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When you Code First, you start with developing Objects and then map them onto your database. When you Model First, you are modelling your application using tools (for example UML) and generate database schema and PHP code from this model. When you have a Database First, you already have a database schema and generate the corresponding PHP code from it.
This getting started guide is in development.
Development of new applications often starts with an existing database schema. When the database schema is the starting point for your application, then development is said to use the Database First approach to Doctrine.
In this workflow you would modify the database schema first and then regenerate the PHP code to use with this schema. You need a flexible code-generator for this task and up to Doctrine 2.2, the code generator hasn't been flexible enough to achieve this.
We spinned off a subproject, Doctrine CodeGenerator, that will fill this gap and allow you to do Database First development.