What is the simplest method in MongoDB to retrieve an array containing only one specific field from the documents?

Let's say I have the following documents in my collection:

{
  _id: xxx,
  region: "sw"
},
{
  _id: yyy,
  region: "nw"
}

I am aiming to create an array like this:

['sw', 'nw']

Despite attempting mongodb aggregation/group and mapreduce, I keep ending up with an array of documents that need to be looped through again to get a single array. Is there a method to achieve this outcome in just one mongodb query or will it always necessitate initial querying followed by additional processing?

Answer №1

Here is a suggestion to try:

db.foo.aggregate(
    {$group: {_id: null, region: {$push: "$region"}}}
).result[0].region

If you prefer using map-reduce instead:

db.foo.mapReduce(
    function() { emit(null, this.region)},
    function(key, values) {
        result = {region: []};
        values.forEach(function(x){ result.region.push(x); });
        return result;
    },
    {out: {inline: 1}}
).results[0].value.region

Another option is to use group (credit to @Travis for this one):

db.foo.group({
  reduce: function(curr, result) {
    result.regions.push(curr.region);
  },
  initial: { regions: [] }
})[0].regions

Note

Remember when using any of these methods to consider the BSON Document Size Limits

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