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

6 files changed, 15 insertions, 18 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"]
diff --git a/lib/gitlab/database/grant.rb b/lib/gitlab/database/grant.rb
index d32837f5793..7d334a79009 100644
--- a/lib/gitlab/database/grant.rb
+++ b/lib/gitlab/database/grant.rb
@@ -2,6 +2,8 @@ module Gitlab
   module Database
     # Model that can be used for querying permissions of a SQL user.
     class Grant < ActiveRecord::Base
+      include FromUnion
+
       self.table_name =
         if Database.postgresql?
           'information_schema.role_table_grants'
@@ -42,9 +44,7 @@ module Gitlab
               .where("GRANTEE = CONCAT('\\'', REPLACE(CURRENT_USER(), '@', '\\'@\\''), '\\'')")
           ]
 
-          union = SQL::Union.new(queries).to_sql
-
-          Grant.from("(#{union}) privs").any?
+          Grant.from_union(queries, alias_as: 'privs').any?
         end
       end
     end
diff --git a/lib/gitlab/group_hierarchy.rb b/lib/gitlab/group_hierarchy.rb
index b74e7b10448..8fbfa1a86bf 100644
--- a/lib/gitlab/group_hierarchy.rb
+++ b/lib/gitlab/group_hierarchy.rb
@@ -89,14 +89,14 @@ module Gitlab
       ancestors_table = ancestors.alias_to(groups_table)
       descendants_table = descendants.alias_to(groups_table)
 
-      union = SQL::Union.new([model.unscoped.from(ancestors_table),
-                              model.unscoped.from(descendants_table)])
-
       relation = model
         .unscoped
         .with
         .recursive(ancestors.to_arel, descendants.to_arel)
-        .from("(#{union.to_sql}) #{model.table_name}")
+        .from_union([
+          model.unscoped.from(ancestors_table),
+          model.unscoped.from(descendants_table)
+        ])
 
       read_only(relation)
     end
diff --git a/lib/gitlab/project_authorizations/with_nested_groups.rb b/lib/gitlab/project_authorizations/with_nested_groups.rb
index e3da1634fa5..448c3f3a7d8 100644
--- a/lib/gitlab/project_authorizations/with_nested_groups.rb
+++ b/lib/gitlab/project_authorizations/with_nested_groups.rb
@@ -49,13 +49,11 @@ module Gitlab
             .where('p_ns.share_with_group_lock IS FALSE')
         ]
 
-        union = Gitlab::SQL::Union.new(relations)
-
         ProjectAuthorization
           .unscoped
           .with
           .recursive(cte.to_arel)
-          .select_from_union(union)
+          .select_from_union(relations)
       end
 
       private
diff --git a/lib/gitlab/project_authorizations/without_nested_groups.rb b/lib/gitlab/project_authorizations/without_nested_groups.rb
index 7d0c00c7f36..ed2287dcc7e 100644
--- a/lib/gitlab/project_authorizations/without_nested_groups.rb
+++ b/lib/gitlab/project_authorizations/without_nested_groups.rb
@@ -24,11 +24,9 @@ module Gitlab
           user.groups.joins(:shared_projects).select_for_project_authorization
         ]
 
-        union = Gitlab::SQL::Union.new(relations)
-
         ProjectAuthorization
           .unscoped
-          .select_from_union(union)
+          .select_from_union(relations)
       end
     end
   end
diff --git a/lib/gitlab/user_extractor.rb b/lib/gitlab/user_extractor.rb
index 09652a4fb5e..bd0d24e4369 100644
--- a/lib/gitlab/user_extractor.rb
+++ b/lib/gitlab/user_extractor.rb
@@ -18,7 +18,7 @@ module Gitlab
     def users
       return User.none unless @text.present?
 
-      @users ||= User.from("(#{union.to_sql}) users")
+      @users ||= User.from_union(union_relations)
     end
     # rubocop: enable CodeReuse/ActiveRecord
 
@@ -43,13 +43,13 @@ module Gitlab
 
     private
 
-    def union
+    def union_relations
       relations = []
 
       relations << User.by_any_email(emails) if emails.any?
       relations << User.by_username(usernames) if usernames.any?
 
-      Gitlab::SQL::Union.new(relations)
+      relations
     end
   end
 end