There are three ways to define database relationships in Yii: one-to-one, one-to-many, and many-to-many. 1. Use the hasOne() method one-to-one, such as the User model obtains the corresponding Profile through getProfile(); 2. Use the hasMany() method one-to-many, such as the Customer model obtains all orders through getOrders(); 3. Use viaTable() for many-to-many, such as the User model obtains multiple roles through getRoles(), and use the intermediate table user_role to improve query efficiency and keep the code tidy.

When working with Yii models, defining database relationships is a key part of building clean and efficient applications. You don't need to write raw SQL joins every time — Yii's ActiveRecord makes it easy to set up one-to-one, one-to-many, and many-to-many relationships directly in your models.

One-to-One Relationships

This type of relationship means that one record in a table corresponds to exactly one record in another table. A common example is a user and their profile — each user has one profile, and each profile belongs to one user.

To define this in Yii, you use the hasOne() or hasMany() method inside your model, depending on which side of the relationship you're on.

For example, if you have a User model and a Profile model:

 // In User.php model
public function getProfile()
{
    return $this->hasOne(Profile::className(), ['user_id' => 'id']);
}

Here, Profile has a foreign key user_id pointing to User.id . When you call $user->profile , Yii will fetch the related profile automatically.

On the flip side, in the Profile model:

 public function getUser()
{
    return $this->hasOne(User::className(), ['id' => 'user_id']);
}

You can then access the user from a profile like $profile->user .

One-to-Many Relationships

A one-to-many relationship occurs when one record in a table can be associated with multiple records in another. For instance, a customer may have many orders.

Let's say you have a Customer model and an Order model. Each order belongs to one customer, but a customer can have multiple orders.

In the Customer model:

 public function getOrders()
{
    return $this->hasMany(Order::className(), ['customer_id' => 'id']);
}

Now, calling $customer->orders will return all orders linked to that customer.

And in the Order model (optional):

 public function getCustomer()
{
    return $this->hasOne(Customer::className(), ['id' => 'customer_id']);
}

This allows you to get the customer for a specific order using $order->customer .

Many-to-Many Relationships

This is used when records in one table are related to multiple records in another, and vice versa — think users and roles. A user can have multiple roles, and a role can belong to multiple users.

To handle this in Yii, you typically use a junction table (eg, user_role ) and define the relationship using the viaTable() method.

Assuming you have:

• User model
• Role model
• user_role table with columns user_id and role_id

In the User model:

 public function getRoles()
{
    return $this->hasMany(Role::className(), ['id' => 'role_id'])
                ->viaTable('user_role', ['user_id' => 'id']);
}

Then, you can get all roles for a user by calling $user->roles .

Similarly, in the Role model:

 public function getUsers()
{
    return $this->hasMany(User::className(), ['id' => 'user_id'])
                ->viaTable('user_role', ['role_id' => 'id']);
}

So now, $role->users give you all users assigned to that role.

Some things to keep in mind:

• Make sure the foreign keys are correctly referenced.
• Use singular names for one-to-one/one-to-many methods ( getOrder() , not getOrders() if it's a hasOne).
• For many-to-many, always use plural names ( getRoles() ), since it's inherently multiple.
• Use eager loading ( with() ) when fetching multiple related records to avoid performance issues.

Defining these relationships properly in your models make querying and organizing data much easier. It also keeps your code clean and readable without having to manually write joins every time.

Basically that's it.

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