PHP development framework Yii Framework tutorial (27) Database-associated Active Record example

We have seen how to use Active Record (AR) to get data from a single data table. In this section, we explain how to use AR to connect multiple related data tables and retrieve the joined data set.

In order to use relational AR, we recommend defining primary key-foreign key constraints in the tables that need to be related. These constraints can help ensure the consistency and integrity of related data.

This example introduces how to use Active Record for multiple related tables by modifying the database-Query Builder example of Yii Framework Development Tutorial (25).

Before we use AR to perform related queries, we need to let AR know how one AR class is related to another.

The relationship between two AR classes is directly related through the relationship between the data tables represented by the AR classes. From a database perspective, there are three types of relationships between tables A and B: one-to-many (such as tbl_user and tbl_post), one-to-one (such as tbl_user and tbl_profile) and many-to-many Many (many-to-many such as tbl_category and tbl_post). In AR, there are four relationships:

BELONGS_TO (belongs to): If the relationship between tables A and B is one-to-many, then table B belongs to table A (for example, Post belongs to User);

HAS_MANY (there are multiple): If the relationship between tables A and B is one-to-many, then A has multiple Bs (for example, User has multiple Posts);

HAS_ONE (there is one) : This is a special case of HAS_MANY. A can have at most one B (for example, User can have at most one Profile);

MANY_MANY: This corresponds to the many-to-many relationship in the database. Since most DBMS do not directly support many-to-many relationships, a relationship table is required to split the many-to-many relationship into a one-to-many relationship. In our example data structure, tbl_post_category is used for this purpose. In AR terms, we can interpret MANY_MANY as the combination of BELONGS_TO and HAS_MANY. For example, Post belongs to many (belongs to many) Category, and Category has many (has many). The relationship defined in Post.

AR needs to override the relations() method in CActiveRecord. This method returns an array of relationship configurations. Each array element represents a single relationship in the following format.

In Query Builder we used the following SQL query statement

SELECT c.FirstName, c.LastName , c.Address,c.Email
FROM customer c
INNER JOIN
employee e
ON c.SupportRepId=e.EmployeeId

WHERE e.EmployeeId=4 involves two tables Employee and Customer, and there is a pair between Employee and Customer Multiple relationships mean that one employee can be responsible for multiple customers. The relationship between Employee and Customer is HAS_MANY, and the relationship between Customer and Employee is HAS_ONE. Therefore, Employee and Customer can be defined as follows:

//Customer.phpclass Customer extends CActiveRecord{
public static function model($className=__CLASS__){return parent::model($className);}
public function tableName(){return 'Customer';}
}
//Employee.phpclass Employee extends CActiveRecord{
public static function model($className=__CLASS__){return parent::model($className);}
public function tableName(){return 'Employee';}
public function relations(){return array('customers'=>array(self::HAS_MANY, 'Customer', 'SupportRepId'),
);}}

Because this example only uses the Customer corresponding to the Employee query, only the relations method is defined for the class. The corresponding tables and foreign keys are Customer and SupportRepId.
Then modify the indexAction method of SiteController:

public function actionIndex(){
$employee=Employee::model()->findByPk(4);
$this->render('index', array('model' => $employee->customers,
));}

The relationship definition in the AR class implicitly adds an attribute to the class for each relationship. After a correlation query is executed, the corresponding properties will be populated with the associated AR instance. Therefore, the Customers record corresponding to Employee can be queried from $employee->customers.
The simplest way to perform a correlation query is to read the correlation attribute in an AR instance. If this property has not been accessed before, a join query will be initialized that joins the two tables and filters using the primary key of the current AR instance. The query results are saved to the property as an instance of the associated AR class. This is the legendary lazy loading (also translated as lazy loading) method. For example, a related query is only executed when the related object is accessed for the first time.
This example uses lazy loading, which is not efficient in some cases. If we want to get the authors of N posts, using this lazy loading will result in N join queries being executed. In this case, we should use eager loading instead.
The eager loading method will obtain the associated AR instance while obtaining the main AR instance. This is done by using the with method when using the find or findAll methods in AR. For example:

$employee=Post::model()->with ('customers')->findAll();

Finally modify the code of the View that displays the results:

$customer){
echo 'First Name:' . $customer->FirstName . '';
echo 'Last Name:' . $customer->LastName . '';
echo 'Address:' . $customer->Address . '';
echo 'Email:' . $customer->Email . '';
echo '----------------------';}
?>

Different data handles the case of column names differently. Some databases are case-sensitive, just to be on the safe side. , the Customer attribute uses the same case as the column definition.

This example introduces the most basic usage of associated Active Record. For other functions and properties, please refer to Yii Chinese documentation. In addition, if you use tools like CodeSmith, if you can automatically Generating database-defined ActiveRecord code can greatly reduce programmers' manual coding workload.

In addition, the convenience of using Active Record comes at the cost of performance. Generally, the performance of using Active Record is one level worse than using DAO to read and write databases. The following table is a reference value to find 200 actors and 1000 movies.

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