Get the nth item of JSON array in MongoDB - json

Question

Using MongoDb how do you get back the date, and 3rd "obs" back from below?
{ "data" : [
{ "val" : [
{ "obs" : "2/3/2016"
},
{ "obs" : 41.8599992990494
},
{ "obs" : 41.3111999630928
},
{ "obs" : 5.048
}
]
},
{ "val" : [
{ "obs" : "2/4/2016"
},
{ "obs" : 39.394998550415
},
{ "obs" : 41.8486998975277
},
{ "obs" : NumberInt(0)
}
]
},
{ "val" : [
{ "obs" : "2/5/2016"
},
{ "obs" : NumberInt(0)
},
{ "obs" : 40.2090013027191
},
{ "obs" : 24.2410004138947
},
{ "obs" : 3.629
}
]
}
]
}
Started with this:
db.myColl.find({},{"_id":0, "data.val.obs": 1, })
would like:
["2/3/2016", 41.3111], ["2/4/2016", 41.8486]

Answer 1

Here is how you could do this in MongoDB starting from v 3.4
db.getCollection('test').aggregate([
{
$addFields: {
data: {
$map: {
input: "$data",
as: "item",
in: {$concatArrays: [{$slice: ['$$item.val', 1]}, {$slice: ['$$item.val', 2, 1]}]}
}
}
}
}
]);
So basically I'm using $addFields not to lose other properties of a root document (as you might need them). If you don't need them you can switch to $project.
Example: collection records look like this: {_id: ..., data: [...], data_2: [...]}.
If you run the query as is you'll have 'data' array filtered. But you'll still have data_2 unchanged. If you replace $addFields with $project you'll lose data_2. (or you need to explicitly tell mongo to keep it by passing data_2: true)
Then I'm mapping each element of 'data' array and assign the result back to 'data' array so in fact data property is overridden by filtered array.
To get 1st and 3rd elements I use $slice (each $slice returns an array of one document). And then I join them into a single array by $concatArrays.