Laravel Nested Eager Loading with Constraints - php

Question

I have a Question Eloquent Model, a Course Eloquent Model, a University Eloquent Model. A One to Many relationship exists between the University and the Course. A Many to Many relationship exists between the Question and the Course. The Three models are shown below:
Question Model
namespace App;
use Illuminate\Database\Eloquent\Model;
class Question extends Model
{
/**
* The database table that the Model uses
* #var string
*/
protected $table = "questions";
/**
* The fields that are mass assignable
* #var array
*/
protected $fillable = ['title','body','images','quality_score','deactivated','creator_id','reviewer_id'];
/**
* Images is stored as serialized json.
* So we cast it to a PHP array.
* See: http://laravel.com/docs/5.1/eloquent-mutators#attribute-casting
*/
protected $casts = [
'images' => 'array',
];
public function courses(){
return $this->belongsToMany('App\Course');
}
}
Course Model
namespace App;
use Illuminate\Database\Eloquent\Model;
class Course extends Model
{
/**
* The database table used by the model
* #var string
*/
protected $table = "courses";
/**
* The fields that can be mass assigned
* #var array
*/
protected $fillable = ['name', 'instructor', 'acronym', 'university_id', 'creator_id', 'reviewer_id'];
/**
* There exists a many to one relationship between the Course and User
* This user is the creator of the course
*
* #method void
*
*/
public function creator(){
return $this->hasOne('App\User','creator_id');
}
/**
* There exists a many to one relationship between the Course and User
* This user is the reviewer of the course
* The reviewer of the Course will always be an admin
* If an Admin is the creator, then the reviewer is also the same admin
*
* #method void
*/
public function reviewer(){
return $this->hasOne('App\User','reviewer_id');
}
/**
* There exists a one to many relationship between the University and the Course
* This university is where the course is held
* Courses may float i.e. not be associated to any university
*
* #method void
*/
public function university(){
return $this->belongsTo('App\University');
}
/**
* This method is an accessor. It automatically changes the acronym to be all capitals
* regardless of how it is stored in the database.
* See: http://laravel.com/docs/5.1/eloquent-mutators#accessors-and-mutators
* #param $value (String from Database)
* #return string (Capitalized String)
*/
public function getAcronymAttribute($value){
return strtoupper($value);
}
}
University Model
namespace App;
use Illuminate\Database\Eloquent\Model;
class University extends Model
{
/**
* The database table used by the model
* #var string
*/
protected $table = "universities";
/**
* The fields that can be mass assigned
* name = Name of the University (Example: University of Illinois at Urbana Champaign)
* acronym = Acronym of the University (Example: UIUC)
* creator_id = Id of User that created the University
* reviewer_id = Id of User that reviewed and approved the University
*
* Universities will not be displayed to users without admin role unless they have been reviewed.
*
* #var array
*/
protected $fillable = ['name','acronym','creator_id','reviewer_id'];
/**
* This method is an accessor. It automatically changes the acronym to be all capitals
* regardless of how it is stored in the database.
* See: http://laravel.com/docs/5.1/eloquent-mutators#accessors-and-mutators
* #param $value (String from Database)
* #return string (Capitalized String)
*/
public function getAcronymAttribute($value){
return strtoupper($value);
}
}
On my home page I am showing a list of questions and allow filters for course and university. The controller method is shown here:
public function getHome(Request $request){
/**
* Eager Load with Course and University
*/
$questions = Question::with('courses.university')->get();
/*
* Filter Questions to remove unwanted entries based on course id
*/
if($request->has('course_id') && $request->input('course_id') != -1){
$questions = $questions->filter(function($question) use ($request){
foreach($question->courses as $course){
if ($course->id == $request->input('course_id')){
return true;
}
}
});
}
/*
* Filter Questions to remove unwanted entries based on university id
*/
if($request->has('university_id') && $request->input('university_id') != -1){
$questions = $questions->filter(function($question) use ($request){
foreach($question->courses as $course){
if ($course->university->id == $request->input('university_id')){
return true;
}
}
});
}
/*
* Return the Welcome View with Pagination on the Questions Displayed
* List of Courses and List of Universities
*/
return view('welcome',[
'questions' => $questions,
'courses' => Course::all(),
'universities' => University::all(),
'selected_university_id' => $request->input('university_id',-1),
'selected_course_id' => $request->input('course_id',-1)
]);
}
What I am doing above is returning all the questions from the database and them combing them through to remove all the ones that don't match the filters. This is obviously quite inefficient. I want to use Nested Eager Loading with constraints except I am having a lot of trouble defining what the constraint would look like. Further, I want to use server side paginate to make the client experience better on lower speed internet connections.
Here is one of my attempts:
$questions = Question::with(['courses.university' => function($query) use ($request){
if($request->has('university_id') && $request->input('university_id') != -1) {
$query->where('id', $request->input('university_id'));
}
if($request->has('course_id') && $request->input('course_id') != -1){
$query->where('courses.id',$request->input('course_id'));
}
}])->paginate(10);
This works fine when I don't have any filters.
When I do have a university_id defined, I get the error: Trying to get property of non-object (View: /var/www/testing.com/resources/views/welcome.blade.php)
When I do have a course_id defined, I get the error: SQLSTATE[42S22]: Column not found: 1054 Unknown column 'courses.id' in 'where clause' (SQL: select * from universities where universities.id in (1, 2) and courses.id = 1)
I expected the error when I have course_id defined (because I took a blind stab at the first argument of the $query->where method.
I am looking for help in defining the Nested Eager Loading Constraints.

Answer 1

I found the solution in a medium article. The solution works for later versions of laravel as it uses whereHas.
// If you want to put the constraint on the second relation
$questions = Question::with(['courses' => function($query) use($request){
return $query->whereHas('university', function($inner_query) use($request){
return $inner_query->where('id', $request->input('university_id'));
});
}, 'courses.university'])->paginate(10);
For your case, a simple whereHas ought to do the trick.
$questions = Question::whereHas('courses', function($query) use ($request){
return $query->where('university_id', $request->input('university_id'));
})->with(['courses.university'])->paginate(10);
I would also recommend using when clauses to reduce the amount of code.
$questions = Question::when(($request->has('course_id') && $request->input('course_id') != -1), function ($query) use($request){
return $query->where('course_id', $request->input('course_id'));
})->when($request->has('university_id') && $request->input('university_id') != -1, function ($outer_query) use($request){
return $outer_query->whereHas('courses', function($query) use($request){
return $query->where('university_id', $request->input('university_id'));
})->with(['courses.university']);
})->with(['courses.university'])->paginate(10);