Three-way many-to-many relationship in Laravel

问题:

I have three models that need to be related in a pivot table: User, Student, Plan. So each user can subscribe a student to a plan.

What I've found so far is to create a pivot for two of the models, say User and Plan, and attach the student_id as an extra field:

$user->plans()->attach([1 => ['student_id' => $student_id]);

One problem with this is that if I try to retrieve the plans for a particular user, I don't get the student model, just the id, so:

return $this->BelongsToMany('App\Plan', 'plans_students_users', 'user_id', 'plan_id')
->withPivot('student_id');

So, I'd have to do a second query to get the student model.

Is there any other way to go about it, given that I'll want to make queries in all directions, e.g:

$user->plans() (attaching the students)
$student->plans() (attaching the user)
$plan->users() (attaching the students)
$plan->students() (attaching the users)

回答1:

I often use another model to abstract a three-way many to many relations.

We have our relation I will call the relation relation.

The db structure:

table relations: id, user_id, student_id, plan_id

The app has the following four models:

• User
• Student
• Plan
• Relation

Here is how we connect the four models using Relationships:

User, Plan, Student:

function relations() {
   return $this->hasMany(Relation::class);
}

Relation:

function student() {
   return $this->belongsToMany(Student::class);
}

function user() {
   return $this->belongsToMany(User::class);
}

function plan() {
   return $this->belongsToMany(Plan::class);
}

You can retrieve the entities like this:

//get the plan of a student related to the user
$user->relations()->where('student_id', $student)->first()->plan();

//get all entities from the relation
foreach ($user->relations as $relation) {
    $plan = $relation->plan;
    $student = $relation->student;
}

Its the only solution I have found in all the time I have been developing on Laravel.