How can I build such a query using Laravel query builder? - mysql

Question

There is a table of MySQL "Categories".
The Category model has a "hasMany" relationship that looks like this.
public function children(){
return $this->hasMany(self::class, 'parent_id');
}
On the category editing page, in order to assign the parent category to the current one (for this I need to fill in its parent_id field), I have to generate a Select tag that will contain all rows from the category table, except for the current one and all its child categories.
category_id | parent_id
------------------------
1 | NULL
2 | 1
3 | 2
4 | 3
5 | NULL
6 | 5
For example,
for category_id 1, should be selected lines with category_id [5, 6]
for category_id 2, should be selected lines with category_id [1, 5, 6]
for category_id 3, should be selected lines with category_id [1, 2, 5, 6]
for category_id 4, should be selected lines with category_id [1, 2, 3, 5, 6]
for category_id 5, should be selected lines with category_id [1, 2, 3, 4]
for category_id 6, should be selected lines with category_id [1, 2, 3, 4, 5]
If a category has parent_id is NULL, it means that it has no parent category.

Answer 1

Short answer: you need so select all records EXCEPT recursive records from the current category.
For the recursive part, you could be used this: https://github.com/staudenmeir/laravel-adjacency-list
Look at the descendantsAndSelf() relation it includes by default.
Next you need to build the EXCEPT query. The query builder does not include an except function, so you need to build one yourself.
For example I have this in one of my controllers:
I have an eloquent model called Thread, which is a model for holding message threads. People have different view rights on these threads so I need several queries to filter out the threads they can and cannot see.
I prebuild these separate queries using the element query builder, and some I UNION. Than there is one query which explicitly excludes threads. And than I combine the unioned and $excludeExplicit queries:
$query = Thread::query()
->fromRaw(
'(SELECT * FROM ((' . $unioned->toSql() . ') EXCEPT ' . $excludeExplicit->toSql() . ') AS threads) AS threads',
array_merge($unioned->getBindings(), $excludeExplicit->getBindings())
);
(See https://stackoverflow.com/a/64231210/9258054 )
The trick of this raw query is in the merging of the parameter bindings. Also, the result of this query are Eloquent models. Note that when using queries like UNION or EXCEPT you need to make sure that the SELECT statement of both return the same columns.

Answer 2

After digging around a bit in the code, I found a more elegant solution.
I am making a selection from the database using the "categories" relationship created in the "Article" model
public function categories(){
return $this->morphToMany(Category::class, 'categoryable');
}
In doing so, I only select those items that do not have a parent category.
But thanks to the "categories" relationship, all the children go to the "Article" collection, and the tree of items is built correctly, and I don't have to do complex queries with "Join".
public function topMenu(){
View::composer('layouts.header', function ($view){
$view->with('categories', Category::whereNull('parent_id')->where('published', 1)->get());
});
}