DDD and MVC: Difference between 'Model' and 'Entity'

By | March 30, 2018

I’m seriously confused about the concept of the ‘Model’ in MVC. Most frameworks that exist today put the Model between the Controller and the database, and the Model almost acts like a database abstraction layer. The concept of ‘Fat Model Skinny Controller’ is lost as the Controller starts doing more and more logic.

In DDD, there is also the concept of a Domain Entity, which has a unique identity to it. As I understand it, a user is a good example of an Entity (unique userid, for instance). The Entity has a life-cycle — it’s values can change throughout the course of the action — and then it’s saved or discarded.

The Entity I describe above is what I thought Model was supposed to be in MVC? How off-base am I?

To clutter things more, you throw in other patterns, such as the Repository pattern (maybe putting a Service in there). It’s pretty clear how the Repository would interact with an Entity — how does it with a Model?

Controllers can have multiple Models, which makes it seem like a Model is less a “database table” than it is a unique Entity.

UPDATE: In this post the Model is described as something with knowledge, and it can be singular or a collection of objects. So it’s sound more like an Entity and a Model are more or less the same. The Model is an all encompassing term, where an Entity is more specific. A Value Object would be a Model as well. At least in terms of MVC. Maybe???

So, in very rough terms, which is better?

No “Model” really …

class MyController {
    public function index() {
        $repo = new PostRepository();
        $posts = $repo->findAllByDateRange('within 30 days');
        foreach($posts as $post) {
            echo $post->Author;

Or this, which has a Model as the DAO?

class MyController {
    public function index() {
        $model = new PostModel();
        // maybe this returns a PostRepository?
        $posts = $model->findAllByDateRange('within 30 days');
        while($posts->getNext()) {
            echo $posts->Post->Author;

Both those examples didn’t even do what I was describing above. I’m clearly lost. Any input?

Category: php

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