Why is it recommended to avoid bidirectional relations in ORM?

Question

What are the technical reasons that bidirectional relations between entities are not recommended? Does it impact an ORM's performance? (If so, why?)

Source:

http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/reference/best-practices.html#constrain-relationships-as-much-as-possible

https://ocramius.github.io/doctrine-best-practices/#/86

Answer 1

In that first source you refer to are three reasons mentioned:

This has several benefits:

• Reduced coupling in your domain model
• Simpler code in your domain model (no need to maintain bidirectionality properly)
• Less work for Doctrine

In the second:

BI-DIRECTIONAL ASSOCIATIONS ARE OVERHEAD

I assume those are the whys. "Less work doctrine" and "are overhead" most likely means that it impacts performance, I wouldn't know how else to interpret that...

Makes sense since the ORM needs to update both sides whenever you change something in a bi-directional relationship.

Answer 2

As well as the reasons mentioned in the source you provided (and Wilt's answer) having a lot of relationships between entities makes it easier to violate single responsibility and can make your code more complex.

Take this example, I want to update a user's phone number from a certain part of the code. I currently only have access to an organization the user belongs to. If I have a full path of connections between entities I can do this:

foreach ($organization->getDepartments() as $department) {
  if ($department->getName() == 'sales') {
    foreach ($department->getMembers() as $member) {
      if ($member->getName == 'Kevin') {
        $member->setPhoneNumber(012343929394);
      }
    }
  }
}

It's a personal preference but I think that making this sort of thing hard to do is a good idea. Instead you would fetch the member based on name from the database in a dedicated service for editing user info. This means your logic is more encapsulated. A new developer working on the code will be more likely to look for the UserEditService if they don't have access to everything from everywhere.