The main difference is which side of the relationship holds the relationship's foreign key. The model that calls $this->belongsTo() is the owned model in one-to-one and many-to-one relationships and holds the key to the owning model.
//Example one-to-one relationship:
class User extends Model {
public function car() {
// user has at maximum one car,
// so $user->car will return a single model
return $this->hasOne('Car');
}
}
class Car extends Model {
public function owner() {
// cars table has owner_id field that stores id of related user model
return $this->belongsTo('User');
}
}
//Example one-to-many relationship:
class User extends Model {
public function phoneNumbers() {
// user can have multiple phone numbers,
// so $user->phoneNumbers will return a collection of models
return $this->hasMany('PhoneNumber');
}
}
class PhoneNumber extends Model {
public function owner() {
// phone_numbers table has owner_id field that stores id of related user model
return $this->belongsTo('User');
}
}