mongotemplate aggregation with condition

Question

I have a collection where the documents look like this:

{
  _id: "545b9fa0dd5318a4285f7ce7",
  owner: "admin",  
  messages: [
    {
      id: "100",
      status: "sent",
      note: ""
    },
    {
      id: "100",
      status: "pending",
      note: ""
    },
    {
      id: "101",
      status: "sent",
      note: ""
    },
    {
      id: "102",
      status: "sent",
      note: ""
    },
    {
      id: "101",
      status: "done",
      note: ""
    }
  ]
}

(This is just a short example, in my case the sub array is very large)

I need to query the collection and get some statistics for a specific document. So in this example if I query the doucment that has id: "545b9fa0dd5318a4285f7ce7" I should get this result:

{
   sent: 3,
   pending: 1,
   done: 1
}

How can I do such aggregation with spring mongotemplate ?

Answer 1

To do this sort of thing you need the $cond operator in the aggregation framework. Spring Data MongoDB does not do that yet, and there are many things lacking from common $group operations that are even only implemented under $project.

The issue for tracking implementation of any $cond support is here:

https://jira.spring.io/browse/DATAMONGO-861

For the rest of the world it looks like this:

db.collection.aggregate([
    { "$match": { "_id": ObjectId("545b9fa0dd5318a4285f7ce7") } },
    { "$unwind": "$messages" },
    { "$group": {
        "_id": "$_id",
        "sent": {
            "$sum": {
                "$cond": [
                    { "$eq": [ "$mesages.status", "sent" ] },
                    1,
                    0
                ]
            }
        },
        "pending": {
            "$sum": {
                "$cond": [
                    { "$eq": [ "$messages.status", "pending" ] },
                    1,
                    0
                ]
            }
        },
        "done": {
            "$sum": {
                "$cond": [
                    { "$eq": [ "$messages.status", "done" ] },
                    1,
                    0
                ]
            }
        }
    }}
])

To get that sort of thing to work under a mongotemplate aggregation you need a class extending aggregation operations that can be built from a DBObject:

public class CustomGroupOperation implements AggregationOperation {
    private DBObject operation;

    public CustomGroupOperation (DBObject operation) {
        this.operation = operation;
    }

    @Override
    public DBObject toDBObject(AggregationOperationContext context) {
        return context.getMappedObject(operation);
    }
}

Then you can define the "$group" as a DBObject and implement in the aggregation pipeline:

   DBObject myGroup = (DBObject)new BasicDBObject(
        "$group", new BasicDBObject(
            "_id","$_id"
        ).append(
            "sent", new BasicDBObject(
                "$sum", new BasicDBObject(
                    "$cond", new Object[]{
                        new BasicDBObject(
                            "$eq", new Object[]{ "$messages.status", "sent"}
                        ),
                        1,
                        0
                    }
                )
            )
        ).append(
            "pending", new BasicDBObject(
                "$sum", new BasicDBObject(
                    "$cond", new Object[]{
                        new BasicDBObject(
                            "$eq", new Object[]{ "$messages.status", "pending"}
                        ),
                        1,
                        0
                    }
                )
             )
        ).append(
            "done", new BasicDBObject(
                "$sum", new BasicDBObject(
                    "$cond", new Object[]{
                         new BasicDBObject(
                            "$eq", new Object[]{ "$messages.status", "done"}
                         ),
                         1,
                         0
                    }
                 )
            )
        )
     );


   ObjectId myId = new ObjectId("545b9fa0dd5318a4285f7ce7");

   Aggregation aggregation = newAggregation(
           match(Criteria.where("_id").is(myId)),
           unwind("messges"),
           new CustomGroupOperation(myGroup)
   );

That allows you to come up with a pipeline that is basically the same as the shell representation above.

So it would seem that for now, where certain operations and sequences are not supported the best case is to implement a class on the AgggregationOperation interface that can be fed a DBObject or otherwise internally construct one from your own custom methods.