mongo shell: return only 1 element of nested array

Question

I have the following data on a mongodb:

{
    "name" : "bla",
    "log" : [
        {
            "A" : 1,
            "B" : 10
        },
        {
            "A" : 2,
            "B" : 20
        }
    ]
}

I understand how to return all values of A from the mongoshell:

 db.test.find({},{'name':1,'log.A':1})
{ "_id" : ObjectId("52712539c99a2fc6f6088cd4"), "name" : "bla", "log" : [ { "A" : 1 }, { "A" : 2 } ] }

but how can I limit the output of A to only the first element? This is the output that I extect to have:

{ "_id" : ObjectId("52712539c99a2fc6f6088cd4"), "name" : "bla", "log.A" : 1, "log.B":10}

I don't mind in having log.A or just A, or even having some [ ] in the output, as soon as it is always only one entry for A and for B

how can I do it?

Answer 1

You can use the $slice array projection operator to do that:

db.test.find({}, {name: 1, 'log.A':1, log: {$slice: 1}})

Outputs:

{ "_id" : ObjectId("..."), "name" : "bla", "log" : [  {  "A" : 1 } ] }