http://www.symfony-project.org/book/trunk/08-Inside-the-Model-Layer#Generating+a+YAML+Data+Model+from+an+Existing+Database

Symfony can use the Creole database access layer to generate a schema.yml file from an existing database, thanks to introspection (the capability of databases to determine the structure of the tables on which they are operating). This can be particularly useful when you do reverse-engineering, or if you prefer working on the database before working on the object model.

In order to do this, you need to make sure that the project propel.ini file points to the correct database and contains all connection settings, and then call the propel-build-schema command:

symfony propel-build-schema

A brand-new schema.yml file built from your database structure is generated in the config/ directory. You can build your model based on this schema.

The schema-generation command is quite powerful and can add a lot of database-dependent information to your schema. As the YAML format doesn't handle this kind of vendor information, you need to generate an XML schema to take advantage of it. You can do this simply by adding an xml argument to the build-schema task:

symfony propel-build-schema xml

Instead of generating a schema.yml file, this will create a schema.xml file fully compatible with Propel, containing all the vendor information. But be aware that generated XML schemas tend to be quite verbose and difficult to read.

	The propel-build-sql and propel-build-schema tasks don't use the connection settings defined in the databases.yml file. Rather, these tasks use the connection settings in another file, called propel.ini and stored in the project config/ directory

documented on: 2007.02.09