MongoDb-聚合：如何使用$ elemMatch对子文档数组进行$ filter数组文档过滤？

Question

I'm trying use $filter on an aggregation. I need to filter an array called 'extras' that intersect at least one of his 'seasons' with the giving 'dates'.

Here an example:

var from = new Date(2016, 0, 1);
var to = new Date(2016, 0, 5)

var projection = {
    name: 1,
    units: 1,
    capacity: 1,
    availabilitySeasons: { // This works
      $filter: {
        input: '$availabilitySeasons',
        as: 'avSeason',
        cond: {
          $and: [{
            $lte: ['$$avSeason.from', to]
          }, {
            $gte: ['$$avSeason.to', from]
          }]
        }
      }
    },
    rate: 1,
    features: 1,
    extras: {
      $filter: { // $elemMatch is no valid
        input: '$extras',
        as: 'extra',
        cond: {
          $elemMatch: ['$$extra.seasons', {
            from: {
              $lte: from
            },
            to: {
              $gte: to
            }
          }]
        }
      }
    }
  };

It's possible to use $filter in this case? Do I have to rush through a $unwind/$group hell?

Answer 1

Yes, you can do it. As for the following example, use a condition with $lte and $gte inside a $and condition.

$project: {
       items: {
           $filter: {
              input: "$items",
              as: "item",
              cond: { 
                 $and: [ 
                    $lt: [ "$$item.price", 100 ],
                    $gt: [ "$$item.price", 50 ]
                 ] }
            }
       }
 }