Sort By length of array mongodb query

问题

I have MongoDB document like below: is there a query to Sort the documents with the length of subscribedGroups? which can be ascending or descending.

{ 
    "_id" : ObjectId("58c29d9ec79d585c0e16b110"), 
    "subscribedGroups" : [
      ObjectId("5b28a9190c8c0d0014b03a0c"), 
      ObjectId("5b2930650b813a0014a3294d"), 
      ObjectId("5b29f1b5243d470014d6d351")
    ]
}

回答1:

You can find the size of the subscribedGroups using $size and then easily $sort with the length of the array

db.collection.aggregate([
  {
    $addFields: {
      subscribedGroupsLength: {
        $size: "$subscribedGroups"
      }
    }
  },
  {
    $sort: {
      subscribedGroupsLength: -1
    }
  }
])

See the sample

来源：https://stackoverflow.com/questions/50942173/sort-by-length-of-array-mongodb-query