MongoDB sort with a custom expression or function

Question

One of my collections is a list of operations (or tasks) with some status. For example, a list of documents can look like this

{
  _id: '57befe57fc956d2e3252890c',
  task: 'Go buy milk',
  status: 'pending'
},
{
  _id: '57befe5efc956d2e3252890d',
  task: 'Cook dinner',
  status: 'complete'
},
{
  _id: '57befe5efc956d2e3252890e',
  task: 'Play games',
  status: 'new'
}

I want to sort this list based on a their status, where new > pending > complete.

How can I do that with MongoDB without having to create an extra field? I am asking as in my case the sorting order may be pre-configured (i.e. users could have their preferences to pending > new > complete for example)

Answer 1

Based on what @chirdam said here is what the implementation looks like. Also the new sorted field is not present in the result as requested.

DataSet :

{
  _id: '57befe57fc956d2e3252890c',
  task: 'Go buy milk',
  status: 'pending'
},
{
  _id: '57befe5efc956d2e3252890d',
  task: 'Cook dinner',
  status: 'complete'
},
{
  _id: '57befe5efc956d2e3252890e',
  task: 'Play games',
  status: 'new'
}

Query :

db.task.aggregate([
    { "$project" : {
        "_id" : 1,
        "task" : 1,
        "status" : 1,
        "order" : {
            "$cond" : {
                if : { "$eq" : ["$status", "new"] }, then : 1,
                else  : { "$cond" : {
                    "if" : { "$eq" : ["$status", "pending"] }, then : 2, 
                    else  : 3
                    }
                }
            }
        }
    } }, 
    {"$sort" : {"order" : 1} },
    { "$project" : { "_id" : 1, "task" : 1, "status" : 1 } }
])

Output:

{ "_id" : "57befe5efc956d2e3252890e", "task" : "Play games", "status" : "new" }
{ "_id" : "57befe57fc956d2e3252890c", "task" : "Go buy milk", "status" : "pending" }
{ "_id" : "57befe5efc956d2e3252890d", "task" : "Cook dinner", "status" : "complete" }