Let’s say I have two objects that map in CRUD fashion to two database tables that are in a one-to-many relationship. A good example would be blog posts and comments. A blog post can have many comments, but a comment can only refer to one blog post.

Given this scenario, what’s the best way to get an array of Comment objects for a specific post?

Should the Post class have a method that returns the array of Comment objects or should the Comment class have a class method that returns the array of Comment objects (based on a post id parameter)? Or neither? Or both? Or it depends?

The former seems to me to be most immediately natural:

$comments = $post->getComments();

as compared to the latter:

$comments = Comment::byPostId($post_id);

The former also seems more consistent with a corresponding method I’d imagine would exist in the Comment class to fetch its parent post:

$post = $comment->getPost();

However, the latter design does have one benefit in that the SQL for accessing comment records can all be conveniently located in the Comment class. This is helpful if our child Comment class has more than one type of parent. Rather than implementing getComments() methods in several different objects (and thus files), they can all live together in the Comment class, e.g.

$comments = Comment::byPostId($post_id);
$comments = Comment::byAuthorId($author_id);

It also occurs to me that implementing getComments() in the Post class means that I have to first instantiate the post to get its comments. I might already have the post object I want laying around, but if not, that means another call to the database.

Thoughts? Suggestions?

• I heart foreach
• Surprising change in PHP’s protected visibility between 5.1 and 5.2
• Escalating the war on comment spam
• Load object by id + database call or by array?
• No late static binding in PHP 5 :(

name

blog (optional)

comment