Q:

Comment entity related to Article or Product

I have entities "Article" and "Product". Now I want to add comments to these 2 entities. Should I create 2 different entities "ArticleComment" and "ProductComment" with the same properties, and build a ManyToOne relation to their respective entity, or create a single "Comment" entity and find a way to build a relation to both "Article" and "Product" entities. Considering solution #2, how could I do that ?

A:

Considering solution #2, how could I do that ?

One way would be to use Single Table Inheritance

Single Table Inheritance is an inheritance mapping strategy where all classes of a hierarchy are mapped to a single database table. In order to distinguish which row represents which type in the hierarchy a so-called discriminator column is used.

This means that you can easily create two separate entities ArticleComment and ProductComment both extending Comment. Then you use the advantages DiscriminatorMap column provides.
Your Comment entity could hold a relation called parent for instance that would refer to either your Article or Product entities. By creating new instance of ArticleComment or ProductComment your discriminator map field would be automatically populated depending on which type you're using.
This would also give you advantages with using DQL to query related comments by their type. Example from documentation:
$query = $em->createQuery('SELECT u FROM Doctrine\Tests\Models\Company\CompanyPerson u WHERE u INSTANCE OF Doctrine\Tests\Models\Company\CompanyEmployee');

And more. You can read that chapter here. Of course this is just a sample and you can use completely different approach. 

