I don't think the parse() function needs to have the Document parameter because you can get to a Document/Email from a Stream. You just need to check if the stream was attached to Document or Email.

The alternative would be to have a separate parseDocument(Document $document) and parseEmail(Email $email). I think this approach is closer to your design where you have separate tables for Document and Email.

If you want to do processing based on Stream like in your example, I think the database design can be made clearer by removing the Documents and Emails tables and put the text inside Streams and add a DocumentData to keep your Documents name and path. This way, the code doesn't need to care about the distinction, except when it needs to save/send which can be looked up quickly through a foreign key to DocumentData.

To allow the parse() function parse both Document and Email, you can create an interface, e.g. TextContainer that Document and Email implement. Something like:

interface TextContainer { function getText(): string; function setText(string $text); function save(); } class ApplyParsingRules { public function parse(Stream $stream, TextContainer $container) { $text = $container->getText(); $fields = $stream->fields()->get(); //1. get all $parsing_rules for each $fields. //2. parse $text by using each $parsing_rule //3. save the end result of $text by document/email specific and field. $container->setText($text); $container->save(); } } 

I actually prefer parse() to return the result and let the caller decide what to do with the result, i.e.

class ApplyParsingRules { public function parse(Stream $stream, TextContainer $container): string { $text = $container->getText(); $fields = $stream->fields()->get(); //1. get all $parsing_rules for each $fields. //2. parse $text by using each $parsing_rule return $text; } }