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<Document> 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<String, Object>) ((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<Document> 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<ResultDao> 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<Document> documents;
private static final String DUMMY_KEY = "dummy";
public JsonOperation(String json) {
documents = parseJson(json);
}
static final List<Document> 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<Document> toPipelineStages(AggregationOperationContext context) {
return documents;
}
@Override
public String getOperator() {
return documents.iterator().next().keySet().iterator().next();
}
}
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 a 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<Document>).
Also note that I have skipped the step of transforming Document instances in toPipelineStages() method as it is not necessary in my case:
@Override
public List<Document> toPipelineStages(AggregationOperationContext context) {
return documents.stream().map(document -> context.getMappedObject(document)).collect(Collectors.toList());
}
