How to Retrieve Custom Objects from Spring Data JPA Grouped Query Results?

How to Retrieve Custom Objects from Spring Data JPA Grouped Query Results

Overview

Spring Data JPA provides mechanisms to execute JPQL queries and retrieve results as custom objects.

JPQL Queries

Step 1: Define a Bean Class

Create a simple bean class to represent the desired object structure:

<code class="java">public class SurveyAnswerStatistics {
  private String answer;
  private Long count;

  // Constructor for object instantiation
}</code>

Step 2: Use the Bean in the Repository Method

Modify the repository method to return instances of the bean class:

<code class="java">public interface SurveyRepository extends CrudRepository<Survey, Long> {
    @Query("SELECT new com.path.to.SurveyAnswerStatistics(v.answer, COUNT(v)) FROM Survey v GROUP BY v.answer")
    List<SurveyAnswerStatistics> findSurveyCount();
}</code>

Native Queries

If using native queries, JPA-specific syntax is not supported. Instead, use:

Step 1: Create a Projection Interface

Define a projection interface to specify the properties of the desired object:

<code class="java">public interface SurveyAnswerStatistics {
  String getAnswer();
  int getCnt();
}</code>

Step 2: Map Result Fields in the Query

Use the SQL AS keyword in the query to map result fields to projection properties:

<code class="java">@Query(nativeQuery = true, value =
          "SELECT v.answer AS answer, COUNT(v) AS cnt FROM Survey v GROUP BY v.answer")
    List<SurveyAnswerStatistics> findSurveyCount();
}</code>

Considerations

• Ensure the bean class path is fully-qualified.
• Use the new keyword when instantiating bean classes.
• Attribute order in the constructor must match the query's field order.
• Use projection interfaces for native queries.
• Map each result field using AS in native queries.

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