Spot\Mapper::hasOne PHP Method

    public function hasOne(EntityInterface $entity, $foreignEntity, $foreignKey)
    {
        $localKey = $this->primaryKeyField();
        if (!is_subclass_of($foreignEntity, 'Spot\\EntityInterface')) {
            throw new \InvalidArgumentException("Related entity name must be a " . "valid entity that extends Spot\\Entity. Given '" . $foreignEntity . "'.");
        }
        // Return relation object so query can be lazy-loaded
        return new Relation\HasOne($this, $foreignEntity, $foreignKey, $localKey, $entity->{$localKey});
    }

 public static function relations(Mapper $mapper, Entity $entity)
 {
     return ['search' => $mapper->hasOne($entity, 'SpotTest\\Entity\\Event\\Search', 'event_id')];
 }