Get the first element from an array with mongodb java client

Question

I have a JavaScript code.

db.xxx.aggregate([
    {$project: {object:{$first:'$object.array'}, _id:0}}
])

I don't know how to overwrite it by java client.

import org.springframework.data.mongodb.core.MongoTemplate;
...
Aggregation.newAggregation(
    Aggregation.project().andExclude("_id").???
)

And where to find the usage?

massimo lauri · Accepted Answer · 2022-03-15 09:35:21Z

1

This is TestDocument

public class TestArrayDocument {
    @Id
    String id;
    Integer[] x;
}

Inject MongoTemplate

@Autowired
MongoTemplate mongoTemplate;

This is aggregate method

  TypedAggregation<TestArrayDocument> aggregation  = Aggregation.newAggregation(TestArrayDocument.class,Aggregation
            .project()
            .andExclude("_id")
            .andExpression("first(x)").as("object"));

    List<Map> mapData=  mongoTemplate
            .aggregate(aggregation,Map.class)
            .getMappedResults();

x is your $object.array.

return in LinkedHashMap "object"=>"firstElement".

answered Mar 15, 2022 at 9:35

massimo lauri

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