MySQL: fix change_column on not-null columns that don't accept dfeault values of ''. Closes #6663.

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@7790 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

activerecord/CHANGELOG

@@ -1,5 +1,7 @@
 *SVN*
 
+* MySQL: fix change_column on not-null columns that don't accept dfeault values of ''.  #6663 [Jonathan Viney, Tarmo Tänav]
+
 * validates_uniqueness_of behaves well with single-table inheritance.  #3833 [Gabriel Gironda, rramdas, François Beausoleil, Josh Peek, Tarmo Tänav]
 
 * Raise ProtectedAttributeAssignmentError in development and test environments when mass-assigning to an attr_protected attribute.  #9802 [Henrik N]

activerecord/lib/active_record/connection_adapters/mysql_adapter.rb

@@ -408,8 +408,8 @@ module ActiveRecord
 
       def change_column(table_name, column_name, type, options = {}) #:nodoc:
         unless options_include_default?(options)
-          if result = select_one("SHOW COLUMNS FROM #{table_name} LIKE '#{column_name}'")
-            options[:default] = result['Default']
+          if column = columns(table_name).find { |c| c.name == column_name.to_s }
+            options[:default] = column.default
           else
             raise "No such column: #{table_name}.#{column_name}"
           end

activerecord/test/migration_test.rb

@@ -429,6 +429,16 @@ if ActiveRecord::Base.connection.supports_migrations?
         Person.connection.add_column("people", "first_name", :string) rescue nil
       end
     end
+    
+    def test_change_type_of_not_null_column
+      assert_nothing_raised do
+        Topic.connection.change_column "topics", "written_on", :datetime, :null => false
+        Topic.reset_column_information
+        
+        Topic.connection.change_column "topics", "written_on", :datetime, :null => false
+        Topic.reset_column_information
+      end
+    end
 
     def test_rename_table
       begin