How to $lookup by avoiding null values in mongodb aggregate - mongodb

Question

In here i'm using $lookup to to a left join from other collections, the query works fine but when some records missing values it returns
errmsg : $in requires an array as a second argument, found: null
Heres the querying document structure :
{
"no" : "2020921008981",
"sale" : {
"soldItems" : [
{
"itemId" : "5b55ac7f0550de00210a3b24",
},
{
"itemId" : "5b55ac7f0550de00215584re",
}
],
"bills" : [
{
"billNo" : "2020921053467",
"insurancePlanId" : "160",
},
{
"billNo" : "2020921053467",
"insurancePlanId" : "170",
}
],
"visitIds" : [
5b55ac7f0550de00210a3b24, 5b55ac7f0550de00210a3b24
]
}
}
the query :
db.case.aggregate([
{
$lookup: {
from: "insurance",
let: { ipids: "$sale.bill.insurancePlanId" },
pipeline: [
{
$unwind: "$coveragePlans"
},
{
$match: { $expr: { $in: ["$coveragePlans._id", "$$ipids"] } }
},
{
$project: { _id: 0, name: 1 }
}
],
as: "insurances"
}
},
{
$lookup: {
from: "item",
let: { iid: "$salesOrder.purchaseItems.itemRefId" },
pipeline: [
{
$match: {
$expr: {
$in: ["$_id", {
$map: {
input: "$$iid",
in: { $toObjectId: "$$this" }
}
}
]
}
}
}
],
as: "items"
}
}
])
insurance collection :
{
"_id" : ObjectId("5b55aca20550de00210a6d25"),
"name" : "HIJKL"
"coveragePlans" : [
{
"_id" : "160",
"name" : "UVWZ",
},
{
"_id" : "161",
"name" : "LMNO",
}
]
},
{
"_id" : ObjectId("5b55aca20550de00210a6d25"),
"name" : "WXYZ"
"coveragePlans" : [
{
"_id" : "169",
"name" : "5ABC",
},
{
"_id" : "170",
"name" : "4XYZ",
}
]
}
item collection :
{
"_id" : ObjectId("5b55ac7f0550de00210a3b24"),
"code" : "ABCDE"
},
{
"_id" : ObjectId("5b55ac7f0550de00215584re"),
"code" : "PQRST"
}
How to avoid this and do null checks effectively before pipe-lining into the next stages? Tried with { $match: { "fieldName": { $exists: true, $ne: null } } } but it returns mongo error regarding the format. If its the way to go please mention the stage i should put that.. Thanks in advance

Answer 1

You can use $ifNull operator
let: { ipids: {$ifNull:["$sale.bill.insurancePlanId", [] ]} },
EDIT: To skip empty "$salesOrder.purchaseItems.itemRefId" values
let: { iid: {$filter: {input:"$salesOrder.purchaseItems.itemRefId", cond:{$ne:["$$this", ""]}}} },

Answer 2

You can get around that by not using $in.
It looks like this $map is executed separately for every document in the items collection. If you were to run the map in an $addFields stage, you could used the simple form of lookup to match the added field to _id, which would automagically handle missing, null, and array.
Remove the added field with a $project stage if necessary.
db.case.aggregate([
{$lookup: {
from: "insurance",
let: { ipids: "$sale.bill.insurancePlanId" },
pipeline: [
{$unwind: "$coveragePlans"},
{$match: { $expr: { $in: ["$coveragePlans._id", "$$ipids"] } }},
{$project: { _id: 0, name: 1 }}
],
as: "insurances"
}}
{$addFields:{
matchArray:{$map: {
input: "$$iid",
in: { $toObjectId: "$$this" }
}}
}},
{$lookup: {
from: "item",
localField: "matchArray",
foreignField:"_id",
as: "items"
}},
{$project:{
arrayField: 0
}}
])