如何在Spring Data Mongo DB中聚合嵌套对象并避免PropertyReferenceException？

Question

I am creating a new endpoint in springboot that will return simple stats on users generated from an aggregate query in a mongo database. However I get a PropertyReferenceException . I have read multiple stackoverflow questions about it, but didn't find one that solved this problem.

We have a mongo data scheme like this:

{ 
"_id" : ObjectId("5d795993288c3831c8dffe60"), 
"user" : "000001", 
"name" : "test", 
"attributes" : { 
    "brand" : "Chrome",
    "language" : "English" }
}

The database is filled with multiple users and we want using Springboot aggregate the stats of users per brand . There could be any number of attributes in the attributes object.

Here is the aggregation we are doing

Aggregation agg = newAggregation(
    group("attributes.brand").count().as("number"),
    project("number").and("type").previousOperation()
);

AggregationResults<Stats> groupResults
    = mongoTemplate.aggregate(agg, Profile.class, Stats.class);

return groupResults.getMappedResults();

Which produces this mongo query which works:

> db.collection.aggregate([ 
{ "$group" : { "_id" : "$attributes.brand" , "number" : { "$sum" : 1}}} ,
{ "$project" : { "number" : 1 , "_id" : 0 , "type" : "$_id"}} ])

{ "number" : 4, "type" : "Chrome" }
{ "number" : 2, "type" : "Firefox" }

However when running a simple integration test we get this error:

org.springframework.data.mapping.PropertyReferenceException: No property brand found for type String! Traversed path: Profile.attributes.

From what I understand, it seems that since attributes is a Map<String, String> there might be a schematic problem. And in the mean time I can't modify the Profile object.

Is there something I am missing in the aggregation, or anything I could change in my Stats object?

For reference, here are the data models we're using, to work with JSON and jackson.

The Stats data model:

@Document
public class Stats {

  @JsonProperty
  private String type;

  @JsonProperty
  private int number;

  public Stats() {}

  /* ... */
}

The Profile data model:

@Document
public class Profiles {

  @NotNull
  @JsonProperty
  private String user;

  @NotNull
  @JsonProperty
  private String name;

  @JsonProperty
  private Map<String, String> attributes = new HashMap<>();

  public Stats() {}

  /* ... */
}

Answer 1

I found a solution, which was a combination of two problems:

The PropertyReferenceException was indeed caused because attributes is a Map<String, String> which means there is no schemes for Mongo.

The error message No property brand found for type String! Traversed path: Profile.attributes. No property brand found for type String! Traversed path: Profile.attributes. means that the Map object doesn't have a brand property in it.

In order to fix that without touching my orginal Profile class, I had to create a new custom class which would map the attributes to an attributes object having the properties I want to aggreate on like:

public class StatsAttributes {

  @JsonProperty
  private String brand;

  @JsonProperty
  private String language;

  public StatsAttributes() {}

  /* ... */
}

Then I created a custom StatsProfile which would leverage my StatsAttributes and would be similar to the the original Profile object without modifying it.

@Document
public class StatsProfile {

  @JsonProperty
  private String user;

  @JsonProperty
  private StatsAttributes attributes;

  public StatsProfile() {}

  /* ... */
}

With that I made disapear my problem with the PropertyReferenceException using my new class StatsAggregation in the aggregation:

AggregationResults<Stats> groupResults 
     = mongoTemplate.aggregate(agg, StatsProfile.class, Stats.class);

However I would not get any results. It seems the query would not find any document in the database. That's where I realied that production mongo objects had the field "_class: com.company.dao.model.Profile" which was tied to the Profile object.

After some research, for the new StatsProfile to work it would need to be a @TypeAlias("Profile") . After looking around, I found that I also needed to precise a collection name which would lead to:

@Document(collection = "profile")
@TypeAlias("Profile")
public class StatsProfile {

/* ... */
}

And with all that, finally it worked!

I suppose that's not the prettiest solution, I wish I would not need to create a new Profile object and just consider the attributes as a StatsAttributes.class somehow in the mongoTemplate query. If anyone knows how to, please share 🙏