在laravel 5.5上急切加载冲突

Question

I do not really understand what is the method of laravel for eager loading and solve the problem of N+1 query and what is going under hood. So I decided the test it myself to see result!

So I designed a simple many to many relationship that every tgchannel(telegram channel) has many tags and every tag has many tgchannel.

namespace App;

use Illuminate\Database\Eloquent\Model;

class tgchannel extends Model
{    
    public function tags() {
        return $this->belongsToMany('App\tag');
    }
}

/////////////////////////////

class tag extends Model
{
    //
    public function tgchannels() {
        return $this->belongsToMany('App\tgchannel');
    }

I used eager loading to load tags relationship in a controller method, according to laravel documentation and output resulting sql with this technique:

public function test()
{
    $all_channels = tgchannel::with(['tags'])->toSql();
    echo $all_channels.'<br>';

    $all_channels = tgchannel::with(['tags'])->get();

    foreach ($all_channels as $channel) {
        $sql = $channel->tags()->toSql();
        echo $sql.'<br>';
    }

}

in the test database, I had 12 tgchannels that each one had a tag. and finally this is the result:

select * from `tgchannels`
select * from `tags` inner join `tag_tgchannel` on `tags`.`id` = `tag_tgchannel`.`tag_id` where `tag_tgchannel`.`tgchannel_id` = ?
select * from `tags` inner join `tag_tgchannel` on `tags`.`id` = `tag_tgchannel`.`tag_id` where `tag_tgchannel`.`tgchannel_id` = ?
select * from `tags` inner join `tag_tgchannel` on `tags`.`id` = `tag_tgchannel`.`tag_id` where `tag_tgchannel`.`tgchannel_id` = ?
select * from `tags` inner join `tag_tgchannel` on `tags`.`id` = `tag_tgchannel`.`tag_id` where `tag_tgchannel`.`tgchannel_id` = ?
select * from `tags` inner join `tag_tgchannel` on `tags`.`id` = `tag_tgchannel`.`tag_id` where `tag_tgchannel`.`tgchannel_id` = ?
select * from `tags` inner join `tag_tgchannel` on `tags`.`id` = `tag_tgchannel`.`tag_id` where `tag_tgchannel`.`tgchannel_id` = ?
select * from `tags` inner join `tag_tgchannel` on `tags`.`id` = `tag_tgchannel`.`tag_id` where `tag_tgchannel`.`tgchannel_id` = ?
select * from `tags` inner join `tag_tgchannel` on `tags`.`id` = `tag_tgchannel`.`tag_id` where `tag_tgchannel`.`tgchannel_id` = ?
select * from `tags` inner join `tag_tgchannel` on `tags`.`id` = `tag_tgchannel`.`tag_id` where `tag_tgchannel`.`tgchannel_id` = ?
select * from `tags` inner join `tag_tgchannel` on `tags`.`id` = `tag_tgchannel`.`tag_id` where `tag_tgchannel`.`tgchannel_id` = ?
select * from `tags` inner join `tag_tgchannel` on `tags`.`id` = `tag_tgchannel`.`tag_id` where `tag_tgchannel`.`tgchannel_id` = ?
select * from `tags` inner join `tag_tgchannel` on `tags`.`id` = `tag_tgchannel`.`tag_id` where `tag_tgchannel`.`tgchannel_id` = ?

and you can see that the queries that executed is not 2 and is N+1 query executed that similar to common lazy loading!

what is the problem??

Answer 1

You're creating additional queries instead of using loaded data, so use $channel->tags collection instead of $channel->tags():

foreach ($all_channels as $channel) {
    echo 'Channel: ' . $channel->id . '<br>';
    foreach ($channel->tags as $tag) {
        echo 'Tag: ' . $tag->id . '<br>';
    }
}