我正在寻找一种使用room创建一对多关系的方法。问题是要获取特定数据。我在官方文档中找到了推荐的方法。
Customer.class
@Entity(tableName = "customers")
data class Customer(
val name: String
) {
@PrimaryKey(autoGenerate = true) val id: Long = 0
}
Order.class
@Entity(tableName = "orders")
data class Order(
val customerId: Long,
val date: Long,
val description: String
) {
@PrimaryKey(autoGenerate = true) val id: Long = 0
}
CustomerWithOrders.class
class CustomerWithOrders() {
@Embedded
lateinit var customer: Customer
@Relation(parentColumn = "id", entityColumn = "customerId", entity = Order::class)
var orders: List<Order> = arrayListOf()
}
Dao.interface
@Transaction
@Query("SELECT * FROM customers")
fun getCustomersWithOrders(): LiveData<List<CustomerWithOrders>>
但是官方文档没有说明如何在保持一对多关系的同时有条件地提出请求。 我写了一个示例查询，但是它提供了不正确的数据。
查询不正确的数据。
@Transaction
@Query("SELECT * FROM customers LEFT JOIN orders ON orders.customerId=customers.id WHERE date BETWEEN :dateStart AND :dateEnd")
fun getCustomersWithOrdersByDate(dateStart: Calendar, dateEnd: Calendar): LiveData<List<CustomerWithOrders>>
请告诉我如何正确完成查询，或告诉源代码正在执行类似操作的位置。而且无论如何。谢谢。
答案 0 :(得分：1)
您可以使用以下内容作为基础（java而不是kotlin）：-
CustomerWithOrders.java
public class CustomerWithOrders {
Customer customer;
List<Order> orders;
//<<<<<<<<<<< this constructor >>>>>>>>>>
public CustomerWithOrders(Customer customer, List<Order> orders) {
this.customer = customer;
this.orders = orders;
}
public Customer getCustomer() {
return customer;
}
public void setCustomer(Customer customer) {
this.customer = customer;
}
public List<Order> getOrders() {
return orders;
}
public void setOrders(List<Order> orders) {
this.orders = orders;
}
}
与（在日期之间添加到WHERE子句中）（在 PersonDao.java 中）
@Query("SELECT * FROM Orders WHERE customerId=:customerId")
abstract List<Order> getCustomerOrders(long customerId);
，然后使用类似：-
List<Customer> allCustomers = mPDB.personDao().getAllCustomers(); //<<<<<<<< Get the required Customers (all in this example)
ArrayList<CustomerWithOrders> allCWO = new ArrayList<>(); //<<<<<<<<<< Empty CustomerWithOrders array
for (Customer c: allCustomers) {
allCWO.add(new CustomerWithOrders(c,mPDB.personDao().getCustomerOrders(c.getId()))); //<<<<<<<< tie the orders to the customer
}
//<<<<<<<<< for demo >>>>>>>>>>
String TAG = "CWOINFO";
StringBuilder sb = new StringBuilder();
for (CustomerWithOrders cwo: allCWO ) {
sb.append("\nCustomer is " + cwo.getCustomer().getName());
for (Order o: cwo.getOrders()) {
sb.append("\n\tOrder is " + o.getDescription());
}
}
Log.d(TAG,sb.toString());
结果（列出所有要检查的订单）：-
2019-06-29 18:27:49.368 D/ORDERINFO: OrderID = 1Description = Order1 for Customer1Cust ID = 1
2019-06-29 18:27:49.368 D/ORDERINFO: OrderID = 2Description = Order2 for Customer1Cust ID = 1
2019-06-29 18:27:49.368 D/ORDERINFO: OrderID = 3Description = Order3 for Customer1Cust ID = 1
2019-06-29 18:27:49.368 D/ORDERINFO: OrderID = 4Description = Order1 for Customer2Cust ID = 2
2019-06-29 18:27:49.368 D/ORDERINFO: OrderID = 5Description = Order2 for Customer2Cust ID = 2
2019-06-29 18:27:49.368 D/ORDERINFO: OrderID = 6Description = Order3 for Customer2Cust ID = 2
2019-06-29 18:27:49.368 D/ORDERINFO: OrderID = 7Description = Order4 for Customer2Cust ID = 2
2019-06-29 18:27:49.368 D/ORDERINFO: OrderID = 8Description = Order1 for Customer3Cust ID = 3
<<<<<<<<<< The output from the above code >>>>>>>>>>
2019-06-29 18:27:49.377 D/CWOINFO: Customer is Customer1
Order is Order1 for Customer1
Order is Order2 for Customer1
Order is Order3 for Customer1
Customer is Customer2
Order is Order1 for Customer2
Order is Order2 for Customer2
Order is Order3 for Customer2
Order is Order4 for Customer2
Customer is Customer3
Order is Order1 for Customer3
通过将以下方法添加到 CustomerWithOrders.java
public void addOrder(Order newOrder) {
this.orders.add(newOrder);
}
并将以下内容添加到Dao中：-
@Query("SELECT * FROM customers WHERE id=:customerId")
abstract Customer getCustomerById(long customerId);
@Query("SELECT * FROM orders LEFT JOIN customers ON orders.customerId = customers.id WHERE date BETWEEN :dateStart AND :dateEnd ORDER BY customerId")
abstract List<Order> getOrders(long dateStart, long dateEnd);
@Transaction
public List<CustomerWithOrders> getCustomersWithOrdersInDateRange(long dateStart, long dateEnd) {
List<Order> ordersWithCustomer = getOrders(dateStart,dateEnd);
ArrayList<CustomerWithOrders> cwo = new ArrayList<>();
long currentCustomerId = 0;
for (Order o: ordersWithCustomer) {
if (o.getCustomerId() != currentCustomerId) {
currentCustomerId = o.getCustomerId();
cwo.add(new CustomerWithOrders(getCustomerById(o.getCustomerId()),new ArrayList<Order>()));
}
cwo.get(cwo.size()-1).addOrder(o);
}
return cwo;
}
您可以按照以下代码使用代码：-
List<CustomerWithOrders> selectedCustomersWithOrders = mPDB.customerOrderDao().getCustomersWithOrdersInDateRange(0,99999999999999999L);
笔记没有经过全面测试，因为我通常使用allowMainThreadQueries()作为答案
@Query("SELECT * FROM orders LEFT JOIN customers ON orders.customerId = customers.id WHERE date BETWEEN :dateStart AND :dateEnd ORDER BY customerId")
abstract LiveData<List<Order>> getOrders(long dateStart, long dateEnd); //<<<<<<<<<<< LD
@Transaction
public List<CustomerWithOrders> getCustomersWithOrdersInDateRange(long dateStart, long dateEnd) {
LiveData<List<Order>> ordersWithCustomer = getOrders(dateStart,dateEnd); //<<<<<<<<<<< LD
ArrayList<CustomerWithOrders> cwo = new ArrayList<>();
long currentCustomerId = 0;
for (Order o: ordersWithCustomer.getValue()) { //<<<<<<<<<<< LD
if (o.getCustomerId() != currentCustomerId) {
currentCustomerId = o.getCustomerId();
cwo.add(new CustomerWithOrders(getCustomerById(o.getCustomerId()),new ArrayList<Order>()));
}
cwo.get(cwo.size()-1).addOrder(o);
}
return cwo;
}