database modeling: null foreign keys by entity type - database

Question

I need some help designing my database relationships.
I have these tables: documents, companies, individuals and users. A document can be internal or external.
If it is external, it can be signed by a company or an individual.
If it is internal it has to be signed by a user.
In any case, the document is signed by a single entity (company, individual or user). I was thinking of creating the documents table in the following way:
documents
----------
id_document
...
type
id_user
id_company
id_indiv
where type can be 0: internal, 1: external and id_user, id_company, id_indiv are foreign keys of the respective tables and can be null. Is this logic okay? Can someone suggest me a better idea?

Answer 1

Laravel's Eloquent ORM provides the Polymorphic relationship to handle this kind of problem.
To be more specific, you can place two fields; documentable_type and documentable_id inside your document table. And then add these relationship methods to each model;
class Document extends Model
{
/**
* Get the owning imageable model.
*/
public function signer()
{
return $this->morphTo();
}
}
class User extends Model
{
/**
* Get the documents signed by this user.
*/
public function documents()
{
return $this->morphMany('App\Models\Document', 'documentable');
}
}
class Company extends Model
{
/**
* Get the documents signed by this company.
*/
public function documents()
{
return $this->morphMany('App\Models\Document', 'documentable');
}
}
class Individual extends Model
{
/**
* Get the documents signed by this individual.
*/
public function documents()
{
return $this->morphMany('App\Models\Document', 'documentable');
}
}
Then you can use the following snippets;
$document->signer; // This returns either user or company or individual
$company->documents; // This returns a collection of document models which is signed by this company
$individual->documents;
$user->documents;
For more details, see this link; https://laravel.com/docs/8.x/eloquent-relationships#polymorphic-relationships