Spot\Mapper::belongsTo PHP Method

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

 public static function relations(Mapper $mapper, Entity $entity)
 {
     return ['tags' => $mapper->hasManyThrough($entity, 'SpotTest\\Entity\\Tag', 'SpotTest\\Entity\\PostTag', 'tag_id', 'post_id'), 'comments' => $mapper->hasMany($entity, 'SpotTest\\Entity\\Post\\Comment', 'post_id')->order(['date_created' => 'ASC']), 'author' => $mapper->belongsTo($entity, 'SpotTest\\Entity\\Author', 'author_id')];
 }