Spring data mongo - Get sum of array of object - spring

Question

I have the following document:
{
pmv: {
budgets: [
{
amount: 10
},
{
amount: 20
}
]
}
}
and I need to sum the amount field from every object in budgets. But it's also possible that the budget object doesn't exist so I need to check that.
How could I do this? I've seen many questions but with projections, I just need a integer number which in this case would be 30.
How can I do it?
Thanks.
EDIT 1 FOR PUNIT
This is the code I tried but its giving me and empty aray
AggregationOperation filter = match(Criteria.where("pmv.budgets").exists(true).not().size(0));
AggregationOperation unwind = unwind("pmv.budgets");
AggregationOperation sum = group().sum("budgets").as("amount");
Aggregation aggregation = newAggregation(filter, unwind, sum);
mongoTemplate.aggregate(aggregation,"Iniciativas",String.class);
AggregationResults<String> aggregationa = mongoTemplate.aggregate(aggregation,"Iniciativas",String.class);
List<String> results = aggregationa.getMappedResults();

Answer 1

You can do this with aggregation pipeline
db.COLLECTION_NAME.aggregate([
{"pmv.budgets":{$exists:true,$not:{$size:0}}},
{$unwind:"$pmv.budgets"},
{amount:{$sum:"$pmv.budgets"}}
]);
This pipeline contains three queries:
get document having non-null and non-empty budgets
$unwind basically open the array and create one document for each array element. e.g. if one document of budgets has 3 elements then it will create 3 document and fill budgets property from each of the array element. You can read more about it here
sum all the budgets property using $sum operator
You can read more about aggregation pipeline here
EDIT: as per comments, adding code for java as well.
AggregationOperation filter = match(Criteria.where("pmv.budgets").exists(true).not().size(0));
AggregationOperation unwind = unwind("pmv.budgets");
AggregationOperation sum = group().sum("pmv.budgets").as("amount");
Aggregation aggregation = newAggregation(filter, unwind, sum);
mongoTemplate.aggregate(aggregation,COLLECTION_NAME,Output.class);
You can do this in more inline way as well but I wrote it like this so that it will be easy to understand.
I hope this answer your question.