mongoexport csv输出最后一个数组值

Question

Inspired by this question in Server Fault https://serverfault.com/questions/459042/mongoexport-csv-output-array-values

I'm using mongoexport to export some collections into CSV files, however when I try to target fields which are the last members of an array I cannot get it to export correctly.

Command I'm using

mongoexport -d db -c collection -fieldFile fields.txt --csv > out.csv

One item of my collection:

{
    "id": 1,
    "name": "example",
    "date": [
        {"date": ""},
        {"date": ""},
    ],
    "status": [
         "true",
         "false",
    ],
}

I can access to the first member of my array writing the fields like the following

name
id
date.0.date
status.0

Is there a way to acess the last item of my array without knowing the lenght of the array?

Because the following doesn't work:

name
id
date.-1.date
status.-1

Any idea of the correct notation? Or if it's simply not possible?

Answer 1

It's not possible to reference the last element of the array without knowing the length of the array, since the notation is array_field.index where the index is in [0, length - 1]. You could use the aggregation framework to create the view of the data that you want to export, save it temporarily into a collection with $out, and then mongoexport that. For example, for your documents you could do

db.collection.aggregate([
    { "$unwind" : "$date" },
    { "$group" : { "_id" : "$_id", "date" : { "$last" : "$date" } } },
    { "$out" : "temp-for-csv" }
])

in order to get just the last date for each document and output it to the collection temp-for-csv.

You can return just the last elements in an array with the $slice projection operator, but this isn't available in aggregation and mongoexport only takes a query specification, not a projection specification, since the --fields and --fieldFile option are supposed to suffice. Might be a good feature request to ask for using a query with a projection for mongoexport.