Problem with Project and Match array element with $expr

[Akshat_Gupta3 (Akshat Gupta)]

Hi Cuong,

In order to access a specific array element you need to use $arrayElemAt operator.

@Cuong_Chau

db.collection.aggregate([
  {
   $project: {all: "$ROOT", 
             a0: {$slice: ["$arry", 0, 1]}, 
             a1: {$slice: ["$arry", 1,1]}
            }
  }, 
  {
   $match: {$expr: {$lt: ["$a0", "$a1"]}}
  }
])

In the above code, $slice operator returns a subarray and the $lt operator then compares two subarrays so it eventually works as expected.

@Cuong_Chau

db.collection.aggregate([
  {
   $project: {all: "$ROOT", 
             a0: {$slice: ["$arry", 0, 1]}, 
             a1: {$slice: ["$arry", 1,1]}
            }
  }, 
  {
   $match: {$expr: {$lt: ["$a0", "$all.d"]}}
  }
])

But in this code $lt operator now has to compare a array with a normal value so it fails.

The proper way of doing this would be -

Mongo playground