How laravel relationship internally works? - php

Question

I have really surprised about laravel relationship becasue it gives result without duplicate entries while if compare to left join.
for example recently i tried self join has many to same table
public function parentMenu(){
return $this->hasMany(Page::class,'parent_page_id','id');
}
and it return following result
Array
(
[0] => Array
(
[id] => 1
[page_name] => Branch Details
[page_url] => #
[parent_page_id] => 0
[page_type] => 1
[created_at] => 2018-06-23 23:45:34
[updated_at] => 2018-06-23 23:45:34
[deleted_at] =>
[slug] => branch_detail
[parent_menu] => Array
(
[0] => Array
(
[id] => 2
[page_name] => Add Branch Detail
[page_url] => add-branch
[parent_page_id] => 1
[page_type] => 1
[created_at] => 2018-06-23 23:45:54
[updated_at] => 2018-06-23 23:45:54
[deleted_at] =>
[slug] => add_branch_detail
)
[1] => Array
(
[id] => 11
[page_name] => View Branch Detail
[page_url] => list-branch
[parent_page_id] => 1
[page_type] => 1
[created_at] => 2018-06-23 23:46:08
[updated_at] => 2018-06-23 23:46:08
[deleted_at] =>
[slug] =>
)
)
)
also i have tried to figure out the query
Array
(
[0] => Array
(
[query] => select * from `pages` where `pages`.`deleted_at` is null
[bindings] => Array
(
)
[time] => 0.82
)
[1] => Array
(
[query] => select * from `pages` where `pages`.`parent_page_id` in (?, ?, ?, ?, ?, ?, ?, ?, ?, ?) and `pages`.`deleted_at` is null
[bindings] => Array
(
[0] => 1
[1] => 2
[2] => 4
[3] => 5
[4] => 7
[5] => 8
[6] => 9
[7] => 10
[8] => 11
[9] => 12
)
[time] => 0.86
)
)
Now my question is suppose if i try to get same result what will be the mysql query .Since i want to know how internally relation query works ? will it do forloop internally or else in query itself it will produce result .
Also will it execute separate query in loop to retrieve child menu list ?

Answer 1

Laravel's active record implementation doesn't use joins for its relationships (well is does for some of them but that's for relationships like belongsToMany where it needs to use a pivot/intermediate table).
What will actually happen is Eloquent will get the related keys from the table and perform another query to get the required relationship e.g.
Post has many Comment and there are 3 posts in the db with ids 1, 2 and 3.
Post::with('comments')->get();
Will result in:
Query 1
SELECT * from `posts`
Query 2
SELECT * FROM `comments` WHERE `comment`.`post_id` in ('1', '2', '3')
It will hen go through the results from the 2nd query and match them to the models loaded from the first.
If you want to see what queries are being run from your Laravel app then I would suggest installing either:
Laravel Debugbar
Clockwork
Amongst other things, either of these tools will show you the queries that are being run.