allow the "USING" statement to be specified on change column calls

activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb

@@ -433,7 +433,9 @@ def change_column(table_name, column_name, type, options = {})
           quoted_table_name = quote_table_name(table_name)
           sql_type = type_to_sql(type, options[:limit], options[:precision], options[:scale])
           sql_type << "[]" if options[:array]
-          execute "ALTER TABLE #{quoted_table_name} ALTER COLUMN #{quote_column_name(column_name)} TYPE #{sql_type}"
+          sql = "ALTER TABLE #{quoted_table_name} ALTER COLUMN #{quote_column_name(column_name)} TYPE #{sql_type}"
+          sql << " USING #{options[:using]}" if options[:using]
+          execute sql
 
           change_column_default(table_name, column_name, options[:default]) if options_include_default?(options)
           change_column_null(table_name, column_name, options[:null], options[:default]) if options.key?(:null)

activerecord/test/cases/adapters/postgresql/change_schema_test.rb

@@ -0,0 +1,25 @@
+require 'cases/helper'
+
+module ActiveRecord
+  class Migration
+    class PGChangeSchemaTest < ActiveRecord::TestCase
+      attr_reader :connection
+
+      def setup
+        super
+        @connection = ActiveRecord::Base.connection
+        connection.create_table(:strings) do |t|
+          t.string :somedate
+        end
+      end
+
+      def teardown
+        connection.drop_table :strings
+      end
+
+      def test_change_string_to_date
+        connection.change_column :strings, :somedate, :timestamp, using: 'CAST("somedate" AS timestamp)'
+      end
+    end
+  end
+end