1 files changed, 45 insertions, 32 deletions

diff --git a/activerecord/lib/arel/visitors/mysql.rb b/activerecord/lib/arel/visitors/mysql.rb
index 37bfb661f0..32f6705d04 100644
--- a/activerecord/lib/arel/visitors/mysql.rb
+++ b/activerecord/lib/arel/visitors/mysql.rb
@@ -9,20 +9,20 @@ module Arel # :nodoc: all
             collector << "( "
           end
 
-          collector =   case o.left
-                        when Arel::Nodes::Union
-                          visit_Arel_Nodes_Union o.left, collector, true
-                        else
-                          visit o.left, collector
+          case o.left
+          when Arel::Nodes::Union
+            visit_Arel_Nodes_Union o.left, collector, true
+          else
+            visit o.left, collector
           end
 
           collector << " UNION "
 
-          collector =    case o.right
-                         when Arel::Nodes::Union
-                           visit_Arel_Nodes_Union o.right, collector, true
-                         else
-                           visit o.right, collector
+          case o.right
+          when Arel::Nodes::Union
+            visit_Arel_Nodes_Union o.right, collector, true
+          else
+            visit o.right, collector
           end
 
           if suppress_parens
@@ -37,6 +37,10 @@ module Arel # :nodoc: all
           visit o.expr, collector
         end
 
+        def visit_Arel_Nodes_UnqualifiedColumn(o, collector)
+          visit o.expr, collector
+        end
+
         ###
         # :'(
         # http://dev.mysql.com/doc/refman/5.0/en/select.html#id3482214
@@ -52,28 +56,6 @@ module Arel # :nodoc: all
           super
         end
 
-        def visit_Arel_Nodes_UpdateStatement(o, collector)
-          collector << "UPDATE "
-          collector = visit o.relation, collector
-
-          unless o.values.empty?
-            collector << " SET "
-            collector = inject_join o.values, collector, ", "
-          end
-
-          unless o.wheres.empty?
-            collector << " WHERE "
-            collector = inject_join o.wheres, collector, " AND "
-          end
-
-          unless o.orders.empty?
-            collector << " ORDER BY "
-            collector = inject_join o.orders, collector, ", "
-          end
-
-          maybe_visit o.limit, collector
-        end
-
         def visit_Arel_Nodes_Concat(o, collector)
           collector << " CONCAT("
           visit o.left, collector
@@ -82,6 +64,37 @@ module Arel # :nodoc: all
           collector << ") "
           collector
         end
+
+        # In the simple case, MySQL allows us to place JOINs directly into the UPDATE
+        # query. However, this does not allow for LIMIT, OFFSET and ORDER. To support
+        # these, we must use a subquery.
+        def prepare_update_statement(o)
+          if o.offset || has_join_sources?(o) && has_limit_or_offset_or_orders?(o)
+            super
+          else
+            o
+          end
+        end
+        alias :prepare_delete_statement :prepare_update_statement
+
+        # MySQL is too stupid to create a temporary table for use subquery, so we have
+        # to give it some prompting in the form of a subsubquery.
+        def build_subselect(key, o)
+          subselect = super
+
+          # Materialize subquery by adding distinct
+          # to work with MySQL 5.7.6 which sets optimizer_switch='derived_merge=on'
+          unless has_limit_or_offset_or_orders?(subselect)
+            core = subselect.cores.last
+            core.set_quantifier = Arel::Nodes::Distinct.new
+          end
+
+          Nodes::SelectStatement.new.tap do |stmt|
+            core = stmt.cores.last
+            core.froms = Nodes::Grouping.new(subselect).as("__active_record_temp")
+            core.projections = [Arel.sql(quote_column_name(key.name))]
+          end
+        end
     end
   end
 end