laravel - with（）加载关系但仍调用数据库

I want use eager-loading to load sources of my relation so I do:
MODEL

public function files() 
{
    return $this->belongsToMany('App\Files', 'product_files', 'product_id', 'file');
}

Method

Products::with(['files', 'promotion', ... etc.])->where('code', $product)->first();

And I have loaded relations:

  #relations: array:7 [▼
    "files" => Collection {#762 ▶}
    "promotion" => Collection {#755 ▶}
    ....

but in view still connecting to database: 500 statements were executed, 474 of which were duplicated

Duplicated query

select id from file_entries inner join product_files on file_entries.id = product_files.file where product_files.product_id = '77627'

Where is the problem?

View

                @foreach ($filesCategory as $element => $file)
                    @if (in_array($file->id, $product->files()->pluck('id')->toArray()))
                        <option selected="selected" value="{{ $file->id }}">@if( $file->name ) {{ $file->name }} @else {{ $file->oryginal_name }} @endif</option>
                    @else
                        <option value="{{ $file->id }}">@if( $file->name ) {{ $file->name }} @else {{ $file->oryginal_name }} @endif</option>
                    @endif
                @endforeach

$product->files()->pluck('id')->toArray()

This is your problem.

$product->files is the eager-loaded, already fetched collection of files. That's what you're going to want to use here to benefit from eager loading. As a collection, it has the same sort of pluck() etc. functionality available (https://laravel.com/docs/5.4/collections#available-methods).

$product->files() is a brand-new Eloquent query builder, so using it is going to cause a new query that's not using the eager-loaded data.