Projecting a field within an association fails with `MappingException`

Originally reported by kliarist at spring-projects/spring-graphql#746:

I am using spring-graphql along with spring-data-mongo and querydsl and faced with the below issue

org.springframework.data.mapping.MappingException: Invalid path reference articles.title; Associations can only be pointed to directly or via their id property
	at org.springframework.data.mongodb.core.convert.QueryMapper$MetadataBackedField.getPath(QueryMapper.java:1275) ~[spring-data-mongodb-4.1.1.jar:4.1.1]
	at org.springframework.data.mongodb.core.convert.QueryMapper$MetadataBackedField.<init>(QueryMapper.java:1134) ~[spring-data-mongodb-4.1.1.jar:4.1.1]
	at org.springframework.data.mongodb.core.convert.QueryMapper$MetadataBackedField.<init>(QueryMapper.java:1111) ~[spring-data-mongodb-4.1.1.jar:4.1.1]
	at org.springframework.data.mongodb.core.convert.QueryMapper.createPropertyField(QueryMapper.java:365) ~[spring-data-mongodb-4.1.1.jar:4.1.1]
	at org.springframework.data.mongodb.core.convert.QueryMapper.lambda$mapFieldsToPropertyNames$0(QueryMapper.java:234) ~[spring-data-mongodb-4.1.1.jar:4.1.1]
	at java.base/java.util.Map.forEach(Map.java:713) ~[na:na]
	at org.springframework.data.mongodb.core.convert.QueryMapper.mapFieldsToPropertyNames(QueryMapper.java:232) ~[spring-data-mongodb-4.1.1.jar:4.1.1]
	at org.springframework.data.mongodb.core.convert.QueryMapper.getMappedFields(QueryMapper.java:220) ~[spring-data-mongodb-4.1.1.jar:4.1.1]
	at org.springframework.data.mongodb.core.QueryOperations$QueryContext.getMappedFields(QueryOperations.java:363) ~[spring-data-mongodb-4.1.1.jar:4.1.1]
	at org.springframework.data.mongodb.core.MongoTemplate.doFind(MongoTemplate.java:2573) ~[spring-data-mongodb-4.1.1.jar:4.1.1]
	at org.springframework.data.mongodb.core.ExecutableFindOperationSupport$ExecutableFindSupport.doFind(ExecutableFindOperationSupport.java:177) ~[spring-data-mongodb-4.1.1.jar:4.1.1]
	at org.springframework.data.mongodb.core.ExecutableFindOperationSupport$ExecutableFindSupport.all(ExecutableFindOperationSupport.java:135) ~[spring-data-mongodb-4.1.1.jar:4.1.1]
	at org.springframework.data.mongodb.repository.support.SimpleMongoRepository$FluentQueryByExample.all(SimpleMongoRepository.java:391) ~[spring-data-mongodb-4.1.1.jar:4.1.1]
	at org.springframework.graphql.data.query.QueryByExampleDataFetcher$ManyEntityFetcher.getResult(QueryByExampleDataFetcher.java:751) ~[spring-graphql-1.2.1.jar:1.2.1]

Here is my set up :

@AllArgsConstructor
@NoArgsConstructor
@Getter
@Setter
@Builder
@EqualsAndHashCode
@QueryEntity
@Document(collection = "articles")
public class Article {

    @Id
    private String id;
    private String title;
    private Integer minutesRead;
    @DBRef(lazy = true)
    private User author;
}

@AllArgsConstructor
@NoArgsConstructor
@Getter
@Setter
@EqualsAndHashCode
@Builder
@QueryEntity
@Document(collection = "users")
public class User {

    @Id
    private String id;

    private String name;
    private Integer age;
    private Date createdAt;
    private String nationality;
    private List<String> friendsIds;
    @DBRef(lazy = true)
    private List<Article> articles;
}

@GraphQlRepository
public interface UserRepository extends MongoRepository<User, String>, QuerydslPredicateExecutor<User> {
}

here is my graphql schema:

type Query {
    users: [User]
     ...
}

👍 So while the following graphql seems to work fine:

query {
  users {
    id
    age
    name
    articles {
      id
    }
  }
}	

❌ adding extra properties on the nested articles object does not:

query {
  users {
    id
    age
    name
    articles {
      id
      title
    }
  }
}	

throwing org.springframework.data.mapping.MappingException: Invalid path reference articles.title; Associations can only be pointed to directly or via their id property.

Can you please shed some light as to what is the problem here ?

Also, I have noticed that when using MongoRepository along with QuerydslPredicateExecutor then QueryByExampleDataFetcher takes precedence over QuerydslDataFetcher. Is this expected behaviour?