Demystifying Spring Data JPA: Mapping Nested Entities for Optimal Query Performance

Introduction:

In the world of Spring Data JPA, optimizing queries and fetching only the required data can significantly enhance performance. However, when it comes to using projections with nested entities, things can get a bit perplexing. In this blog post, we will explore various approaches to efficiently map nested entities in Spring Data JPA and overcome the limitations of projections.

Understanding the Challenge:

To start with, let’s examine the issue at hand. We have a scenario where we want to retrieve specific columns from nested entities using projections. However, it seems that projections with nested projections expose all columns, making further nesting impossible. This poses a challenge in optimizing queries and fetching only the necessary data.

Exploring Possible Solutions:

1. Attempting @Query: One approach is to utilize the @Query annotation. However, it’s challenging to map nested lists using this approach. Even if we eliminate the nesting of questions, we encounter issues like “Only one expression can be specified in the select list when the subquery is not introduced with EXISTS.” Additionally, mapping the test field as a String leads to IllegalAccessException.
2. Exploring @EntityGraph: Another option is using @EntityGraph, which allows us to specify the desired columns. However, it seems challenging to request only specified columns with this approach, as it retrieves the entire entity graph.
3. Utilizing @SqlResultSetMapping: We can also consider @SqlResultSetMapping to handle mapping nested lists. However, mapping nested lists using this approach can be complex and may not provide the desired outcome.
4. Manual Mapping: If all else fails, we might resort to receiving List<Object[]> and manually mapping the data. While this approach can be time-consuming and error-prone, it offers flexibility in mapping nested entities.

An Alternative Approach:

Using Projections: To address this challenge, we can adopt an alternative approach using projections. By defining projection interfaces, we can request only the required columns and structure our data accordingly. Let’s consider an example:

public interface TestAttemptList {
Long getId();
Test getTest();

interface Test {
    String getName();

    List<Question> getQuestions();

    interface Question {
        String getName();
    }
}

}

By employing this approach and utilizing the TestAttemptRepository interface, we can fetch data in the desired structure:

List getAllByTargetUserId(Long targetUserId);