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Populate from One Model to Another Using Mongoose/MongoDB in Node/Express Application

I am struggling to understand the populate method in Mongoose, while having trouble populating fields from one Mongoose model to another. The first model Schema is:

var MprnSchema = new mongoose.Schema({
mprNo: {type: Number, unique: true, required: true},
siteName: String,
buildingNo: Number,
streetAddress: String,
secondAddress: String,
townCity: String,
postCode: String,
supplier: String,
siteContactName: String,
siteContactNo: String,
});

module.exports = mongoose.model("Mprn", MprnSchema);


And another Schema is:

var FaultSchema = new mongoose.Schema({
jobRef: Number,
mprNo: Number,
requestedDate: {type: Date, default: Date.now},
attendedDate: {type: Date, default: null},
siteDetails: {type: mongoose.Schema.Types.ObjectId, ref: 'Mprn'},
faultIssue: String,
});


However, when I try to populate the 'siteDetails' with the contents of Mprn, I get when using thhe code below:

Fault.find(reportQuery).populate('siteDetails', 'siteName
postCode').exec((err, faultResults) => {
if(err){
req.flash("error", err.message);
res.redirect("/");
}

console.log(faultResults);
res.render("reportResults", { queryResults: faultResults });

});


The object passed to be rendered (queryResults) only contains the Fault data and NOT the 'siteDetails' that should be populated. I think the problem may be when I am using the Fault.Create() method in the first place, should I be saving a reference to the MPRN model at this point, so that the there is an Id to be referenced when populating?

My samples collections are:

Fault

{
"jobRef": 60000,
"mprNo": 123456,
"faultIssue": "Test"
}


Mprn

{
"mprNo": 123456,
"siteName": "Smithson Gates",
"buildingNo": 76,
"streetAddress": "Garden Place",
"secondAddress": "Greater Manchester",
"townCity": "Salford",
"postCode": "M5 3AP",
"supplier": "test",
"siteContactName": "Mr B Jones",
"siteContactNo": "0161 000 0000"
}


1 Answer
1

Well you are using latest version of mongodb... So, You can try below $lookup aggregation... and the plus point of using this is you don't need to store _id of Mprn collection in the Fault... You can easily join both the collections with mprn number

$lookup

_id

Mprn

Fault

mprn

Fault.aggregate([
{ "$match": reportQuery },
{ "$lookup": {
"from": Mprn.collection.name,
"let": { "mprNo": "$mprNo" },
"pipeline": [
{ "$match": { "$expr": { "$eq": [ "$mprNo", "$$mprNo" ] } } }
],
"as": "siteDetails"
}},
{ "$unwind": "$siteDetails" }
])


Fault

Mprn.collection.name

Mprn

Fault

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