MongoDB:Spring data:- Embedded Array Count - mongodb

Question

How to identify the number of elements in the particular embedded document (or) how to find the number of elements in the embedded array?
Award
{
"brand" : [
{
"name" : "multi",
"descr" : "Multpple"
},
{
"name" : "multi",
"descr" : "two"
},
{
"name" : "multi",
"descr" : "three"
}
],
"name" : "Test",
"narname" : "Nar"
}
For Eg: In the above document how to find the number of elements that is inside the embedded array BRAND using Spring Data.?
Any pointers would be greatly appreciated !

Answer 1

I don't think there is a method which can get the answer directly.
You can use aggregate to implement it. For example, if you want to get the count of elements in array brand in a specific document, this way should be available (run on mongo shell):
db.Award.aggregate({$match:{_id:id}}, {$unwind:"$brand"}, {$group:{_id:"$_id", count:{$sum:1}}});
count is the result you want.
Then implement that using spring-data-mongodb syntax.
-------------- APPENDED ---------------------
// You can find the relative aggregation method from MongoTemplate.java file to handle your requirements.
// For exmaple:
// public <O> AggregationResults<O> aggregate(Aggregation aggregation, Class<?> inputType, Class<O> outputType)
// The version is around spring-data-mongodb-1.5.0 or higher.
// Below I call the mongo-java-driver directly because I find it needs some time to learn it from spring-data-mongodb. :)
protected int getArraySize(Object id, String collName) {
// Attention: make sure id is in the correct data type because the following statement would not convert it automatically.
// Issue codes according to this command line:
// db.Award.aggregate({$match:{_id:id}}, {$unwind:"$brand"}, {$group:{_id:"$_id", count:{$sum:1}}});
DBObject match = BasicDBObjectBuilder.start().push("$match").append("_id", id).get();
DBObject unwind = new BasicDBObject("$unwind", "$brand");
DBObject group = BasicDBObjectBuilder.start().push("$group").append("_id", "$_id").push("count").append("$sum", 1).get();
List<DBObject> pipeline = Arrays.asList(match, unwind, group);
// This aggregate method is supported in higher version of mongo-java-driver, here I use is 2.12.3
AggregationOutput aggr = this.mongoTemplate.getCollection(collName).aggregate(pipeline);
for (DBObject dbo : aggr.results()) {
Object count = dbo.get("count");
if (count instanceof Number) {
return ((Number)count).intValue();
}
}
return 0;
}