java - lookup in mongodb aggregation

Question

The following bson is personaddress collection:

{ 
        "id" : "123456", 
        "name" : "foo", 
        "address" : [
            {
                "local" : "yes", 
                "location" : [
                   {
                        "place" : {
                            "_id":"VZG", 

                        }, 
                        "place_lat" : "18", 
                        "place_lan" : "83", 

                },
                {
                        "place" : {
                            "name" : "kerala", 
                            "district" : "palakkad", 
                            "pincode" : "5203689", 

                        }, 
                        "place_lat" : "18", 
                        "place_lan" : "83",      
                    }
                ]
            }
        ]
    } 

I have an another places collection:

 {
     "_id":"VZG",
     "name" : "vizag", 
     "district" : "Visakhaptanam, 
     "pincode" : "568923",
 }

Using lookup in mongodb aggregation, I want to embed places collection in personaddress collection

I tried using

Aggregation aggregation = newAggregation(lookup("places", "address.location.place._id", "_id", "myplaces"), unwind("myplaces"));

AggregationResults<OutputDocument> aggResults = mongoTemplate.aggregate(aggregation, PersonAddressDocument.class, OutputDocument.class);

Can anyone help me?

Answer 1

Since you have nested arrays, you need to apply the $unwind operator first in order to denormalise the embedded documents before using the $lookup pipeline (unless you have already flattened them in your aggregation operation):

db.personaddress.aggregate([
    { "$unwind": "$address" },
    { "$unwind": "$address.location" },
    {
        "$lookup": {
            "from": "places", 
            "localField": "address.location.place._id", 
            "foreignField": "_id", 
            "as": "address.location.place", 
        }
    }
])

which can then be implemented as (untested):

LookupOperation lookupOperation = LookupOperation.newLookup()
    .from("places")
    .localField("address.location.place._id")
    .foreignField("_id")
    .as("address.location.place");

Aggregation agg = newAggregation(
    unwind("address"),
    unwind("address.location"),
    lookupOperation  
);

AggregationResults<OutputDocument> aggResults = mongoTemplate.aggregate(
    agg, PersonAddressDocument.class, OutputDocument.class
);

---

If your Spring Data version does not support this, a workaround is to implement the AggregationOperation interface to take in 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 implement the $lookup operation as a DBObject in the aggregation pipeline:

DBObject lookupOperation = (DBObject)new BasicDBObject(
    "$lookup", new BasicDBObject("from", "places")
        .append("localField", "address.location.place._id")
        .append("foreignField", "_id")
        .append("as", "address.location.place")       
);

which you can then use as:

Aggregation agg = newAggregation(
    unwind("address"),
    unwind("address.location"),
    lookupOperation  
);

AggregationResults<OutputDocument> aggResults = mongoTemplate.aggregate(
    agg, PersonAddressDocument.class, OutputDocument.class
);