Problem with Project and Match array element with $expr

Working with Data
,
1

I have this simple test document

[
{
arry: [0,1],
d: 1
},
{
arry: [1,0],
d:0
}
]

I’d like to compare one array element to another. I tried using the $expr with dot notation and it didn’t work
db.col.find({$expr: {$gt: [‘$arry.0’, ‘$arry.1’]}})

Then I tried this and it worked
db.col.aggregate({$project: {all: "$$ROOT, a0: {$slice: [‘$arry’, 0, 1]}, a1: {$slice: [‘$arry’, 1,1]}}}, {$match: {$expr: {$lt: [‘$a0’, ‘$a1’]}}}])

But this failed and I don’t know why
db.col.aggregate({$project: {all: "$$ROOT, a0: {$slice: [‘$arry’, 0, 1]}, a1: {$slice: [‘$arry’, 1,1]}}}, {$match: {$expr: {$lt: [‘$a0’, ‘$all.d’]}}}])

I also tried using $arrayElemAt to extract the array elements but same result.

Please tell me what I’m doing wrong here. Thank you!

2

Please read Formatting code and log snippets in posts and then update your post.

Once we can easily read and cut-n-paste your code and documents we might be able to help.

3

My apologies for not formatting the code…

I have a simple test collection:

[
  {
    arry: [0,1],
    d: 1
  },
  {
    arry: [1,0],
    d: 0
  }
]

I’d like to compare one array element to another. I tried using the $expr with the dot notation but it didn’t work

db.collection.find({$expr: {$gt: ["$arry.0", "$arry.1"]}})

Then I tried the following. It worked but I’d like to understand why it behaves differently from the previous query

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

In my usecase I also have a need to compare one array element to another field but why doesn’t the following work?

db.collection.aggregate([
  {
   $project: {all: "$$ROOT", 
             a0: {$slice: ["$arry", 0, 1]}, 
             a1: {$slice: ["$arry", 1,1]}
            }
  }, 
  {
   $match: {$expr: {$lt: ["$a0", "$all.d"]}}
  }
])
4

Hi Cuong,

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

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

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 -

2 Likes
5

Just to add an observation,

I think that

could be done more efficiently with

Unless you really need $$ROOT to be renamed all, but in your case you could $match with $d rather than $all.d. This is true especially if you have a $replaceRoot:{newRoot:$all} later in the real pipeline.

6

Thank you Steeve and Akshat!

Do you know why the following does not work?

db.collection.find({$expr: {$lt: ["$arry.0", "$arry.1"]}})
7

Please share sample documents for which you expect it to work but that it does not.

8

Looks like the dot notation does not work in this case. So

1 Like
9

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