How to GROUP and SUM a pivot table column in Eloquent relationship?

The best way that you can use is:

$project->parts->sum('pivot.count');

I faced the same problem, but this solved my issue.

From the code source:

We need to alias all of the pivot columns with the "pivot_" prefix so we can easily extract them out of the models and put them into the pivot relationships when they are retrieved and hydrated into the models.

So you can do the same with select method

public function parts()
{
    return $this->belongsToMany('Part', 'project_part', 'project_id', 'part_id')
        ->selectRaw('parts.*, sum(project_part.count) as pivot_count')
        ->withTimestamps()
        ->groupBy('project_part.pivot_part_id')
}

Try to sum in Collection,

$project->parts->sum('pivot.count');

This is best way I found. It's clean (easy to read) and able to re-use all of your scope, ordering and relation attribute caching in parts many-to-many defination.

@hebron No N+1 problem for this solution if you use with('parts') to eager load. Because $project->parts (without funtion call) is a cached attribute, return a instance of Collection with all your data. And sum('pivot.count') is a method of Collection which contains pure funcional helpers (not relative to database, like underscore in js world).

Full example:

Definition of relation parts:

class Project extends Model
{
    public function parts()
    {
        return $this->belongsToMany('Part', 'project_part', 'project_id', 'part_id')
            ->withPivot('count')
            ->withTimestamps();
    }
}

When you use it (note that eager load is important to avoid N+1 problem),

App\Project::with('parts')->get()->each(function ($project) {
    dump($project->parts->sum('pivot.count'));
});

Or you can define the sum function in Project.php,

class Project extends Model
{
    ...

    /**
     * Get parts count.
     *
     * @return integer
     */
    public function partsCount()
    {
        return $this->parts->sum('pivot.count');
    }
}

If you want to avoid with('parts') on caller side (eager load parts by default), you can add a $with attribute

class Project extends Model
{
    /**
     * The relations to eager load on every query.
     *
     * @var array
     */
    protected $with = ['parts'];

    ...
}