db\BaseActiveRecord hasOne()

Declares a has-one relation.

The declaration is returned in terms of a relational yii\db\ActiveQuery instance through which the related record can be queried and retrieved back.

A has-one relation means that there is at most one related record matching the criteria set by this relation, e.g., a customer has one country.

For example, to declare the country relation for Customer class, we can write the following code in the Customer class:

public function getCountry()
{
    return $this->hasOne(Country::className(), ['id' => 'country_id']);
}

Note that in the above, the 'id' key in the $link parameter refers to an attribute name in the related class Country, while the 'country_id' value refers to an attribute name in the current AR class.

Call methods declared in yii\db\ActiveQuery to further customize the relation.