public function hasOne($accessor, $foreignClass, $foreignColumn = null, $selfColumn)
{
// foreignColumn is default to foreignClass.primary key
return $this->relations[$accessor] = new Relationship($accessor, array('type' => Relationship::HAS_ONE, 'self_schema' => $this->getCurrentSchemaClass(), 'self_column' => $selfColumn, 'foreign_schema' => $this->resolveSchemaClass($foreignClass), 'foreign_column' => $foreignColumn));
}