A general question

[farideh_gorji]

when should we use “$” sign in front of a collection’s field names in aggregation pipelines?
It’s very confusing.

[SourabhBagrecha]

HI @farideh_gorji,
You have to use the $ sign every time you want to access the value of a field in the document that was passed to that stage.

Suppose you have the following document that is storing some user data:

{
  "name": {
    "first": "Sourabh",
    "last": "Bagrecha"
  },
  "email": "sourabh@example.com",
  "phone": "1234567890",
  "address": {
    "street": "123 Main St",
    "city": "New York",
    "state": "NY",
    "zip": "10001"
  }
}

So if you want to get the full name of every user, you can achieve that using the following aggregation pipeline syntax:

[
  {
    "$project": {
      "fullName": {
        "$concat": ["$name.first", " ", "$name.last"]
      }
    }
  }
]

You can see how we utilized the $ to access the value of a field from the document.

If you have any doubts, please feel free to reach out to us.

Thanks and Regards.
Sourabh Bagrecha,
MongoDB