**Course Title:** Mastering Yii Framework: Building Scalable Web Applications **Section Title:** Database Management with Active Record **Topic:** Understanding relations in Active Record (one-to-one, one-to-many, many-to-many) **Overview** In the previous topics, we covered the basics of Active Record and how to perform CRUD operations using it. However, in most real-world applications, you'll encounter complex relationships between tables, such as one-to-one, one-to-many, and many-to-many relationships. In this topic, we'll explore how to define and manage these relationships using Active Record. **One-to-One Relationships** A one-to-one relationship occurs when one record in a table is related to only one record in another table. This type of relationship is often used when you need to store additional information about a record in a separate table. **Example:** Suppose we have a `User` table and a `Profile` table. Each user has one profile, and each profile belongs to one user. We can define a one-to-one relationship between these two tables using the following code: ```php // User.php namespace app\models; use yii\db\ActiveRecord; class User extends ActiveRecord { public static function tableName() { return 'user'; } public function getProfile() { return $this->hasOne(Profile::className(), ['user_id' => 'id']); } } // Profile.php namespace app\models; use yii\db\ActiveRecord; class Profile extends ActiveRecord { public static function tableName() { return 'profile'; } public function getUser() { return $this->hasOne(User::className(), ['id' => 'user_id']); } } ``` In the above code, we define a `getProfile()` method in the `User` model and a `getUser()` method in the `Profile` model. These methods establish the one-to-one relationship between the two tables. **One-to-Many Relationships** A one-to-many relationship occurs when one record in a table is related to multiple records in another table. This type of relationship is often used when you need to store multiple related records for a single record in another table. **Example:** Suppose we have a `Category` table and a `Product` table. Each category can have multiple products, and each product belongs to one category. We can define a one-to-many relationship between these two tables using the following code: ```php // Category.php namespace app\models; use yii\db\ActiveRecord; class Category extends ActiveRecord { public static function tableName() { return 'category'; } public function getProducts() { return $this->hasMany(Product::className(), ['category_id' => 'id']); } } // Product.php namespace app\models; use yii\db\ActiveRecord; class Product extends ActiveRecord { public static function tableName() { return 'product'; } public function getCategory() { return $this->hasOne(Category::className(), ['id' => 'category_id']); } } ``` In the above code, we define a `getProducts()` method in the `Category` model and a `getCategory()` method in the `Product` model. These methods establish the one-to-many relationship between the two tables. **Many-to-Many Relationships** A many-to-many relationship occurs when multiple records in a table are related to multiple records in another table. This type of relationship is often used when you need to store multiple related records for multiple records in another table. **Example:** Suppose we have a `User` table and a `Group` table. Each user can belong to multiple groups, and each group can have multiple users. We can define a many-to-many relationship between these two tables using the following code: ```php // User.php namespace app\models; use yii\db\ActiveRecord; class User extends ActiveRecord { public static function tableName() { return 'user'; } public function getGroups() { return $this->hasMany(Group::className(), ['id' => 'group_id']); } } // Group.php namespace app\models; use yii\db\ActiveRecord; class Group extends ActiveRecord { public static function tableName() { return 'group'; } public function getUsers() { return $this->hasMany(User::className(), ['id' => 'user_id']); } } ``` In the above code, we define a `getGroups()` method in the `User` model and a `getUsers()` method in the `Group` model. These methods establish the many-to-many relationship between the two tables. **Conclusion** In this topic, we covered how to define and manage one-to-one, one-to-many, and many-to-many relationships using Active Record. We also provided examples of how to establish these relationships using the `hasOne()` and `hasMany()` methods. **Practice Exercise** Try to create a many-to-many relationship between the `Book` and `Author` tables. Each book can have multiple authors, and each author can write multiple books. **Additional Resources** * Yii Framework documentation: [Active Record](https://www.yiiframework.com/doc/guide/2.0/en/db-active-record) * Yii Framework documentation: [Relationships](https://www.yiiframework.com/doc/guide/2.0/en/db-active-record#relationships) **Leave a comment or ask for help if you have any questions or need further clarification on any of the concepts covered in this topic.**