Support updates with joins. Fixes #522.

activerecord/lib/active_record/connection_adapters/abstract/database_statements.rb

@@ -306,6 +306,15 @@ module ActiveRecord
         end
       end
 
+      # The default strategy for an UPDATE with joins is to use a subquery. This doesn't work
+      # on mysql (even when aliasing the tables), but mysql allows using JOIN directly in
+      # an UPDATE statement, so in the mysql adapters we redefine this to do that.
+      def join_to_update(update, select) #:nodoc:
+        subselect = select.clone
+        subselect.ast.cores.last.projections = [update.ast.key]
+        update.wheres = [update.ast.key.in(subselect)]
+      end
+
       protected
         # Returns an array of record hashes with the column names as keys and
         # column values as values.

activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb

@@ -577,6 +577,10 @@ module ActiveRecord
         where_sql
       end
 
+      def join_to_update(update, select) #:nodoc:
+        update.table select.ast.cores.last.source
+      end
+
       protected
         def quoted_columns_for_index(column_names, options = {})
           length = options[:length] if options.is_a?(Hash)

activerecord/lib/active_record/connection_adapters/mysql_adapter.rb

@@ -491,6 +491,10 @@ module ActiveRecord
         execute("RELEASE SAVEPOINT #{current_savepoint_name}")
       end
 
+      def join_to_update(update, select) #:nodoc:
+        update.table select.ast.cores.last.source
+      end
+
       # SCHEMA STATEMENTS ========================================
 
       def structure_dump #:nodoc:

activerecord/lib/active_record/relation.rb

@@ -217,13 +217,18 @@ module ActiveRecord
         where(conditions).apply_finder_options(options.slice(:limit, :order)).update_all(updates)
       else
         stmt = arel.compile_update(Arel.sql(@klass.send(:sanitize_sql_for_assignment, updates)))
+        stmt.key = table[primary_key]
 
-        if limit = arel.limit
-          stmt.take limit
+        if joins_values.any?
+          @klass.connection.join_to_update(stmt, arel)
+        else
+          if limit = arel.limit
+            stmt.take limit
+          end
+
+          stmt.order(*arel.orders)
         end
 
-        stmt.order(*arel.orders)
-        stmt.key = table[primary_key]
         @klass.connection.update stmt, 'SQL', bind_values
       end
     end

activerecord/test/cases/relations_test.rb

@@ -965,4 +965,12 @@ class RelationTest < ActiveRecord::TestCase
   def test_ordering_with_extra_spaces
     assert_equal authors(:david), Author.order('id DESC , name DESC').last
   end
+
+  def test_update_all_with_joins
+    comments = Comment.joins(:post).where('posts.id' => posts(:welcome).id)
+    count    = comments.count
+
+    assert_equal count, comments.update_all(:post_id => posts(:thinking).id)
+    assert_equal posts(:thinking), comments(:greetings).post
+  end
 end