Aggregation query with lookup in Spring

Question

I'm using the Spring framework to perform an aggregation on my mongodb. However, the lookup keeps failing and I can't understand why. Here's the query:

Aggregation aggregation = newAggregation(
    match(Criteria.where("idOfUser").is(loggedInAccount.getId())),
    group("imgID"),
    new CustomAggregationOperation(
        new BasicDBObject("$lookup",
        new BasicDBObject("from","img")
            .append("localField","_id")
            .append("foreignField","_id")
            .append("as","uniqueImgs")
        )
    ),
    limit(pageable.getPageSize()),
    skip(pageable.getPageSize()*pageable.getPageNumber())
);

AggregationResults aggregationResults = mongo.aggregate(aggregation, "comment", String.class); //Using String at the moment just to see the output clearly.

CustomAggregationOperation is as follows:

public class CustomAggregationOperation implements AggregationOperation {
    private DBObject operation;

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

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

The Spring MongoDB version of lookup isn't recognised which is why I'm using this CustomAggregationOperation. AFAIK it shouldn't affect it.

Ideally what I want to happen is:

Get all the comments of the user.
Make sure that the imgID is distinct for the comments (so there are only the id's of the imgs that have been commented on)
Get the actual img objects related to these ids.
Paginate the returned imgs.

At the moment, step 3 doesn't work, and I think 4 wouldn't work either since limit and skip won't be applied to the objects in "uniqueImgs". What is returned is:

[{ "_id" : "570e2f5cb1b9125510a443f5" , "uniqueImgs" : [ ]}]

How can I fix this?

EDIT the imgID stored isn't an ObjectID whereas the _id in the img collection is. Would that have any effect?

If the two "types" are not the same then the $lookup will not be successful, just as it would also fail with any query where you used an incorrect BSON type. So you will need to change the "imgID" data to be ObjectId values instead. — Neil Lunn
– Neil Lunn, Commented Apr 16, 2016 at 8:40
@NeilLunn Thanks, I think another reason why it isn't working is because I've just seen that my version of mongo doesn't support $lookup. — Tometoyou
– Tometoyou, Commented Apr 16, 2016 at 8:54
If it didn't support it then you would have a big "error" as opposed to an empty array. If a pipeline operator is not supported, then that is exactly the error that is thrown. These things are not silent. — Neil Lunn
– Neil Lunn, Commented Apr 16, 2016 at 9:04

chridam · Accepted Answer · 2016-12-20 14:29:54Z

6

The current release (at the time of writing 1.9.5) has support for the $lookup operator and can be implemented as (untested):

LookupOperation lookupOperation = LookupOperation.newLookup()
    .from("img")
    .localField("_id")
    .foreignField("_id")
    .as("uniqueImgs");

Aggregation agg = newAggregation(
    match(Criteria.where("idOfUser").is(loggedInAccount.getId())),
    group("imgID"),
    lookupOperation,
    limit(pageable.getPageSize()),
    skip(pageable.getPageSize()*pageable.getPageNumber())
);

AggregationResults aggregationResults = mongo.aggregate(agg, "comment", String.clas);

answered Dec 20, 2016 at 14:29

chridam

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2 Comments

PAA Over a year ago

Could you please guide me here: stackoverflow.com/questions/61953464/… ?

abdul rashid Over a year ago

Limit is applied to comment collection, What If I want to limit the results from img collection ?

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