[EDIT] 収支IDでカテゴリエンティティを取得するメソッドを作成しました。

src/main/java/com/example/demo/app/domain/service/CategoryService.java

@@ -11,4 +11,6 @@ public interface CategoryService {
 	public CategoryModel findOne(Long categoryId);
 	
 	public CategoryModel findOneByCategoryName(String categoryName);
+	
+	public CategoryModel findByTransactionId(Long transactionId);
 }

src/main/java/com/example/demo/app/domain/service/impl/CategoryServiceImpl.java

@@ -48,4 +48,9 @@ public class CategoryServiceImpl implements CategoryService {
 
 	}
 
+	public CategoryModel findByTransactionId(Long transactionId) {
+		return CategoryModel.convertFrom(categoryRepository.findByTransactionId(transactionId));
+	}
+	
+	
 }

src/main/java/com/example/demo/app/infra/repository/CategoryRepository.java

@@ -12,4 +12,7 @@ public interface CategoryRepository extends JpaRepository<CategoryEntity, Long>
 	@Query("select c from CategoryEntity c where c.categoryName = :categoryName")
 	public CategoryEntity findByCategoryName(@Param("categoryName") String categoryName);
 	
+	@Query("select c from CategoryEntity c inner join TransactionEntity t on c.categoryId = t.categoryEntity.categoryId where t.transactionId = :transactionId")
+	public CategoryEntity findByTransactionId(Long transactionId);
+	
 }