Doctrine persists extra entities at update/merge fix

If you have batch create and update action simultaneously (persisting entity if it doesn't exists in DB or update it otherwise), made like in this documentation, and you noticed that it creates redundant entities, it means Doctrine think they are new. Enable "detach" cascade (transitional) operation in Doctrine like this:

/**
* @OneToMany(targetEntity="Customer", mappedBy="product", cascade={"detach"})
*/
protected $customers;

If you are not in dev environment, do not forget to run orm:clear-cache:metadata command.