
+end
diff --git a/rubocop/rubocop.rb b/rubocop/rubocop.rb
index 46bd7d3bec6..9d6cc73fc3b 100644
--- a/rubocop/rubocop.rb
+++ b/rubocop/rubocop.rb
@@ -3,6 +3,7 @@ require_relative 'cop/gitlab/module_with_instance_variables'
 require_relative 'cop/gitlab/predicate_memoization'
 require_relative 'cop/gitlab/httparty'
 require_relative 'cop/gitlab/finder_with_find_by'
+require_relative 'cop/gitlab/union'
 require_relative 'cop/include_sidekiq_worker'
 require_relative 'cop/avoid_return_from_blocks'
 require_relative 'cop/avoid_break_from_strong_memoize'