Mongo projection to set value based on if a key exists or not

Question

I have a mongo document that looks something like:

{
  bookings: [
    {complete: true, name: "John", checklist: {a: 1, b: 2}},
    {complete: false, name: "Kate"},
    {complete: true, name: "Mary", checklist: {a: 1, b: 2}}
  ]
}

I have a aggregation with a projection that looks like:

{
  $project: {
    'bookings.complete': 1,
    'bookings.name': 1
  }
}

Which returns the array of bookings, with only the complete and name keys.

I now want to add a key called hasChecklist that is true if the checklist exists, and false otherwise.

But I'm stuck because this projection will always return true for some reason :(

{
  $project: {
    'bookings.complete': 1,
    'bookings.name': 1
    'bookings.hasChecklist': { $ne: ['$bookings.checklist', null] }
  }
}

Basically I get

{
  bookings: [
    {complete: true, name: "John", hasChecklist: true},
    {complete: false, name: "Kate", hasChecklist: true},
    {complete: true, name: "Mary", hasChecklist: true}
  ]
}

When I want

{
  bookings: [
    {complete: true, name: "John", hasChecklist: true},
    {complete: false, name: "Kate", hasChecklist: false},
    {complete: true, name: "Mary", hasChecklist: true}
  ]
}

Anyone know what the correct expression in the projection should be?

Possible duplicate of [How to project whether field exists](https://stackoverflow.com/questions/38720388/how-to-project-whether-field-exists) — charliehorse55, Oct 23 '17 at 22:49
Not exactly, because having arrays in this case makes the difference. — cbartosiak, Oct 24 '17 at 10:18

score 2 · Accepted Answer · answered Oct 23 '17 at 22:52

Try the $project below:

{
    "bookings": {
        "$map": {
            "input": "$bookings",
            "as": "booking",
            "in": {
                "complete": "$$booking.complete",
                "name": "$$booking.name",
                "hasChecklist": { "$gt": [ "$$booking.checklist", null ] }
            }
        }
    }
}

It should return the following result:

{
    "bookings" : [
        {
            "complete" : true, 
            "name" : "John", 
            "hasChecklist" : true
        }, 
        {
            "complete" : false, 
            "name" : "Kate", 
            "hasChecklist" : false
        }, 
        {
            "complete" : true, 
            "name" : "Mary", 
            "hasChecklist" : true
        }
    ]
}

Mongo projection to set value based on if a key exists or not

1 Answers1