Nested conditions in $cond aggregate

Question

I'm trying to create a computed status field in my Mongo query (statuses: created, payment received, shipped, received, finished).

db.orders.aggregate( [
   { $project: { status: {
      $cond: { if: { $ne: ["$feedback", null] }, 
      then: 'finished', else: {
         $cond: { if: { $ne: ["$received", null] }, 
         then: 'received', else: {
            $cond: { if: { $ne: ["$shipped", null] }, 
            then: 'shipped', else: {
               $cond: { if: { $ne: ["$payment", null] }, 
                  then: 'payment received', else: 'created' }
            } }
         } } 
      } }
   } } },
   { $match: {  } }
] )

Example data:

{
    "_id" : "xxxxxx0",
    "payment" : ISODate("2016-02-03T10:45:00.011Z"),
    "shipped" : ISODate("2016-02-03T11:55:00.011Z"),
    "received" : ISODate("2016-02-03T12:45:00.011Z"),
    "feedback" : ISODate("2016-02-03T14:34:00.011Z")
},
{
    "_id" : "xxxxxx1",
    "payment" : ISODate("2016-02-03T10:45:00.011Z"),
    "shipped" : ISODate("2016-02-03T11:55:00.011Z"),
    "received" : ISODate("2016-02-03T12:45:00.011Z")
},
{
    "_id" : "xxxxxx2",
    "payment" : ISODate("2016-02-03T10:45:00.011Z"),
    "shipped" : ISODate("2016-02-03T11:55:00.011Z")
},
{
    "_id" : "xxxxxx3",
    "payment" : ISODate("2016-02-03T10:45:00.011Z")
},
{
    "_id" : "xxxxxx4"
}

For some reason all my results show up as 'finished', am I using $cond wrong? Does it support nested $cond ?

Answer 1

You can't use $eq null if you want to check if the field exists or not, it will always return true There is a trick to do that with $gt. You can check full explanation here (https://docs.mongodb.com/manual/reference/bson-types/#bson-types-comparison-order)

db.orders.aggregate( [
   { $project: { status: {
      $cond: { if: { $gt: ["$feedback", null] }, 
      then: 'finished', else: {
         $cond: { if: { $gt: ["$received", null] }, 
         then: 'received', else: {
            $cond: { if: { $gt: ["$shipped", null] }, 
            then: 'shipped', else: {
               $cond: { if: { $gt: ["$payment", null] }, 
                  then: 'payment received', else: 'created' }
            } }
         } } 
      } }
   } } },
   { $match: {  } }
] )