Data Mapper

Data Mappers moves data between objects and a database while keeping them independent of each other and the mapper itself. If you started with Active Records, you may eventually faced with more complex business objects as your project progresses. When you build an object model with a lot of business logic it's valuable to use these mechanisms to better organize the data and the behavior that goes with it. Doing so leads to variant schemas; that is, the object schema and the relational schema don't match up.

The Data Mapper separates the in-memory objects from the database. Its responsibility is to transfer data between the two and also to isolate them from each other. With Data Mapper the in-memory objects needn't know even that there's a database present; they need no SQL interface code, and certainly no knowledge of the database schema. (The database schema is always ignorant of the objects that use it.)

When to Use It

The primary occasion for using Data Mapper is when you want the database schema and the object model to evolve independently. Data Mapper's primary benefit is that when working on the business (or domain) objects you can ignore the database, both in design and in the build and testing process. The domain objects have no idea what the database structure is, because all the correspondence is done by the mappers.

This helps you in the code because you can understand and work with the domain objects without having to understand how they're stored in the database. You can modify the business models or the database without having to alter either. With complicated mappings, particularly those involving existing databases, this is very valuable.

The price, of course, is the extra layer that you don't get with Active Record, so the test for using these patterns is the complexity of the business logic. If you have fairly simple business logic, an Active Record will probably work. For more complicated logic a Data Mapper may be more suitable.

SqlMap Data Mapper

The SqlMap DataMapper framework makes it easier to use a database with a PHP application. SqlMap DataMapper couples objects with stored procedures or SQL statements using a XML descriptor. Simplicity is the biggest advantage of the SqlMap DataMapper over object relational mapping tools. To use SqlMap DataMapper you rely on your own objects, XML, and SQL. There is little to learn that you don't already know. With SqlMap DataMapper you have the full power of both SQL and stored procedures at your fingertip

alt="SqlMap Data Mapper Overview" id="fig:sqlmap.png" class="figure"/> Here's a high level description of the work flow illustrated in the figure abov. Provide a parameter, either as an object or a primitive type. The parameter can be used to set runtime values in your SQL statement or stored procedure. If a runtime value is not needed, the parameter can be omitted.

Execute the mapping by passing the parameter and the name you gave the statement or procedure in your XML descriptor. This step is where the magic happens. The framework will prepare the SQL statement or stored procedure, set any runtime values using your parameter, execute the procedure or statement, and return the result.

In the case of an update, the number of rows affected is returned. In the case of a query, a single object, or a collection of objects is returned. Like the parameter, the result object, or collection of objects, can be a plain-old object or a primitive PHP type.

Setting up a database connection and initializing the SqlMap

A database connection for SqlMap can be set as follows. See Establishing Database Connection for futher details regarding creation of database connection in general. //create a connection and give it to the SqlMap manager. $dsn = 'pgsql:host=localhost;dbname=test'; //Postgres SQL $conn = new TDbConnection($dsn, 'dbuser','dbpass'); $manager = new TSqlMapManager($conn); $manager->configureXml('my-sqlmap.xml'); $sqlmap = $manager->getSqlMapGateway();

The TSqlMapManager is responsible for setting up the database connection and configuring the SqlMap with given XML file(s). The configureXml() method accepts a string that points to a SqlMap XML configuration file. Once configured, call the getSqlMapGateway() method to obtain an instance of the SqlMap gateway interface (use this object to insert/delete/find records).

SqlMap database connection can also be configured using a <module> tag in the application.xml or config.xml as follows.

The ConfigFile attribute should point to a SqlMap configuration file (to be detailed later) either using absolute path, relative path or the Prado's namespace dot notation path (must omit the ".xml" extension).

To obtain the SqlMap gateway interface from the <module> configuration, simply do, for example, class MyPage extends TPage { public function onLoad($param) { parent::onLoad($param); $sqlmap = $this->Application->Modules['my-sqlmap']->Client; $sqlmap->queryForObject(...); //query for some object } }

A quick example

Let us consider the following "users" table that contains two columns named "username" and "email", where "username" is also the primary key. CREATE TABLE users ( username VARCHAR( 20 ) NOT NULL , email VARCHAR( 200 ) , PRIMARY KEY ( username ) );

Next we define our plain User class as follows. Notice that the User is very simple. class User { public $username; public $email; }

Next, we need to define a SqlMap XMl configuration file, lets name the file as my-sqlmap.xml SELECT username, email FROM users

The <select> tag returns defines an SQL statement. The id attribute will be used as the identifier for the query. The resultClass attribute value is the name of the class the the objects to be returned. We can now query the objects as follows: //assume that $sqlmap is an TSqlMapGateway instance $userList = $sqlmap->queryForList("SelectUsers"); //Or just one, if that's all you need: $user = $sqlmap->queryForObject("SelectUsers");

The above example shows demonstrates only a fraction of the capabilities of the SqlMap Data Mapper. Further details can be found in the SqlMap Manual.

Combining SqlMap with Active Records

The above example may seem trival and it also seems that there is alot work just to retrieve some data. However, notice that the User class is totally unware of been stored in the database, and the database is unware of the User class.

One of advantages of SqlMap is the ability to map complex object relationship, collections from an existing database. On the other hand, Active Record provide a very simple way to interact with the underlying database but unable to do more complicated relationship or collections. A good compromise is to use SqlMap to retrieve complicated relationships and collections as Active Record objects and then using these Active Records to do the updates, inserts and deletes.

Continuing with the previous example, we change the definition of the User class to become an Active Record. class UserRecord extends TActiveRecord { public $username; //the column named "username" in the "users" table public $email; private static $_tablename='users'; //table name /** * @return TActiveRecord active record finder instance */ public static function finder($className=__CLASS__) { return parent::finder($className); } }

We also need to change the definition of the SqlMap XML configuration. We just need to change the value of resultClass attribute to UserRecord. SELECT username, email FROM users

The PHP code for retrieving the users remains the same, but SqlMap returns Active Records instead, and we can take advantage of the Active Record methods. //assume that $sqlmap is an TSqlMapGateway instance $user = $sqlmap->queryForObject("SelectUsers"); $user->email = 'test@example.com'; //change data $user->save(); //save it using Active Record

References

• Fowler et. al. Patterns of Enterprise Application Architecture, Addison Wesley, 2002.
• iBatis Team. iBatis Data Mapper, http://ibatis.apache.org.