Get Sum of Values in MongoDB - node.js

Question

I have an array and each list item in that array is an object the has a value key. What I would like to do is add all of the values to get a total. I want to do this on the back end rather than the front end. I have tried the aggregate method, but haven't had any luck as it returns an empty array. Here is my array:
"budgets" : [
{
"name" : "check",
"value" : "1000",
"expense" : "false",
"uniqueId" : 0.9880268634296954
},
{
"name" : "car",
"value" : "500",
"expense" : "true",
"uniqueId" : 0.1904486275743693
},
{
"name" : "car2",
"value" : "500",
"expense" : "false",
"uniqueId" : 0.23043920518830419
},
{
"name" : "car23",
"value" : "500",
"expense" : "false",
"uniqueId" : 0.014449386158958077
},
{
"name" : "car235",
"value" : "500",
"expense" : "false",
"uniqueId" : 0.831609656335786
}
],
What I want to do is get the total of the values that have "expense" : "true" and the get a separate total of "expense" : "false" how would I do this? Like I said I have tried the aggregate method, but I must be doing it wrong.

Answer 1

If you are new to using the aggregation framework commands you might not yet be aware of the $cond operator. You can use this in order to get your totals separated:
db.collection.aggregate([
// Unwind the array to de-normalize the items
{ "$unwind": "$budgets" },
// Group the totals with conditions
{ "$group": {
"_id": "$_id",
"expense": { "$sum": {
"$cond": [
"$budgets.expense",
"$budgets.value",
0
]
}},
"nonExpense": { "$sum": {
"$cond": [
"$budgets.expense",
0,
"$budgets.value"
]
}}
}}
])
So what that will do is evaluate the true/false condition of expense as the first argument, and when the condition is actually true then the second argument of the $cond will be chosen and passed on to the $sum. If the condition evaluates to false then the second argument is chosen.
But looking at your data you appear to have a problem:
{
"name" : "check",
"value" : "1000",
"expense" : "false",
"uniqueId" : 0.9880268634296954
},
Noting here that all of the fields, and most notably the expense and value items, are all strings. So this is a problem as while we could get around evaluating the true/false values by doing string comparisons rather than direct boolean, you simply cannot co-erce a string as a number that can be passed to $sum.
So firstly you need to fix your data, unless it is not actually in the form as you have represented. But when it meets a correct form, then you can do your aggregation.

Answer 2

First of all covert you datatype of value from string to integer(summable) type and then use
db.collectioName.aggregate(
{$unwind:"$budgets"},
{
$group:{_id: "$budgets.expense",
total: { $sum: "$budgets.value"}}
})
and then you should get result like this
{
"result" : [
{
"_id" : "true",
"total" : 500
},
{
"_id" : "false",
"total" : 2000
}
],
"ok" : 1
}

Answer 3

Try something like this
db.collection.aggregate([ { "$unwind": "$budgets" },
{ $match: { expense: "true" } },
{ $group: { _id: "$_id",
name: { $addToSet: "$name" },
uniqueId: { $addToSet: "$uniqueId" },
total: { $sum: "$$budgets.value" } } }
])