Many To Many relationships in Laravel: belongsToMany() vs. hasManyThrough()

21
votes

What is the difference between using belongsToMany() or hasManyThrough() when defining a Many To Many relationship in Laravel?

Example: User Account Account_User

So, User has a many to many relation to Account via the Account_User table. On top of being just the pivot table that defines the related Users with related Accounts, it also stores a Account_User.role field, which determines which role a given user has in a given account.

What would the implications be of using either User belongsToMany() Account or User hasManyThrough() Account Account_User? Or is this essentially the same?

When decided upon a method, I guess I should use the same method for the reverse relation definition.

2
ormlaravellaravel-4relationshipeloquent
Has many through is not proper for your approach. Negatively, you will have to add a Eloquent model for the pivot table. As I've stated in my answer, they are different things, and for your example has many through is not suitable. Think Account_user is a part of account and user tables, not like a third table. – Arda

2 Answers

27
votes

Let's say you have two models, let's call A and B:

If A may have more than one items of B, and also If B may have more than one items of A (think like blog posts / tags) You have to use belongsToMany();

Now let's say you have 3 models, A, B and C.

A is related to B, and B is related to C. But you need to access all C's which is related to A (over B), then you need to use hasManyThrough() (think like countries -> users -> posts , and you need all posts from specific country)

hasManyThrough() is not totally meant for many to many relationships, it's more like a shortcut.

Check the documentation links, 1, 2 (Laravel 4.2), or 3, 4 (Laravel 5.x).

3
votes

While @Arda's answer is absolutely correct, I found myself needing some time to digest it. So here is my attempt to put the same thing in simpler terms.

hasManyThrough is useful when you have something similar to the following scenario:

  • A company has many offices and each office has many employees working in it. In other words, Company one-to-many Office, and Office one-to-many Employee. And if you want to fetch all employees working for a given company, you need:
// Company Model

public function employees()
{
    return $this->hasManyThrough('App\Employee', 'App\Office');
}

belongsToMany, on the other hand, is useful when you have a many-to-many relationship with a pivot table in-between. For example:

  • A film can have many categories (comedy, drama, etc.) and each category can belong to many films. In other words, Film many-to-many Category. And if you want to fetch all categories for a given film, you need:
// Film Model

public function categories()
{
    return $this->belongsToMany('App\Category', 'pivot_table_name');
}

Given the scenario in question, belongsToMany is the relationship needed to connect many Users to many Accounts. hasManyThrough cannot be applied.