Java ActiveAndroid多对多关系
我目前使用的是ActiveAndroid,在过去的几个小时里,我一直试图让多对多的关系发挥作用,但我就是无法让它发挥作用。我希望你能帮助我: 我有“学生”和“课程”两种模式,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“学生课程”模型中的内容:Java ActiveAndroid多对多关系,java,android,sql,sqlite,activeandroid,Java,Android,Sql,Sqlite,Activeandroid,我目前使用的是ActiveAndroid,在过去的几个小时里,我一直试图让多对多的关系发挥作用,但我就是无法让它发挥作用。我希望你能帮助我: 我有“学生”和“课程”两种模式,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“学生课程”模型中的内容: @Column(name = COURSE) public Course course; @Column(name = STUDENT) public Student student; public StudentCour
@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");
}
但是,我得到以下错误:
java.lang.ClassCastException:com.papinotas.models.StudentCourse不能强制转换为com.papinotas.models.Student
如果我将(学生)的演员阵容更改为(学生课程),则会出现以下错误:
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一个查询中实现这一点。任何帮助都将不胜感激。提前谢谢
PS:我已经看过了几乎所有我能找到的东西:和
如果我将(学生)的演员阵容改为(学生课程),我会得到以下错误
实际上,演员阵容应该是(List)
,但真正的问题在于这里模型的逻辑。您正在调用executeSingle(),但确实需要多个StudentCourse对象,以便获得课程的每个学生-课程关系。您的students()和courses()方法没有多大意义,因为一个StudentCourse对象只有一个Student和一个courses
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);
List enrolments=new Select().from(StudentCourse.class).where(“course=?”,selectedCourse.getId()).execute();
List studentsInCourse=new ArrayList();
学生(课程注册:入学人数)
学生在课程中。添加(注册人数。学生);
我不会为此使用getMany
。我会这样做:
return new Select()
.from(Student.class)
.innerJoin(StudentCourse.class).on("students.id = studentcourses.id")
.where("studentcourses.course = ?", courseId)
.execute();
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();
}
}
公共类DBHelper{
公共列表getRelatedContracts(客户端){
列表合同=空;
if(client!=null&&client.isCreated()){
合同=新选择()
.from(Contract.class)
.innerJoin(ClientContract.class).on(“ClientContract.Contract=Contract.id”)
.where(“ClientContract.Client=?”,Client.getId()
.execute();
}
归还合同;
}
公共列表GetRelatedClient(合同){
列表客户端=null;
if(contract!=null&&contract.isCreated()){
clients=newselect()
.from(Client.class)
.innerJoin(ClientContract.class).on(“ClientContract.Client=Client.id”)
.where(“ClientContract.Contract=?”,Contract.getId()
.execute();
}
返回客户;
}
//获得关系
公共客户端合同getClientContract(客户端,合同){
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();
}
返回客户合同;
}
//加法关系
public ClientContract addClientContract(客户端,合同){
ClientContract ClientContract=getClientContract(客户,合同);
if(client!=null&&contract!=null&&clientContract==null){
if(client.getId()==null)
client.save();
if(contract.getId()==null)
contract.save();
clientContract=新的clientContract();
clientContract.client=client;
clientContract.contract=合同;
clientContract.save();
}
返回客户合同;
}
//删除关系
public void deleteClientContract(客户,合同){
ClientContract ClientContract=getClientContract(客户,合同);
if(clientContract!=null&&contract.isCreated())
clientContract.delete();
}
}
我试图找到一种方法,通过一个查询获得所有学生;如果我没有弄错的话,这就可以进行N次查询PS:我理解你的观点,我同意这没有任何意义,我试图按照我上面发布的链接上的示例进行操作,但是它们并没有像我预期的那样工作。这是一个单选查询,但是它会做一些列表操作,这样你就可以得到一个学生的列表,而不是一个学生课程的列表。哦,“registration.student”不会对数据库进行查询,那么我相信你是对的!根据您使用它的目的,您可以将它保留为StudentCourse对象的列表(也可以更改适配器类或任何您使用它的目的)。如果您确实需要它作为一个学生对象列表,那么您可以执行SQL连接,使它能够更有效地扩展到大量学生。
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();
}
}