Mongodb query variable field name that is defined inside another field

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1

Hi everyone! I’m relatively new to using mongodb with pure queries, so it may be something stupid and obvious, but still I have a trouble dealing with it.

I have some kind of complex query using aggregate pipeline, and in one place, I need to compare two fields of same document. The thing is that one of the fields name is variable. So as far as I’ve read, there is a possibility of using $objectToArray to convert everything to array inside necessary struct and just make $elemMatch. However, one of the defined fields already have a necessary key for this dynamic field.

For example, consider this collection:

[
  {
    "id": 2,
    "field_bb": {
      "id": 2,
    },
    "field_c": "field_bb",
  },
  {
    "id": 2,
    "field_b2": {
      "id": 2,
    },
    "field_c": "field_b2",
  },
  {
    "id": 3,
    "field_bsss": {
      "id": 2,
    },
    "field_c": "field_bsss",
  }
]

And I need to find each document who has equal id and variable_field.id. But actual variable_field is defined inside constant field_c, so there may be some kind of optimizations, not iterating all over the document keys. So for example, in JS doing something like that will look as simple as obj[obj.field_c].id === obj.id.

So, is it actually possible to make such things working smoothly using MongoDB Aggregate? Any help would be appreciated!