Should an object be fully complete before injected as a dependency? - php

Question

This is an extension of this question: Understanding how to inject object dependencies. Since it is a bit different, I wanted to separate them, to make it, hopefully, easier to answer. Also, this is not a real system, just a simplified example that I thought we'd all be familiar with. TIA. :
DB
threads: thread_id, thread_name, etc
posts: post_id, thread_id, post_name, post_contents, post_date, post_user_id, etc
Overview
Basically I'm looking at the most maintainable way to load $post_id and have it cascade and load the other things I want to know about and I'm trying to keep the controller skinny. BUT:
I'm ending up with too many dependencies to inject
I'm passing in initialized but empty objects
I want to limit how many parameters I am passing around
I could inject $post(->many) into $thread(one<-), but on that page I'm not looking at a thread, I'm looking at a post
I could combine/inject them into a new object
Detail
If I am injecting an object into another, is it best to have it fully created first? I'm trying to limit how many parameters I have to pass in to a page, but I end up with a circle.
// 1, empty object injected via constructor
$thread = new Thread;
$post = new Post($thread); // $thread is just an empty object
$post->load($post_id); // I could now do something like $post->get('thread_id') to get everything I want in $post
// 2, complete object injected via constructor
$thread = new Thread;
$thread->load($thread_id); // this page would have to have passed in a $thread_id, too
$post = new Post($thread); // thread is a complete object, with the data I need, like thread name
$post->load($post_id);
// 3, inject $post into $thread, but this makes less sense to me, since I'm looking at a post page, not a thread page
$post = new Post();
$post->load($post_id);
$thread = new Thread($post);
$thread->load(); // would load based on the $post->get('post_id') and combine. Now I have all the data I want, but it's non-intuitive to be heirarchially Thread->Post instead of Post-with-thread-info
// Or, I could inject $post into $thread, but if I'm on a post page,
// having an object with a top level of Thread instead of
// Post-which-contains-thread-info, makes less sense to me.
// to go with example 1
class post
{
public function __construct(&$thread)
{
$this->thread=$thread;
}
public function load($id)
{
// ... here I would load all the post data based on $id
// now include the thread data
$this->thread->load($this->get('thread_id'));
return $this;
}
}
// I don't want to do
$thread = new Thread;
$post = new Post;
$post->load($post_id);
$thread->load($post->get('post_id'));
Or, I could create a new object and inject both $post and $thread into it, but then I have object with an increasing number of dependencies.

Answer 1

These are not dependencies, just parts of the same model, so there is no need to inject anything. Think about your model and it's usage. What are the main entities (building blocks) - post or thread? Or are they equally important/used?
You should deal (load/store) only with main entities, preferably using external class usually called Repository. E.g. you could have method ThreadRepository::findById($id), and that's enough, leave the rest to repository internals... Load and set whatever you want inside repository but never expose it to other parts of application. You could also implement some form of lazy loading (internal resource loader, proxy pattern, etc.) if performance is an issue
If you have free time in your life, download and read this book (or buy original):
http://www.infoq.com/minibooks/domain-driven-design-quickly
Your coding problems are already solved and well explained there.