src/Resources/DataNormalizer.php
protected function normalizeRelation(Relation $relation)
    {
        if ($this->isInstanceOf($relation, [BelongsTo::class, HasOne::class, MorphOne::class, MorphTo::class])) {
            return $relation->first();
        }

        return $relation->get();
    }
protected function normalizeRelation(Relation $relation)
    {
        if ($this->isInstanceOf($relation, [BelongsTo::class, HasOne::class, MorphOne::class, MorphTo::class])) {
            return $relation->first();
        }

        return $relation->get();
    }
Normalize a relationship.

@param  \Illuminate\Database\Eloquent\Relations\Relation $relation
@return \Illuminate\Support\Collection|\Illuminate\Database\Eloquent\Model|null