How to determine if a table relationship is bidirectional or unidirectional in Doctrine 2?

Question

I am in the process of upgrading from Doctrine 1.1.4 to Doctrine 2.0.6 in my Zend application.

Currently, I am working on mapping the associations between entities. In Doctrine 2's Documentation it says 'relationships maybe bidirectional or unidirectional. I am confused as to what these terms mean within the given context.

How do I determine if a relationship is unidirectional or bidirectional?

Appreciate the help.

Answer 1

A relationship is bidirectional if both entities contain a reference to the other.

If you omit one of those references, it's unidirectional.

Consider a typical "posts" and "tags" schema. Typically, you'd implement a bidirectional association:

<?php  class Post {     // ...      /**       * @ManyToMany(targetEntity="Tag",inversedBy="posts")      */      protected $tags;      // ... }  class Tag {     // ...      /**      * @ManyToMany(targetEntity="Post",mappedBy="tags")      */      protected $posts      // ... } 

Now, imagine you decided you never (or rarely) needed to answer questions like "Which posts have Tag 'foo'?". You could omit the $posts association in your Tag entity, converting it to a unidirectional association, and take some load off of the ORM.

You could still answer that kind of question, but you'd have to write code to do it.

In fact, it's probably a good way to go in Posts/Tags scenario, as you wouldn't typically be adding/removing Posts from Tags. Typically, you'd add/remove tags from posts only. You'd only ever go from Tags to Posts when looking for "all posts with tag 'x'", which could be trivially implemented in a service class of some sort.