ActiveRecord::Relation unions [PROPOSAL], closes rails#939

Depends on rails/arel#118.
The accepted implementation could act as
a unions list, or a new top-level object,
which would change this patch proposal.

activerecord/lib/active_record/relation/query_methods.rb

@@ -6,7 +6,7 @@ module QueryMethods
     extend ActiveSupport::Concern
 
     attr_accessor :includes_values, :eager_load_values, :preload_values,
-                  :select_values, :group_values, :order_values, :joins_values,
+                  :select_values, :group_values, :order_values, :joins_values, :unions_values,
                   :where_values, :having_values, :bind_values,
                   :limit_value, :offset_value, :lock_value, :readonly_value, :create_with_value,
                   :from_value, :reordering_value, :reverse_order_value,
@@ -123,6 +123,17 @@ def joins(*args)
       relation
     end
 
+    def union(*args)
+      return self if args.compact.blank?
+
+      relation = clone
+
+      relation.unions_values ||= []
+      relation.unions_values += args
+
+      relation
+    end
+
     def bind(value)
       relation = clone
       relation.bind_values += [value]
@@ -284,6 +295,8 @@ def build_arel
       arel.from(@from_value) if @from_value
       arel.lock(@lock_value) if @lock_value
 
+      build_union(arel, @unions_values) unless @unions_values.empty?
+
       arel
     end
 
@@ -375,6 +388,10 @@ def build_joins(manager, joins)
       manager
     end
 
+    def build_union(arel, unions)
+      arel.union(*unions.map {|union| union.build_arel})
+    end
+
     def build_select(arel, selects)
       unless selects.empty?
         @implicit_readonly = false