Here is a problem I had when adding the sluggable behaviour on existing data. The generated migration was not working because of the unique index it tries to set on the slug columns. Here is how to fix this: Symfony2 Doctrine Migrations with Unique Indexes (Slugs)
If you ever needed to migrate an old project’s users to Symfony2, you probably needed to preserve their passwords. As the passwords are encoded using a specific algorithm, you have to configure your new Symfony2 project to use exactly the same method in order for the old passwords work.
Find out how to do this in this new blog post: How to Create a Custom Symfony2 Password Encoder