Laravel Eloquent：如何获取特定用户未选修的课程（多对多关联）

Hey there! I feel your pain—spending hours searching for the right Eloquent query can be frustrating, especially with many-to-many relationships. Let’s walk through exactly how to get the courses a specific user hasn’t signed up for.

First, make sure your model relationships are set up correctly (I’m assuming you already have this, but it’s worth double-checking):

Step 1: Confirm Model Associations #

In your User model:

class User extends Model
{
    public function courses()
    {
        return $this->belongsToMany(Course::class);
    }
}

In your Course model:

class Course extends Model
{
    public function users()
    {
        return $this->belongsToMany(User::class);
    }
}

Step 2: The Most Efficient Method — whereDoesntHave #

Eloquent has a built-in method called whereDoesntHave that’s perfect for this scenario. It lets you query models that don’t have a specific related model record. Here’s how to use it:

// Replace with your target user's ID
$targetUserId = 5;

// Get all courses the user hasn't selected
$unselectedCourses = Course::whereDoesntHave('users', function ($query) use ($targetUserId) {
    $query->where('users.id', $targetUserId);
})->get();

Let me break this down:

• whereDoesntHave('users') tells Eloquent to look for courses with no linked users...
• ...but the closure narrows it down to only exclude the specific user (so we don’t get courses with zero students entirely).

Alternative Method — whereNotIn #

If you prefer a more explicit approach, you can first grab the IDs of courses the user has selected, then filter courses that aren’t in that list:

$targetUserId = 5;

// Get IDs of courses the user already takes
$selectedCourseIds = User::find($targetUserId)->courses()->pluck('courses.id');

// Get all courses not in that ID list
$unselectedCourses = Course::whereNotIn('id', $selectedCourseIds)->get();

This works, but note that it runs two separate database queries, whereas whereDoesntHave does it in one—so the first method is more efficient, especially with large datasets.

Pro Tip: Handle Non-Existent Users #

Always add a check to make sure the user exists before running the query, to avoid errors:

$user = User::find($targetUserId);

if (!$user) {
    // Handle the case where the user doesn't exist (e.g., return an empty collection or error)
    $unselectedCourses = collect();
} else {
    $unselectedCourses = Course::whereDoesntHave('users', fn($q) => $q->where('id', $user->id))->get();
}

That’s it! Both methods will give you exactly the courses the user hasn’t enrolled in.

内容的提问来源于stack exchange，提问作者Misbah Ahmad