How can create a Nova filter to filter a resource by its relation?

Question

I have a question resource that has many categories (many to many relation). I would like to create a filter the question resource for different categories.

<?php

namespace App\Nova\Filters;

use Illuminate\Http\Request;
use Laravel\Nova\Filters\Filter;

class CategoryType extends Filter
{
    /**
     * Apply the filter to the given query.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  \Illuminate\Database\Eloquent\Builder  $query
     * @param  mixed  $value
     * @return \Illuminate\Database\Eloquent\Builder
     */
    public function apply(Request $request, $query, $value)
    {
        return $query->categories()->where('id', $value);
    }

    /**
     * Get the filter's available options.
     *
     * @param  \Illuminate\Http\Request  $request
     * @return array
     */
    public function options(Request $request)
    {
            $categories = \App\Category::all();
            return $categories->pluck('id', 'name')->all();
    }
}

Answer 1

$query is type \Illuminate\Database\Eloquent\Builder, cannot use Eloquent model method categories().

Try using join like below. category_question is intermediate table. I have not tested, hope this will help you.

return $query
        ->join('category_question as cq', 'questions.id', 'cq.question_id')
        ->where('cq.category_id', $value)
        ->get();

Answer 2

Like querying any relation, you want to use a closure.

public function apply(Request $request, $query, $value)
{
    return $query->whereHas('categories', function($query) use ($value) {
        $query->where('id', $value);
    });
}