Mongodb aggregate ifNull against array elements - mongodb

Question

I have the following dataset:
{
patientId: 228,
medication: {
atHome : [
{
"drug" : "tylenol",
"start" : "3",
"stop" : "7"
},
{
"drug" : "advil",
"start" : "0",
"stop" : "2"
},
{
"drug" : "vitaminK",
"start" : "0",
"stop" : "11"
}
],
}
}
When I execute the following aggregate everything looks great.
db.test01.aggregate(
[
{$match: {patientId: 228}},
{$project: {
patientId: 1,
"medication.atHome.drug": 1
}
},
]);
Results (Exactly what I wanted):
{
"_id" : ObjectId("5a57b7d17af6772ebf647939"),
"patientId" : NumberInt(228),
"medication" : {
"atHome" : [
{"drug" : "tylenol"},
{"drug" : "advil"},
{"drug" : "vitaminK"}
]}
}
We then wanted to add ifNull to change nulls to a default value, but this bungled the results.
db.test01.aggregate(
[
{$match: {patientId: 228}},
{$project: {
patientId: {$ifNull: ["$patientId", NumberInt(-1)]},
"medication.atHome.drug": {$ifNull: ["$medication.atHome.drug", "Unknown"]}
}
},
]);
Results from ifNull (Not what I was hoping for):
{
"_id" : ObjectId("5a57b7d17af6772ebf647939"),
"patientId" : NumberInt(228),
"medication" : {
"atHome" : [
{"drug" : ["tylenol", "advil", "vitaminK"]},
{"drug" : ["tylenol", "advil", "vitaminK"]},
{"drug" : ["tylenol", "advil", "vitaminK"]},
]}
}
What am I missing or not understanding?

Answer 1

To set attributes of documents that are elements of an array to default values you need to $unwind the array and then to group everything up after you check the attributes for null. Here is the query:
db.test01.aggregate([
// unwind to evaluete the array elements
{$unwind: "$medication.atHome"},
{$project: {
patientId: {$ifNull: ["$patientId", -1]},
"medication.atHome.drug": {$ifNull: ["$medication.atHome.drug", "Unknown"]}
}
},
// group to put atHome documents to an array again
{$group: {
_id: {_id: "$_id", patientId: "$patientId"},
"atHome": {$push: "$medication.atHome" }
}
},
// project to get a document of required format
{$project: {
_id: "$_id._id",
patientId: "$_id.patientId",
"medication.atHome": "$atHome"
}
}
])
UPDATE:
There is another more neat query to achieve the same. It uses the map operator to evaluate each array element thus does not require unwinding.
db.test01.aggregate([
{$project:
{
patientId: {$ifNull: ["$patientId", -1]},
"medication.atHome": {
$map: {
input: "$medication.atHome",
as: "e",
in: { $cond: {
if: {$eq: ["$$e.drug", null]},
then: {drug: "Unknown"},
else: {drug: "$$e.drug"}
}
}
}
}
}
}
])