MongoDB Aggregation Projection - mongodb

Question

If I have a collection as follows:
db.cafe.insert({name: "Cafe1", customers: [{name: "David", foods: [{name : "cheese"}, {name: "beef"}]}, {name: "Bill", foods: [{name: "fish"}]} ]})
db.cafe.find().pretty()
{
"_id" : ObjectId("54f5ae58baed23b7a34fccb6"),
"name" : "Cafe1",
"customers" : [
{
"name" : "David",
"foods" : [
{
"name" : "cheese"
},
{
"name" : "beef"
}
]
},
{
"name" : "Bill",
"foods" : [
{
"name" : "fish"
}
]
}
]
}
How can I extract an array containing just the food objects for people called "David".
Desired output is just the array of foods, i.e:
[{name: "cheese"}, {name: "beef"}]
I have tried an aggregation pipeline that unwinds the cafes customers, then matches on name then projects the food, e.g:
db.cafe.aggregate( [{$unwind : "$customers"}, {$match : {"customers.name": "David"}}, {$project : {"customers.foods": 1, _id : 0}
}] ).pretty()
{
"customers" : {
"foods" : [
{
"name" : "cheese"
},
{
"name" : "beef"
}
]
}
}
This seems close to the desired result, however, I'm left with the issue that the foods I want are referenced as an array under the property customers.foods. I would like the result to directly be:
[
{
"name" : "cheese"
},
{
"name" : "beef"
}
]
is there a way I can achieve the desired output?

Answer 1

You are doing your projection wrong.
db.cafe.aggregate( [
{ "$match" : { "customers.name": "David" }},
{ "$unwind" : "$customers" },
{ "$project" : { "foods": "$customers.foods", "_id": 0 }}
])
Output
{ "foods" : [ { "name" : "cheese" }, { "name" : "beef" } ] }

Answer 2

You can also get (something very, very close to) your desired output with a regular query:
> db.cafe.find({ "customers.name" : "David" }, { "customers.$.foods" : 1, "_id" : 0 })
{ "customers" : [ { "name" : "David", "foods" : [ { "name" : "cheese" }, { "name" : "beef" } ] } ] }
Customers will be an array containing just the first object with name : "David". You should prefer this approach to the aggregation as it's vastly more performant. You can extract the foods array in client code.