MongoDB - Query on the last element of an array?

Question

I know that MongoDB supports the syntax find{array.0.field:\"value\"}, but I specifically want to do this for the last element in the array, which means I don\'

Answer 1

Version 3.6 use aggregation to achieve the same.

db.getCollection('deviceTrackerHistory').aggregate([
   {
     $match:{clientId:"12"}
   },
   {
     $project:
      {
         deviceId:1,
         recent: { $arrayElemAt: [ "$history", -1 ] }
      }
   }
])

Answer 2

Starting Mongo 4.4, the aggregation operator $last can be used to access the last element of an array:

---

For instance, within a find query:

// { "myArray": ["A", "B", "C"] }
// { "myArray": ["D"] }
db.collection.find({ $expr: { $eq: [{ $last: "$myArray" }, "C"] } })
// { "myArray": ["A", "B", "C"] }

---

Or within an aggregation query:

db.collection.aggregate([
  { $addFields: { last: { $last: "$myArray" } } },
  { $match: { last: "C" } }
])

Answer 3

For the answer use $arrayElemAt,if i want orderNumber:"12345" and the last element's value $gt than "value"? how to make the $expr? thanks！

For embedded arrays - Use $arrayElemAt expression with dot notation to project last element.

 db.col.find({$expr: {$gt: [{"$arrayElemAt": ["$array.field", -1]}, value]}})