Grouping by multiple fields with the aggregation builder (question)

[speedytwenty]

I'm trying to determine the syntax for grouping by multiple fields using the aggregation builder.

Like this example from https://docs.mongodb.com/manual/reference/operator/aggregation/group/

db.sales.aggregate(
   [
      {
        $group : {
           _id : { month: { $month: "$date" }, day: { $dayOfMonth: "$date" }, year: { $year: "$date" } },
           totalPrice: { $sum: { $multiply: [ "$price", "$quantity" ] } },
           averageQuantity: { $avg: "$quantity" },
           count: { $sum: 1 }
        }
      }
   ]
)

I can elaborate on the question if needed but hoping I'm just missing something simple.

Eg.

How do I do this:
_id : { month: { $month: "$date" }, day: { $dayOfMonth: "$date" }, year: { $year: "$date" } },

With this:
$aggregationBuilder->group()->field('_id')->????

Thanks in advanced for the help guys!

[alcaeus]

You have to create a nested expr object to be used with the field:

$builder
    ->group()
    ->field('_id')
    ->expression(
        $builder
            ->expr()
            ->field('month')
            ->month('$date')
            ->field('day')
            ->dayOfMonth('$date')
            ->field('year')
            ->year('$date')
    )
    ->field('totalPrice')
    ->sum(
        $builder
            ->expr()
            ->multiply('$price', '$quantity')
    )
    ->field('averageQuantity')
    ->avg('$quantity')
    ->field('count')
    ->sum(1);

[speedytwenty]

Awesome. This was exactly what I was looking for. Thanks Andreas!