Eloquent tries to fetch relationship for non persisted model instances.

[jonnsl]

• Laravel Version: 5.5.40 & 5.6.22
• PHP Version: PHP 7.2.4
• Database Driver & Version: Any

Description:

Eloquent will try to fetch relationship even if model isn't persisted yet.

Steps To Reproduce:

Given the Models:

Class A extends Model{}
Class B extends Model
{
    public function a()
    {
        return $this->hasOne(A::class)->withDefault();
    }
}

Trying to access/load the relationship 'a':

(new B)->getRelationValue('a');

Eloquent will try to fetch the related model with this query:

select * from `a_s` where `a_s`.`b_id` is null and `a_s`.`b_id` is not null

Which obviously will return nothing, so a default model will be generated and returned.

Expected Behavior

If the model is not persisted, no query should be issued at all, and the default returned right away.

[laurencei]

But there could be a relation there. For example - if you have events or defaults triggered when a new model is created.

[Miguel-Serejo]

A model doesn't know if it's persisted or not.

This also allows someone to get a model id's relationships without actually fetching the model first:

$student = new Student();
$student->id = 1;
$student->grades;

[jonnsl]

Maybe Relation::getResults() could first verify if it has all foreign keys necessary to fetch the related models before trying to do so.
e.g. for the HasOne relation:

public function getResults()
{
    if (is_null($this->getParentKey()) {
        return $this->getDefaultFor($this->parent);
    }

    return $this->query->first() ?: $this->getDefaultFor($this->parent);
}