Does Spring Data MongoDb support $filter array aggregations operator?

Question

I'm trying to implement in Spring Data using MongoTemplate the following working mongoDb query:

db.answers.aggregate([
        { "$match" : { "object_id" : "1" } },
        { "$project": { 'answer_list': 1, 'profile': { $filter : { input: '$answer_list', as: 'answer', cond: { $eq: [ '$$answer.question', 2 ] } } } } },
        { "$unwind" : "$profile"},
        { "$unwind" : "$answer_list"},
        { "$group" : { "_id" : { "question" : "$answer_list.question", "answer" : "$answer_list.answer", "criteria" : "$profile.answer"}, "count" : { "$sum" : 1 } } },
        { "$sort" : { "_id.question" : 1, "_id.answer" : 1 } }
]);

The collection has this structure:

{
"_id" : ObjectId("..."),
"object_id" : ObjectId("..."),
"answer_list" : [ 
    {
        "question" : NumberLong(0),
        "answer" : NumberLong(0)
    }, 
    {
        "question" : NumberLong(1),
        "answer" : NumberLong(2)
    }, 
    {
        "question" : NumberLong(2),
        "answer" : NumberLong(2)
    }
]}

What I'm trying to do here is a report on a simple survey submission data. The question is "How did the users that answered 0 to the first question answer to the second question?" I spent all day searching the SpringData Mongo Db docs but I found nothing. Can anyone help?

TIA

Thanks Christoph. I hope to find a workaround until it's implemented — optimalab
– optimalab, Commented Sep 13, 2016 at 8:18
As of Spring Boot 3 (maybe also Spring Boot 2.x) the $filter is supported. In the AggregationTest class in the spring-data-mongodb repository you'll find some examples. — Marvin
– Marvin, Commented Jan 25, 2023 at 11:34

Christoph Strobl · Accepted Answer · 2016-09-13 08:56:18Z

5

You can workaround this issue by providing your own AggregationExpression.

ProjectionOperation agg = Aggregation.project() //

      .and(new AggregationExpression() {

        @Override
        public DBObject toDbObject(AggregationOperationContext context) {

          DBObject filterExpression = new BasicDBObject();
          filterExpression.put("input", "$answer_list");
          filterExpression.put("as", "answer");
          filterExpression.put("cond", new BasicDBObject("$eq2", Arrays.<Object> asList("$$answer.question", 2)));

          return new BasicDBObject("$filter", filterExpression);
        }
      }).as("profile");

answered Sep 13, 2016 at 8:56

Christoph Strobl

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15 Comments

optimalab Over a year ago

That looks exactly what I need. The only problem is that interface AggregationExpression is not public, if I'm not missing something...

optimalab Over a year ago

I've created jira.spring.io/browse/DATAMONGO-1492 suggesting that the interface should be public. I'm still searching for a way to implement your suggestion...

Christoph Strobl Over a year ago

doh... my bad - that one should be public - thanks for filing the issue! Since AggregationExpression is package private you could add the package org.springframework.data.mongodb.core.aggregation to your project and create a public class/interface FilterExpression implementing/extending AggregationExpression within that package. Then just use that one from there.

optimalab Over a year ago

I'm getting closer... this is my implementation:

public class FilterExpression implements AggregationExpression {     // private fields and constructor omitted for brevity     @Override     public DBObject toDbObject( AggregationOperationContext context ) {         DBObject filterExpression = new BasicDBObject();         filterExpression.put("input", input);         filterExpression.put("as", as);         filterExpression.put("cond", cond);         return new BasicDBObject("$filter", filterExpression);     } }

optimalab Over a year ago

The expression seems correct (invoking the toDBObject() and a toJson() produces: "{ "$filter" : { "input" : "$answer_list", "as" : "answer", "cond" : { "$eq2" : ["$$answer.question", 2] } } }") but I'm getting thi exception:

org.springframework.data.mapping.PropertyReferenceException: No property answer found for type Opinion! 	at org.springframework.data.mapping.PropertyPath.<init>(PropertyPath.java:77) ...

|

Anshul · Accepted Answer · 2023-02-13 09:12:05Z

4

There is one more alternative using Aggregation

import static org.springframework.data.mongodb.core.aggregation.ComparisonOperators.valueOf;

    Aggregation aggregation = newAggregation(
                               project()
                               .and(filter("answer_list")
                               .as("answer")
                           .by(valueOf("answer.question").equalToValue(2)))
                               .as("profile"));
    AggregationResults<OutputType> profile = mongoTemplate.aggregate(aggregation, InputType.class, OutputType.class);

I may not be able to answer your question correctly but I just wanted to give another approach for aggregation as there are lesser number of examples using Aggregation.

In project() you can specify keys you want in your response, as its a varargs method

edited Feb 13, 2023 at 9:12

answered Jun 22, 2020 at 15:48

Anshul

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2 Comments

takanuva15 Over a year ago

I like this answer better. However, please include static imports you are using in your answer. For anyone else, the valueOf() operator comes from here: import static org.springframework.data.mongodb.core.aggregation.ComparisonOperators.valueOf;

Marvin Over a year ago

In the AggregationTest class in the spring-data-mongodb repository you'll find more examples. E.g. for for filtering within a project see here.

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