calculated field in MongoDB - mongodb

Question

My document is as below.
{
"name" : "MadhuA"+i,
"lastname" : "MohanB"+i,
"created" : new Date(),
"Address" : [
{
"Home Address" : "St:123, Abcd road, cityA, stateB, Pin code123",
"Work Address" : "St:134, xyz road, cityV, stateX, pin code456",
"Billing Address" : "St:156, pppp road, cityZ, stateNm ,Pincode154"
}
],
"tariff" : [
{
"Service" : "Broadband",
"Recurring" : "Monthly",
"Fee" : 100
}
],
"charges" : [ {
"charge_details" : [
{
"start_date" : "01-JAN-2016",
"end_date" : "01-FEB-2016",
"charge_amount" : Math.floor(Math.random()*400),
"Status" : "Outstanding",
"Service" : "Broadband",
"charge_type" : "monthly"
},
{
"start_date" : "01-JAN-2016",
"end_date" : "01-FEB-2016",
"charge_amount" : Math.floor(Math.random()*400),
"Status" : "Outstanding",
"Service" : "VoIP",
"charge_type" : "monthly"
}
] }
]
}
I want the Total amount to be populated with all the charge_amounts in the charges field. Whenever there is a charge_amount added to the document, the total_amount should be changed accordingly.
Please let me know if we have such functionality.
<3

Answer 1

If I understand correctly you are trying to add a property totalAmount such that:
Contain the list of charge_amount values
Changes dynamically when the document changes
If this is the case the best solution is to use Mongo 3.4 with views, writing something like:
db.createView("totalAmount", "collection", [
{
$addFields: {
totalAmount: "$charges.charge_details.charge_amount"
}
},
{
$addFields: {
totalAmount: { $arrayElemAt: ["$totalAmount",0] }
}
},
])
Here I am assuming that your collection is called "collection"
Now, you can query the new view and you will see the totalAmount:
db.totalAmount.find().pretty
shows:
{
"_id" : ObjectId("5856c41fbd0535e2fb8fb2ff"),
"name" : "MadhuA",
"lastname" : "MohanB",
"created" : ISODate("2016-12-18T17:15:11.797Z"),
"Address" : [
{
"Home Address" : "St:123, Abcd road, cityA, stateB, Pin code123",
"Work Address" : "St:134, xyz road, cityV, stateX, pin code456",
"Billing Address" : "St:156, pppp road, cityZ, stateNm ,Pincode154"
}
],
"tariff" : [
{
"Service" : "Broadband",
"Recurring" : "Monthly",
"Fee" : 100
}
],
"charges" : [
{
"charge_details" : [
{
"start_date" : "01-JAN-2016",
"end_date" : "01-FEB-2016",
"charge_amount" : 37,
"Status" : "Outstanding",
"Service" : "Broadband",
"charge_type" : "monthly"
},
{
"start_date" : "01-JAN-2016",
"end_date" : "01-FEB-2016",
"charge_amount" : 398,
"Status" : "Outstanding",
"Service" : "VoIP",
"charge_type" : "monthly"
}
]
}
],
"totalAmount" : [
37,
398
]
}
Moreover, if you add new documents to "collection" or modify any document the view (in particular totalAmount) will change accordingly.

Answer 2

Got this error:
> db.totalAmount.find({"name" : "MadhuA4097899"}).pretty()
Error: error: {
"ok" : 0,
"errmsg" : "$arrayElemAt's first argument must be an array, but is int",
"code" : 28689,
"codeName" : "Location28689"
}