Emit single pair of parens for UNION and UNION ALL

mysql has a great implementation to suppress multiple parens for union
sql statements.

This moves that functionality to the generic implementation
This also introduces that functionality for UNION ALL

1 files changed, 19 insertions, 4 deletions

diff --git a/activerecord/lib/arel/visitors/to_sql.rb b/activerecord/lib/arel/visitors/to_sql.rb
index 8e56fb55a2..34e82975ad 100644
--- a/activerecord/lib/arel/visitors/to_sql.rb
+++ b/activerecord/lib/arel/visitors/to_sql.rb
@@ -268,13 +268,11 @@ module Arel # :nodoc: all
         end
 
         def visit_Arel_Nodes_Union(o, collector)
-          collector << "( "
-          infix_value(o, collector, " UNION ") << " )"
+          infix_value_with_paren(o, collector, " UNION ")
         end
 
         def visit_Arel_Nodes_UnionAll(o, collector)
-          collector << "( "
-          infix_value(o, collector, " UNION ALL ") << " )"
+          infix_value_with_paren(o, collector, " UNION ALL ")
         end
 
         def visit_Arel_Nodes_Intersect(o, collector)
@@ -845,6 +843,23 @@ module Arel # :nodoc: all
           visit o.right, collector
         end
 
+        def infix_value_with_paren(o, collector, value, suppress_parens = false)
+          collector << '( ' unless suppress_parens
+          collector = if o.left.class == o.class
+                        infix_value_with_paren(o.left, collector, value, true)
+                      else
+                        visit o.left, collector
+                      end
+          collector << value
+          collector = if o.right.class == o.class
+                        infix_value_with_paren(o.right, collector, value, true)
+                      else
+                        visit o.right, collector
+                      end
+          collector << ' )' unless suppress_parens
+          collector
+        end
+
         def aggregate(name, o, collector)
           collector << "#{name}("
           if o.distinct