MongoDB get count of particular key in an array - arrays

Question

In mongoDB, how can we get the count of particular key in an array
{
"_id" : ObjectId("52d9212608a224e99676d378"),
"business" : [
{
"name" : "abc",
"rating" : 4.5
},
{
"name" : "pqr"
},
{
"name" : "xyz",
"rating" : 3.6
}
]
}
in the above example, business is an array (with "name" and/or "rating" keys)
How can i get the count of business array with only "rating" key existing ?
Expected output is : 2

Answer 1

Looks like you have to use Aggregation Framework. In particular you need to $unwind your array, then match only elements with rating field included, then $group documents back to original format.
Try something like this:
db.test.aggregate([
{ $match: { /* your query criteria document */ } },
{ $unwind: "$business" },
{ $match: {
"business.rating": { $exists: 1 }
}
},
{ $group: {
_id: "$_id",
business: { $push: "$business" },
business_count: { $sum: 1 }
}
}
])
Result will look like the following:
{
_id: ObjectId("52d9212608a224e99676d378"),
business: [
{ name: "abc", rating: 4.5 },
{ name: "xyz", rating: 3.6 }
],
business_count: 2
}
UPD Looks like OP doesn't want to group results by wrapping document _id field. Unfortunately $group expression must specify _id value, otherwise it fails with exception. But, this value can actually be constant (e.g. plain null or 'foobar') so there will be only one resulting group with collection-wise aggregation.