The schema is used to build the model classes of the ORM layer. To save execution time, these classes are generated with a command-line task called propel-build-model.

> symfony propel-build-model

Tip After building your model, you must remember to clear symfony's internal cache with symfony cc so symfony can find your newly created models.

Typing this command will launch the analysis of the schema and the generation of base data model classes in the lib/model/om/ directory of your project:

  • BaseArticle.php
  • BaseArticlePeer.php
  • BaseComment.php
  • BaseCommentPeer.php

In addition, the actual data model classes will be created in lib/model/:

  • Article.php
  • ArticlePeer.php
  • Comment.php
  • CommentPeer.php

You defined only two tables, and you end up with eight files. There is nothing wrong, but it deserves some explanation.

8.3.1. Base and Custom Classes

Why keep two versions of the data object model in two different directories?

You will probably need to add custom methods and properties to the model objects (think about the getName() method in Listing 8-1). But as your project develops, you will also add tables or columns. Whenever you change the schema.yml file, you need to regenerate the object model classes by making a new call to propel-build-model. If your custom methods were written in the classes actually generated, they would be erased after each generation.

The Base classes kept in the lib/model/om/ directory are the ones directly generated from the schema. You should never modify them, since every new build of the model will completely erase these files.

On the other hand, the custom object classes, kept in the lib/model/ directory, actually inherit from the Base ones. When the propel-build-model task is called on an existing model, these classes are not modified. So this is where you can add custom methods.

Listing 8-4 presents an example of a custom model class as created by the first call to the propel-build-model task.

Listing 8-4 - Sample Model Class File, in lib/model/Article.php

class Article extends BaseArticle

It inherits all the methods of the BaseArticle class, but a modification in the schema will not affect it.

The mechanism of custom classes extending base classes allows you to start coding, even without knowing the final relational model of your database. The related file structure makes the model both customizable and evolutionary.

8.3.2. Object and Peer Classes

Article and Comment are object classes that represent a record in the database. They give access to the columns of a record and to related records. This means that you will be able to know the title of an article by calling a method of an Article object, as in the example shown in Listing 8-5.

Listing 8-5 - Getters for Record Columns Are Available in the Object Class

$article = new Article();
$title = $article->getTitle();

ArticlePeer and CommentPeer are peer classes; that is, classes that contain static methods to operate on the tables. They provide a way to retrieve records from the tables. Their methods usually return an object or a collection of objects of the related object class, as shown in Listing 8-6.

Listing 8-6 - Static Methods to Retrieve Records Are Available in the Peer Class

$articles = ArticlePeer::retrieveByPks(array(123, 124, 125));
// $articles is an array of objects of class Article

Note From a data model point of view, there cannot be any peer object. That's why the methods of the peer classes are called with a :: (for static method call), instead of the usual -> (for instance method call).

So combining object and peer classes in a base and a custom version results in four classes generated per table described in the schema. In fact, there is a fifth class created in the lib/model/map/ directory, which contains metadata information about the table that is needed for the runtime environment. But as you will probably never change this class, you can forget about it.