How to select specific fields from nested relations in Eloquent - php

Question

I have the following query in Eloquent:
public function firstSubsectionIdsOnly()
{
return $this->model->with(['sections' => function ($q) {
$q->with(['subsections' => function ($q2) {
$q2->first();
}
])->first();
}])->whereHas('sections.subsections')->first();
}
This returns something like this:
{
"id": 1,
"name": "Training exercise",
"entry": "<p>sss</p>",
"created_at": "2018-04-20 09:38:36",
"updated_at": "2018-04-20 10:08:27",
"sections": [
{
"id": 1,
"name": "Section 1 Training",
"parent": null,
"position": 1,
"created_at": "2018-05-04 09:37:23",
"updated_at": "2018-05-04 09:37:23",
"pivot": {
"training_exercise_id": 1,
"section_id": 1
},
"subsections": [
{
"id": 2,
"name": "Subsection 1 training",
"parent": 1,
"created_at": "2018-05-04 09:54:09",
"updated_at": "2018-05-04 09:54:09"
}
]
}
]
}
I would like for it to only select the id fields for each relation so it should return something like this:
{
"id": 1,
"name": "Training exercise",
"entry": "<p>sss</p>",
"created_at": "2018-04-20 09:38:36",
"updated_at": "2018-04-20 10:08:27",
"sections": [
{
"id": 1,
"subsections": [
{
"id": 2,
}
]
}
]
}
I have tried adding $q->select('id'); to the subsections nested closure but that returns an empty subsections array.
Any idea on how I can achieve this? I am using Laravel 5.6

Answer 1

As others have stated before, you need to include the foreign key in the values you select. Let's say we have two models. Parent and Child. This is the bare minimum you have to select.
App\Parent::select('id')->with(['children' => function ($query) {
$query->select('id', 'parent_id');
}])->get();
It nets you something like
Illuminate\Database\Eloquent\Collection {
all: [
App\Parent {
id: 1,
children: Illuminate\Database\Eloquent\Collection {
all: [
App\Child {
id: 109,
parent_id: 1,
},
App\Child {
id: 153,
parent_id: 1,
},
],
},
},
],
}
If you're trying to go for the inverse, you still need the foreign key, like so
App\Child::select('id', 'parent_id')->with('parent' => function ($query) {
$query->select('id');
}])->get();
This nets you
Illuminate\Database\Eloquent\Collection {
all: [
App\Child {
id: 1,
parent_id: 58,
parent: App\Parent {
id: 58,
},
},
],
}
I'm using select() instead of get() for legibility on the queries but you must use select() for the subqueries.
If you use with('relation') => function ($query) { $query->get(['id']); } you'll get the whole object.

Answer 2

You have to include the foreign key columns. For the subsections relationship:
$q2->select('id', 'parent');