Multiplication on nested fields on MongoDB - mongodb

Question

In a database in MongoDB I am trying to group some data by their date (one group for each day of the year), and then add an additional field that would be the result of the multiplication of two of the already existing fields.
The data structure is:
{
"_id" : ObjectId("567a7c6d9da4bc18967a3947"),
"units" : 3.0,
"price" : 50.0,
"name" : "Name goes here",
"datetime" : ISODate("2015-12-23T10:50:21.560+0000")
}
I first tried a two stage approach using $project and then $group like this
db.things.aggregate(
[
{
$project: {
"_id" : 1,
"name" : 1,
"units" : 1,
"price" : 1,
"datetime":1,
"unitsprice" : { $multiply: [ "$price", "$units" ] }
}
},
{
$group: {
"_id" : {
"day" : {
"$dayOfMonth" : "$datetime"
},
"month" : {
"$month" : "$datetime"
},
"year" : {
"$year" : "$datetime"
}
},
"things" : {
"$push" : "$$ROOT"
}
}
}
],
)
in this case, the first step (the $project) gives the expected output (with the expected value of unitsprice), but then when doing the second $group step, it outputs this error:
"errmsg":$multiply only supports numeric types, not String",
"code":16555
I tried also turning around things, doing the $group step first and then the $project
db.things.aggregate(
[
{
$group: {
"_id" : {
"day" : {
"$dayOfMonth" : "$datetime"
},
"month" : {
"$month" : "$datetime"
},
"year" : {
"$year" : "$datetime"
}
},
"things" : {
"$push" : "$$ROOT"
}
}
},
{
$project: {
"_id" : 1,
"things":{
"name" : 1,
"units" : 1,
"price" : 1,
"datetime":1,
"unitsprice" : { $multiply: [ "$price", "$units" ] }
}
}
}
],
);
But in this case, the result of the multiplication is: unitsprice:null
Is there any way of doing this multiplication? Also, it would be nice to do it in a way that the output would not have nested fields, so it would look like:
{"_id":
"units":
"price":
"name":
"datetime":
"unitsprice":
}
Thanks in advance
PS:I am running MongoDB 3.2

Answer 1

Finally found the error. When importing one of the fields, a few of the price fields were created as a string. Surprisingly, the error didn't came out when first doing the multiplication in the project step (the output was normal until it reached the first wrong field, then it stopped), but when doing the group step.
In order to find the text fields I used this query:
db.things.find( { price: { $type: 2 } } );
Thanks for the hints