7

我目前正在使用 ActiveAndroid，并且在过去的几个小时里一直在尝试建立多对多的关系，但我就是无法让它发挥作用。我希望你能帮助我：

我有模型“学生”和“课程”，一个学生可以有很多课程，一个课程有很多学生。基本上这就是我在“StudentCourse”模型中的内容：

 @Column(name = COURSE)
 public Course course;

 @Column(name = STUDENT)
 public Student student;

 public StudentCourse(Student student, Course course) {
 super();
 this.student = student;
 this.course = course;
 }
//
 public StudentCourse(){ }

 public List<Course> courses(){
 return getMany(Course.class, "StudentCourse");
 }
 public List<Student> students(){
 return getMany(Student.class, "StudentCourse");
 }

现在我要做的是获取“课程 X 中的所有学生”，使用以下代码：

((Student) new Select().from(StudentCourse.class).where("course = ?",selectedCourse.getId()).executeSingle()).students();

但是我收到以下错误：

java.lang.ClassCastException：com.papinotas.models.StudentCourse 无法转换为 com.papinotas.models.Student

如果我将 (Student) 的演员表更改为 (StudentCourse)，我会收到以下错误：

android.database.sqlite.SQLiteException：没有这样的列：students.StudentCourse（代码1）：，编译时：SELECT * FROM students WHERE students.StudentCourse=1

我的主要目标是希望在 1 个查询中实现这一目标。任何帮助将不胜感激。提前致谢！

PS：我已经看过几乎所有我能找到的东西：Active Android 多对多关系和https://github.com/pardom/ActiveAndroid/issues/46

4

3 回答 3

12

我不会用getMany这个。我会这样做：

return new Select()
    .from(Student.class)
    .innerJoin(StudentCourse.class).on("students.id = studentcourses.id")
    .where("studentcourses.course = ?", courseId)
    .execute();
于 2014-10-05T23:08:46.690 回答
2

像魅力一样工作：

客户端类

 @Table(name = "Client")
 public class Client extends Model{}

合同类

 @Table(name = "Contract")
 public class Contract extends Model{}

客户与合同的关系

 @Table(name = "ClientContract")
 public class ClientContract extends Model {

    @Column(name = "Client", onDelete = Column.ForeignKeyAction.CASCADE)
    public Client client;

    @Column(name = "Contract", onDelete = Column.ForeignKeyAction.CASCADE)
    public Contract contract;
}

数据库助手

public class DBHelper {

 public List<Contract> getRelatedContracts(Client client) {
        List<Contract> contracts = null;

        if (client != null && client.isCreated()) {
            contracts = new Select()
                    .from(Contract.class)
                    .innerJoin(ClientContract.class).on("ClientContract.Contract = Contract.id")
                    .where("ClientContract.Client = ?", client.getId())
                    .execute();
        }
        return contracts;
    }

    public List<Client> getRelatedClients(Contract contract) {
        List<Client> clients = null;

        if (contract != null && contract.isCreated()) {
            clients = new Select()
                    .from(Client.class)
                    .innerJoin(ClientContract.class).on("ClientContract.Client = Client.id")
                    .where("ClientContract.Contract = ?", contract.getId())
                    .execute();
        }
        return clients;
    }

    // get relation
    public ClientContract getClientContract(Client client, Contract contract) {
        ClientContract clientContract = null;
        if (client != null && contract != null && client.isCreated() && contract.isCreated()) {

            clientContract = new Select().from(ClientContract.class)
                    .where("Client = ?", client.getId())
                    .where("Contract = ?", contract.getId())
                    .executeSingle();
        }
        return clientContract;
    }

    // add relation
    public ClientContract addClientContract(Client client, Contract contract) {
        ClientContract clientContract = getClientContract(client, contract);

        if (client != null && contract != null && clientContract == null) {

            if (client.getId() == null)
                client.save();

            if (contract.getId() == null)
                contract.save();

            clientContract = new ClientContract();
            clientContract.client = client;
            clientContract.contract = contract;
            clientContract.save();

        }
        return clientContract;
    }

    // delete relation
    public void deleteClientContract(Client client, Contract contract) {
        ClientContract clientContract = getClientContract(client, contract);
        if (clientContract != null && contract.isCreated())
            clientContract.delete();
    }
}
于 2016-08-29T10:16:36.890 回答
1

如果我将 (Student) 的演员表更改为 (StudentCourse)，我会收到以下错误...

演员表实际上应该是 to (List<StudentCourse>)，但真正的问题在于您的模型的逻辑。您正在调用 executeSingle()，但您确实需要多个 StudentCourse 对象，以便获得课程的每个 Student-Course 关系。您的 students() 和 course() 方法没有多大意义，因为一个 StudentCourse 对象只有一个 Student 和一个 Course。

我会这样做：

List<StudentCourse> enrolments = new Select().from(StudentCourse.class).where("course = ?",selectedCourse.getId()).execute();

List<Student> studentsInCourse = new ArrayList<Student>();
for(StudentCourse enrolment:enrolments)
    studentsInCourse.add(enrolment.student);
于 2014-10-05T22:47:22.880 回答