Question

我在使以下方案工作时遇到问题。学生可以参加考试。一名学生随着时间的推移进行了一些测试并获得了每项测试的分数。每个学生实体都有一个他们已完成映射为@OneToMany的测试列表。

现在我想选择所有已完成一系列分组标准测试的学生。例如，我想搜索所有拥有以下内容的学生：

第1组：完成“测试1”并得分“介于75和100之间”

和/或

第2组：完成“测试2”，得分“在50到80之间”

这是我到目前为止所做的，但它不能满足我的需要（不能通过多个参数进行搜索，这意味着我必须多次执行查询）：

SELECT s FROM Student s JOIN s.tests t WHERE t.score BETWEEN :minScore AND :maxScore AND t.testName = :testName

有没有办法使用单个NamedQuery来实现我想要的？要检索所有已完成与上述至少一个参数组匹配的测试的学生？我一直在尝试连接，但一直跑到墙上。

我在下面制作了一个示例代码框架，以说明我正在尝试做什么。

@Entity
@NamedQueries({
    @NamedQuery(name="Student.findStudentByParams", query="????????") // What should this query look like to satisfy the criteria? (see below for more detail)
})
public class Student {
    // .. Some other variables that are not relevant for this example

    @Id
    private String name;

    @OneToMany(fetch=FetchType.EAGER, mappedBy = "student")
    private List<Test> tests;

    // Setters and getters
}

@Entity
public class Test {
    private double score;
    private String testName;
    // .. Some other variables that are not relevant for this example

    @ManyToOne(cascade=CascadeType.ALL)
    private Student student;

    // Setters and getters
}

public class SearchParameters {
    private double minScore;
    private double maxScore;
    private String testName; 

    public SearchParameters(String minScore, String maxScore, String testName) {
        this.minScore = minScore;
        this.maxScore = maxScore;
        this.testName = testName;
    }

    // Setters and getters
}

public class MainClass {
    public static List<Student> getStudents(List<SearchParameters> searchParams) {

        // Database initialization stuff

        // What should the query look like to find all students that match any of the combined requirements in the searchParams list? 
        // Is it possible to do in a single query or should i make multiple ones?
        // What parameters should i set? Is it possible to put in the entire array and do some sort of join?

        // Retrieve all students which matches any of these search parameters:
        // Have either:
        //      Completed "Test 1" and got a score between 75 and 100
        // and/or:
        //      Completed "Test 2" and got a score between 50 and 80
        Query namedQuery = em.createNamedQuery("Student.findStudentByParams");
        namedQuery.setParameter(??); 

        return (List<Student>)namedQuery.getResultList();

    }
    public static void main() {
        List<SearchParams> searchParams = new ArrayList<SearchParams();
        searchParams.add(new SearchParameters(75,100, "Test 1"));
        searchParams.add(new SearchParameters(50,80, "Test 2"));

        // Retrieve all students which matches any of these search parameters:
        // Have either:
        //      Completed "Test 1" and got a score between 75 and 100
        // and/or:
        //      Completed "Test 2" and got a score between 50 and 80
        ArrayList<Student> students = getStudents(searchParams);
        for(Student s: students) // Print all user that match the criteria
        {
            System.out.println("Name: " + s.getName());
        }
    }   
}

Answer 1

您需要使用Criteria Builder（最终使用规范的Metamodel）。

尝试这样的事情（未经过测试的代码）：

EntityManager em;    // put here your EntityManager instance
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Student> cq = cb.createQuery(Student.class);
Root<Student> student = cq.from(Student.class);
Predicate predicate = cb.disjunction();
for (SearchParams param : searchParams) {
    ListJoin<Student, Test> tests = student.join(Student_.tests);
    Predicate tempPredicate1 = cb.equal(tests.get(Test_.testName), param.getTestName());
    Predicate tempPredicate2 = cb.ge(tests.get(Test_.score), param.getMinScore());
    Predicate tempPredicate3 = cb.le(tests.get(Test_.score), param.getMaxScore());
    Predicate tempPredicate = cb.and(tempPredicate1, tempPredicate2, tempPredicate3);
    predicate = cb.or(predicate, tempPredicate);
}
cq.where(predicate);
TypedQuery<Student> tq = em.createQuery(cq);
return tq.getResultList();

Answer 2

如果不动态编写查询，我看不出怎么可能。请考虑使用Criteria API创建它。

我会像这样设计查询：

select s from Student s where
    exists (select t.id from Test t where t.student.id = s.id and ...)
or
    exists (select t.id from Test t where t.student.id = s.id and ...)
or
    exists (...)

如你所见，有一个重复的模式，所有这些子查询都是相似的，并且被组合成一个分离。

JPA：根据多个子实体的多个标准选择实体

2 个答案: