MongoDB Remove or Limit fields conditional aggregation

Question

I am having some issue writing a find/aggregate mongo query where my requirement is to get all the documents but having condition like:

Suppose I have 2 documents:

{
  _id: 5ccaa76939d95d395791efd2,
  name: 'John Doe',
  email: '[email protected]',
  private: true
}

{
  _id: 5ccaa76939d95d395791efd2,
  name: 'Jane Doe',
  email: '[email protected]',
  private: false
} 

Now the query I am trying to get my head around is if the field private is true then when I query I must get all documents except email fields not included if private is true, like this:

{
 _id: 5ccaa76939d95d395791efd2,
 name: 'John Doe',
 private: true
}

{
 _id: 5ccaa76939d95d395791efd2,
 name: 'Jane Doe',
 email: '[email protected]',
 private: false
} 

Tried $redact, $cond, $$PRUNE, $$DESCEND in aggregate() as well as came across $$REMOVE (looks like it is newest feature) but unable to get the required output. Please help me out with the Query

Answer 1

You can use $$REMOVE to remove a field from returned documents.

db.collection.aggregate([ 
  { "$addFields": {
    "email": {
      "$cond": [
        { "$eq": ["$private", true] },
        "$$REMOVE",
        "$email"
      ]
    }
  }}
])

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