Aggregation with multiple criteria's and sum match based on the count key

问题

I was trying get to get the count of element that is present in the array of objects in another collection. Example:

Collection A:

{
   _id:1,
   name:"Sample1"
}
{
  _id:2,
  name:"Sample 2"
}
{
  _id:3,
  "name":"Sample 3"
}
{
  _id:4,
  "name":"Sample 4"
}

Collection B:

{
   _id:11,
   items:[ {_id:1, name:"sample1",size:1},{_id:3, name:"sample 3",size:5}]
}
{
   _id:12,
   items:[ {_id:1, name:"sample1",size:2},{_id:3, name:"sample 3",size:6}]
}
{
   _id:13,
   items:[ {_id:2, name:"sample2", size:5},{_id:1, name:"sample 1",size:8}],
is_delete:true
}
{
   _id:14,
   items:[ {_id:1, name:"sample1",size:3},{_id:3, name:"sample 3",size:1}]
}

Note: The _id in items is string.

Expected Output:

{
   _id:1,
   name:"Sample1",
  count:6
}
{
  _id:2,
  name:"Sample 2",
  count:0
}
{
  _id:3,
  "name":"Sample 3",
  "count":12
}
{
  _id:4,
  "name":"Sample 4",
  "count":0
}

Please help me to write a mongo query to get the expected out put.

回答1:

Since there are two collections, we need to use

• $lookup to join tow collections. Here I used uncorelated subqueries
• The aggregation is performed in colA collections, but inside the lookup's pipeline we perform aggregation on colB. $unwind helps to de-structure the items. $match helps to eliminate unwanted data (match stage requires $expr).
• Once our join is succeeded, we need to just count the array using $size
• $reduce helps to sum the array value of size

The mongo script is given below.

db.colA.aggregate([
  {
    $lookup: {
      from: "colB",
      let: {
        bid: "$_id"
      },
      pipeline: [
        {
          $match: {
            $or: [
              {
                is_delete: false
              },
              {
                is_delete: {
                  "$exists": false
                }
              }
            ]
          }
        },
        {
          $unwind: "$items"
        },
        {
          $match: {
            $expr: {
              $eq: [
                "$items._id",
                "$$bid"
              ]
            }
          }
        },
        
      ],
      as: "data"
    }
  },
  {
    $project: {
      count: {
        $reduce: {
          input: "$data",
          initialValue: 0,
          in: {
            $add: [
              "$$value",
              "$$this.items.size"
            ]
          }
        }
      }
    }
  }
])

Working Mongo playground

来源：https://stackoverflow.com/questions/64734988/aggregation-with-multiple-criterias-and-sum-match-based-on-the-count-key