Laravel Eloquent: Bind Table to another table via pivot with multiple relationships

Let me start by apologizing for the vague title, I couldn't find a proper way to formulate it.

As for the question, in this instance I want to bind users to a project with a certain role. I'm trying to do this via the following pivot table:

|user_has_projects|
|-----------------|
|id               |
|user_id          |
|projectrole_id   |
|project_id       |
|created_at       |
|updated_at       |
|deleted_at       |
|-----------------|

The users, projects and projectroles tables are what you would expect from said tables.

The current Eloquent relationships per model are as follows:

Project.php:

public function users()
 {
   return $this->belongsToMany(
      "App\User",
      "user_has_projects"
   )->withPivot(
      "user_id"
   )->withTimestamps();
}

public function userProjects()
{
    return $this->hasMany(
        "App\UserProjects",
        "project_id",
        "id"
    );
}

Users.php:

public function projects()
 {
   return $this->belongsToMany(
      'App\Project',
      'user_has_projects'
   )->withPivot(
      'project_id'
   );
}

public function projectRole($id)
{
    return $this->belongsToMany(
        "App\ProjectRole",
        "user_has_projects"
    )->withPivot(
        "projectrole_id"
    )->wherePivot(
        "project_id",
        '=',
        $id
    );
}

ProjectRole.php:

public function user()
{
    return $this->belongsToMany(
        "App\User",
        "user_has_projects"
    )->withPivot(
        "user_id"
    )->withTimestamps();
}

public function projects()
{
    return $this->belongsToMany(
        "App\Project",
        "user_has_projects"
    )->withPivot(
        "project_id"
    )->withTimestamps();
}

UserProjects.php:

public function projectRoles()
{
    return $this->belongsTo(
        'App\ProjectRoles'
    );
}

public function users()
{
    return $this->belongsTo(
        'App\User'
    );
}

public function projects()
{
    return $this->belongsTo(
        'App\Project'
    );
}

Now if I want to retrieve all the users bound to a project and then their roles in the project I would first call $aBoundUsers = $oProject->users(); and then loop through those users to get their roles with

foreach($aBoundUsers as $oUser) {
$role = $oUser->projectRole($oProject->id); 
}

However, when I call $oUser->projectRole(...) I get the following error: "Call to a member function projectRole() on boolean".. After some thorough searching I found out that the only thing returned is {"withTimestamps":true}

What am I doing wrong here? I found some solutions on having pivot tables with multiple models, but none work somehow.