I am writing an application to consume the data in my Mongo database, but I need to process the data in a flat format. My document has a few arrays in it that I would like to turn into a string array, but can't find a way to do so with a mongo query.
Document:
{
name: 'Hello World',
tags: ['simple', 'easy']
}
Desired Output:
{
name: 'Hello World',
tags: 'simple,easy',
}
It can be done natively in MongoDB aggregation with $reduce:
db.myCollection.aggregate([{
$set: {
tags: {
$reduce: {
input: "$tags",
initialValue: "",
in: {
$concat : ["$$value", "," ,"$$this"]
}
}
}
}, {
$project: {
name: 1,
tags: { $substr: [ "$tags", 1, -1 ] }
}
}]);
There is no way to avoid extra separator (leading or trailing) when using $concat, so there is additional step with $substr to remove it.
$concat : ["$$value", "," ,"$$this"] produces string ",a,b,c,d"$substr: [ "$tags", 1, -1 ] makes substring starting at index 1 with length -1, which is "as much as possible" in this context.This is very doable with one stage. addFields will overwrite the $tags element since it already exists. Mongo Playground
{
"$addFields": {
"tags": {
"$reduce": {
"input": "$tags",
"initialValue": "",
"in": {
"$concat": [
//add the running concatination of this array to the front
"$$value",
//add the current value after that
"$$this",
{
//comma. if current index is the last element, don't add a comma to the end.
$cond: {
if: {
"$eq": [
{
$subtract: [
{
$size: "$tags"
},
1
]
},
{
$indexOfArray: [
"$tags",
"$$this"
]
}
]
},
then: "",
else: ","
}
}
]
}
}
}
}
}
The only way I can think of achieve this using a aggregation query is below but its very static.
db.test.aggregate([
{ $unwind: "$tags" },
{ $group: { _id: "$_id", name: {$first: "$name"}, firstTag: {$first: "$tags"}, lastTag: {$last: "$tags"} } },
{ $project: { name: "$name", tags: { $concat: [ "$firstTag", ",", "$lastTag" ] } } }
]);
You can however achive this with a mapReduce:
db.test.mapReduce(
function() {emit(this._id, this);},
function(key, value) {return value}, {
out:"tags",
finalize: function(key, reducedVal){
reducedVal.tags = reducedVal.tags.join();
return reducedVal;
}
}
)
> db.tags.find()
{ "_id" : ObjectId("5849a9f6a4db9c5811299d08"), "value" : { "name" : "Hello World", "tags" : "simple,easy" } }
mapReduceto do that. Probably better to do that client-side if possible.toString().