Question: Can anyone please tell me how can I add a new column of type enum to my schema in order to implement a Doctrine Migration?

Answer: 
1. modify your schema
2. run `./symfony doc:generate-migrations-diff`

this will generate one or more files in lib/migrations/doctrine/
3. run `./symfony doc:migrate`

this will apply the generated migrations to the database
4. run `./symfony doc:build --all-classes`

this works for symfony >= 1.3/1.4 and will rebuild all form/filters/model classes according to the modified schema

remember that the migration is generated comparing the new schema.yml to the current model classes, so if you rebuild your classes before running generate-migrations-diff you're screwed.
In case you need to write the migration script yourself, here's an example of the syntax -- I haven't found a proper specification for the syntax anywhere.

```
$this->addColumn('tablename', 'column_name', 'enum', false, 
                  array('fixed' => 1,
                        'values' => 
                         array(0 => 'auto',
                               1 => 'manual',
                               2 => 'unknown'),
                         'default' => 'unknown',
                         'notnull' => true,
                         'length' => NULL,
                   ));

```