Spring Data MongoDB Lookup with Pipeline Aggregation

Question

How would I convert the following MongoDB query into a query to be used by my Java Spring application? I can\'t find a way to use pipeline with the provided loo

Answer 1

I would like to add this my solution which is repeating in some aspect the solutions posted before.

Mongo driver v3.x

For Mongo driver v3.x I came to the following solution:

import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

import com.mongodb.BasicDBList;
import com.mongodb.BasicDBObject;
import com.mongodb.util.JSON;

import org.bson.Document;
import org.springframework.data.mongodb.core.aggregation.AggregationOperation;
import org.springframework.data.mongodb.core.aggregation.AggregationOperationContext;

public class JsonOperation implements AggregationOperation {

    private List documents;

    public JsonOperation(String json) {
        Object root = JSON.parse(json);

        documents = root instanceof BasicDBObject
                    ? Collections.singletonList(new Document(((BasicDBObject) root).toMap()))
                    : ((BasicDBList) root).stream().map(item -> new Document((Map) ((BasicDBObject) item).toMap())).collect(Collectors.toList());
    }

    @Override
    public Document toDocument(AggregationOperationContext context) {
        // Not necessary to return anything as we override toPipelineStages():
        return null;
    }

    @Override
    public List toPipelineStages(AggregationOperationContext context) {
        return documents;
    }
}

and then provided that aggregation steps are given in some resource aggregations.json:

[
  {
    $match: {
      "userId": "..."
    }
  },
  {
    $lookup: {
      let: {
        ...
      },
      from: "another_collection",
      pipeline: [
        ...
      ],
      as: "things"
    }
  },
  {
    $sort: {
      "date": 1
    }
  }
]

one can use above class as follows:

import static org.springframework.data.mongodb.core.aggregation.Aggregation.newAggregation;

Collection results = mongoTemplate.aggregate(newAggregation(new JsonOperation(resourceToString("aggregations.json", StandardCharsets.UTF_8))), "some_collection", ResultDao.class).getMappedResults();

Mongo driver v4.x

As JSON class was removed from Mongo v4, I have rewritten the class as follows:

import java.util.Collections;
import java.util.List;

import org.bson.Document;
import org.springframework.data.mongodb.core.aggregation.AggregationOperation;
import org.springframework.data.mongodb.core.aggregation.AggregationOperationContext;

public class JsonOperation implements AggregationOperation {

    private List documents;

    private static final String DUMMY_KEY = "dummy";

    public JsonOperation(String json) {
        documents = parseJson(json);
    }

    static final List parseJson(String json) {
        return (json.startsWith("["))
                    ? Document.parse("{\"" + DUMMY_KEY + "\": " + json + "}").getList(DUMMY_KEY, Document.class)
                    : Collections.singletonList(Document.parse(json));
    }

    @Override
    public Document toDocument(AggregationOperationContext context) {
        // Not necessary to return anything as we override toPipelineStages():
        return null;
    }

    @Override
    public List toPipelineStages(AggregationOperationContext context) {
        return documents;
    }
}

but implementation is now a bit ugly because of string manipulations. If somebody has a better idea of how to parse array of objects in more elegant way, please edit this post or drop a comment. Ideally there should be some method in Mongo core that allows to parse either JSON object or list (returns BasicDBObject/BasicDBList or Document/List).

Also node that I have skipped the step of transforming Document instances in toPipelineStages() method as it is not necessary in my case:

@Override
public List toPipelineStages(AggregationOperationContext context) {
    return documents.stream().map(document -> context.getMappedObject(document)).collect(Collectors.toList());
}