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

4 files changed, 12 insertions, 17 deletions

diff --git a/app/finders/members_finder.rb b/app/finders/members_finder.rb
index 48777838d60..f90a7868102 100644
--- a/app/finders/members_finder.rb
+++ b/app/finders/members_finder.rb
@@ -18,7 +18,7 @@ class MembersFinder
       group_members = GroupMembersFinder.new(group).execute(include_descendants: include_descendants) # rubocop: disable CodeReuse/Finder
       group_members = group_members.non_invite
 
-      union = Gitlab::SQL::Union.new([project_members, group_members], remove_duplicates: false)
+      union = Gitlab::SQL::Union.new([project_members, group_members], remove_duplicates: false) # rubocop: disable Gitlab/Union
 
       sql = distinct_on(union)
 
diff --git a/app/finders/snippets_finder.rb b/app/finders/snippets_finder.rb
index 715dffb972f..3528e4228b2 100644
--- a/app/finders/snippets_finder.rb
+++ b/app/finders/snippets_finder.rb
@@ -89,9 +89,7 @@ class SnippetsFinder < UnionFinder
 
     # We use a UNION here instead of OR clauses since this results in better
     # performance.
-    union = Gitlab::SQL::Union.new([authorized_projects.select('projects.id'), visible_projects.select('projects.id')])
-
-    Project.from("(#{union.to_sql}) AS #{Project.table_name}")
+    Project.from_union([authorized_projects, visible_projects])
   end
   # rubocop: enable CodeReuse/ActiveRecord
 
diff --git a/app/finders/todos_finder.rb b/app/finders/todos_finder.rb
index 0d9f16fdce7..74baf79e4f2 100644
--- a/app/finders/todos_finder.rb
+++ b/app/finders/todos_finder.rb
@@ -164,16 +164,13 @@ class TodosFinder
 
   # rubocop: disable CodeReuse/ActiveRecord
   def by_group(items)
-    if group?
-      groups = group.self_and_descendants
-      project_todos = items.where(project_id: Project.where(group: groups).select(:id))
-      group_todos = items.where(group_id: groups.select(:id))
+    return items unless group?
 
-      union = Gitlab::SQL::Union.new([project_todos, group_todos])
-      items = Todo.from("(#{union.to_sql}) #{Todo.table_name}")
-    end
+    groups = group.self_and_descendants
+    project_todos = items.where(project_id: Project.where(group: groups).select(:id))
+    group_todos = items.where(group_id: groups.select(:id))
 
-    items
+    Todo.from_union([project_todos, group_todos])
   end
   # rubocop: enable CodeReuse/ActiveRecord
 
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