Laravel hasMany is Return No Results - php

Question

In Laravel, I am creating a message thread feature. My schema looks like this:
MessageThreads Table
column
id
MessageThreadParticapants Table
column
thread_id
user_id
And I have the corresponding models of MessageThread and MessageThreadParticapant. In the MessageThread model, I have the following relation:
public function users() {
return $this->hasMany(MessageThreadParticapant::class, 'thread_id', 'id');
}
Here is where things get funny. If I do:
MessageThread->users
I get an empty result. But if I do:
MessageThreadParticapant::where('thread_id', $same_thread_id)->get()
I get the correct amount of results back. What am I doing wrong here?
UPDATE
One of the suggestions was "hasMany(Model, 'foreign_key', 'local_key')" to be incorrect. Some more context,its failing my unit tests. I'm testing up a test as such:
public function testUsers() {
$thread1 = MessageThread::factory()->create();
$thread2 = MessageThread::factory()->create();
$this->assertCount(0, $thread1->users);
$this->assertCount(0, $thread2->users);
$user1 = User::factory()->create();
$user2 = User::factory()->create();
$user3 = User::factory()->create();
$user4 = User::factory()->create();
MessageThreadParticapant::factory()->create([
'user_id' => $user1->id,
'thread_id' => $thread1->id
]);
MessageThreadParticapant::factory()->create([
'user_id' => $user2->id,
'thread_id' => $thread1->id
]);
MessageThreadParticapant::factory()->create([
'user_id' => $user2->id,
'thread_id' => $thread2->id
]);
MessageThreadParticapant::factory()->create([
'user_id' => $user3->id,
'thread_id' => $thread2->id
]);
MessageThreadParticapant::factory()->create([
'user_id' => $user4->id,
'thread_id' => $thread2->id
]);
//PASSES!!!!
$this->assertCount(2, MessageThreadParticapant::where('thread_id', $thread1->id)->get());
//FAILS!!!
$this->assertCount(2, $thread1->users);
$this->assertCount(3, $thread2->users);
}
At bottom of my test:
//PASSES!!!!
$this->assertCount(2, MessageThreadParticapant::where('thread_id', $thread1->id)->get());
//FAILS!!!
$this->assertCount(2, $thread1->users);
In other tests, $thread->users works correctly in getting the right amount of users back. Why are these getting different results?

Answer 1

I solved this problem in two steps.
Refresh
Lazy loading apparently only represents that state of the object at the time that it was loaded. Meaning it's not retrieving new data from the DB when the joined property is called. To solve, just do a refresh on the model and then access the joined property.
$model->refresh();
$model->users;
String ID
I'm using UUID in Postegresql. Even those I am using $cast = ['id' => 'string']; in model, this is not enough. I also have to add:
protected $keyType = 'string';