Refactor mysql rename column statement generation

* Simplify code
* Fix to not lose the default value for NOT NULL columns (previous code had a
  bug here)
* Change base property_schema_statement method to only add a default to the
  statement if it is non-NULL

lib/dm-migrations/adapters/dm-do-adapter.rb

@@ -239,7 +239,11 @@ def property_schema_statement(connection, schema)
             statement << "(#{connection.quote_value(length)})"
           end
 
-          statement << " DEFAULT #{connection.quote_value(schema[:default])}" if schema.key?(:default)
+          default = schema[:default]
+          if default
+            statement << " DEFAULT #{connection.quote_value(default)}"
+          end
+
           statement << ' NOT NULL' unless schema[:allow_nil]
 
           statement

lib/dm-migrations/sql/mysql.rb

@@ -42,31 +42,24 @@ def change_column_type_statement(name, column)
     end
 
     def rename_column_type_statement(table_name, old_col, new_col)
-      column_info    = select("SHOW COLUMNS FROM #{quote_name(table_name)} LIKE ?", old_col).first
+      table       = quote_name(table_name)
+      column_info = select("SHOW COLUMNS FROM #{table} LIKE ?", old_col).first
+
       column_options = {
         :name      => column_info.field,
         :primitive => column_info.type,
+        :serial    => column_info.extra == 'auto_increment',
+        :allow_nil => column_info.null == 'YES',
+        :default   => column_info.default,
       }
 
-      if column_info.null == 'YES'
-        column_options[:allow_nil] = true
-        column_options[:default]   = column_info.default
-      else
-        column_options[:allow_nil] = false
-      end
-
-      case column_info.extra
-      when 'auto_increment'
-        column_options[:serial] = true
-      end
-
       column = with_connection do |connection|
         property_schema_statement(connection, column_options)
       end
 
-      column_name, column_definition = column.split(' ', 2)
+      column_definition = column.split(' ', 2).last
 
-      "ALTER TABLE #{quote_name(table_name)} CHANGE #{quote_name(old_col)} #{quote_name(new_col)} #{column_definition}"
+      "ALTER TABLE #{table} CHANGE #{quote_name(old_col)} #{quote_name(new_col)} #{column_definition}"
     end
 
     class Table

spec/integration/sql_spec.rb

@@ -165,7 +165,7 @@
         when :mysql
           before do
             # create the table so the existing column schema can be instrospected
-            @adapter.execute('CREATE TABLE `people` (name VARCHAR(50) NOT NULL)')
+            @adapter.execute("CREATE TABLE `people` (name VARCHAR(50) DEFAULT 'John' NOT NULL)")
 
             @modifier = DataMapper::Migration::TableModifier.new(@adapter, :people) do
               rename_column :name, :first_name
@@ -177,7 +177,7 @@
           end
 
           it "should change the column" do
-            @modifier.to_sql.should == %q{ALTER TABLE `people` CHANGE `name` `first_name` varchar(50) NOT NULL}
+            @modifier.to_sql.should == %q{ALTER TABLE `people` CHANGE `name` `first_name` varchar(50) DEFAULT 'John' NOT NULL}
           end
         end
       end