MongoDB projection - take first element from Array

Question

My field "Field1.Field2" is an array and I want to take only first element of this array, i.e. "Field1.Field2.0" and from this I want to take any other given field for example: "Field1.Field2.0.Field3". Unfortunately when I use this projection in PyMongo then it doesn't work (this "0" make problems). The projection doesn't throw an error but the field "0" doesnt contain any values after projection while it contain the values before projection. What may be the reason?

you can not access it by index, you have to use operators, $first or $arrayElemAt — turivishal
– turivishal, Commented Jun 24, 2022 at 15:19

R2D2 · Accepted Answer · 2022-06-24 20:41:44Z

2

Option 1:aggregation/$arrayAt or $first

db.collection.aggregate([
{
  $addFields: {
     x: {
      "$arrayElemAt": [
        "$x",
        0
       ]
     }
   }
 }
])

Playground 1

Option 2:aggregation/$slice

db.collection.aggregate([
{
 $addFields: {
  x: {
    "$slice": [
      "$x",
      1
    ]
   }
  }
 }
])

Playground 2

Option 3:find/project/$slice

db.collection.find({},
{
  x: {
    $slice: 1
  }
})

Playground 3

answered Jun 24, 2022 at 20:41

R2D2

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