Spring Mongo Aggregation give a conversion error - spring

Question

I'm trying to use Mongo aggregation but I receive an error that I don't understand.
This is my domain:
#Document(collection = "tapes")
public class Tape {
#Id
private String id;
private String area;
private Integer tape;
private String tapeModel;
// follow getters e setters
The mongo shell command and the output is the following:
> db.tapes.aggregate([{ $group: { _id: { "area":"$area"}, tapes: {$push: {tape: "$tape"}}}} ])
{ "_id" : { "area" : "free" }, "tapes" : [ { "tape" : 1 }, { "tape" : 2 } ] }
{ "_id" : { "area" : "Qnap" }, "tapes" : [ { "tape" : 3 } ] }
The following is an attempt to re-create the aggregation in Spring:
AggregationOperation group = Aggregation.group("area").push("tape").as("tape");
Aggregation aggregation = Aggregation.newAggregation(group);
AggregationResults<Tape> results = mongoTemplate.aggregate(aggregation, "tapes", Tape.class);
//List<Tape> tapes = mongoTemplate.aggregate(aggregation, mongoTemplate.getCollectionName(Tape.class), Tape.class).getMappedResults();
List<Tape> tapes = results.getMappedResults();
System.out.println(tapes);
But I obtain the following error:
Cannot convert [3] of type class java.util.ArrayList into an instance of class java.lang.Integer! Implement a custom Converter<class java.util.ArrayList, class java.lang.Integer> and register it with the CustomConversions. Parent object was: it.unifi.cerm.cermadminspring.domain.Tape#2b84da07 -> null
org.springframework.data.mapping.MappingException: Cannot convert [3] of type class java.util.ArrayList into an instance of class java.lang.Integer! Implement a custom Converter<class java.util.ArrayList, class java.lang.Integer> and register it with the CustomConversions. Parent object was: it.unifi.cerm.cermadminspring.domain.Tape#2b84da07 -> null
I don't understand why, I searched for aggregation examples and all are more or less similar to mine.
Someone can help me?

Answer 1

First, for making life easier, a more simplified aggregation can be used:
> db.tapes.aggregate([ {$group: {_id: "$area", tapes: {$push: "$tape"}}} ])
Which should yield:
{ "_id" : "free", "tapes" : [ 1 , 2 ] }
{ "_id" : "Qnap", "tapes" : [ 3 ] }
Which should be matched by a change to the Java group aggregation operation:
AggregationOperation group = Aggregation.group("area").push("tape").as("tapes");
Note that I've changed to plural: as("tapes")
Then, notice that you are actually returning a document which doesn't have the same structure as the one you've mapped in the Tape class. That document contains two fields, the String id and a List<Integer> tapes fields.
This is the reason for the shorthand group aggregation I've suggested above, for making mapping easier:
public class TapesForArea {
private String id; // which is the area
private List<Integer> tapes;
// getters, setters ...
}
You don't need to map this class using spring-data-mongodb annotations.
Finally, have the aggregation results return the right type:
AggregationResults<TapesForArea> results =
mongoTemplate.aggregate(aggregation, "tapes", TapesForArea.class);
List<TapesForArea> tapes = results.getMappedResults();
BTW, the error comes from the fact that you try to map the single item tape array [ 3 ] into private Integer tape; property of Tape class.