[英]How do I set up a Foreign Key
我设置的是两个表,一个用于用户创建的帐户,另一个用于让用户购买产品。
我把两张桌子都设置成这样
客户表
@PrimaryKey(autoGenerate = true)
private int custId;
@ColumnInfo(name = "user_name")
private String userName;
@ColumnInfo(name = "password")
private String password;
@ColumnInfo(name = "first_name")
private String firstName;
@ColumnInfo(name = "last_name")
private String lastName;
@ColumnInfo(name = "address")
private String address;
@ColumnInfo(name = "city")
private String city;
@ColumnInfo(name = "postal_code")
private String postalCode;
@ColumnInfo(name = "country")
private String country;
电话表
@PrimaryKey(autoGenerate = true)
private int productId;
private String phoneMake;
private String phoneModel;
private String phoneColor;
private String storageCapacity;
private Float price;
我设置的是两个外键,每个表一个。 我的最后一张表用于订购电话,这需要使用每个表中的两个主键。 我觉得我需要的是一个 ForeignKey,类似于已经创建的 PrimaryKey。 问题是我不确定如何将其实现到程序中。 我尝试做的一切都不起作用。 我查看了文档,但没有任何点击。 我希望你能帮助我提供正确的屏幕截图。 如果需要更多,请告诉我(这段代码是用 Java 代码编写的)
如果您只是希望客户拥有 1 部电话,那么您只有一个列(成员变量)用于存储电话产品 ID 的关系。
例如
private int mapToPhone; //<<<<< ADDED no need for @ColumnInfo the column name will be as per the variable name.
很明显,您将值设置为适当的值。
然后让客户获得电话的详细信息,然后您有一个 POJO,它使用@Embedded
批注嵌入父级(客户),并使用@Relation
批注嵌入子级(电话)。
例如:-
class CustomerWithPhoneDetails {
@Embedded
Customer customer;
@Relation(
entity = Phone.class,
parentColumn = "mapToPhone",
entityColumn = "productId"
)
Phone phoneDetails;
}
然后,您可以在 @Dao 注释接口/抽象 class 中使用一个方法来查询父表,但返回 POJO 或 POJO 的列表/数组,例如:-
@Query("SELECT * FROM Customer")
abstract List<CustomerWithPhoneDetails> getAllCustomersWithPhoneDeytails();
例子
根据您的代码和附加示例代码以及带@Database
注释的摘要 class:-
@Database(entities = {Customer.class,Phone.class}, version = 1, exportSchema = false)
abstract class TheDatabase extends RoomDatabase {
abstract AllDao getAllDao();
private static volatile TheDatabase instance = null;
public static TheDatabase getInstance(Context context) {
if (instance == null) {
instance = Room.databaseBuilder(context,TheDatabase.class,"the_database.db")
.allowMainThreadQueries()
.build();
}
return instance;
}
}
和一个活动,例如:-
public class MainActivity extends AppCompatActivity {
TheDatabase db;
AllDao dao;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
db = TheDatabase.getInstance(this);
dao = db.getAllDao();
long phone01ProductId = dao.insert(new Phone("PhoneMaker001","Model001","Color001","100Mb",111.11F));
long phone02ProductId = dao.insert(new Phone("PhoneMaker002","Model002","Color002","200Mb",222.22F));
dao.insert(new Customer("c001","password001","firstname001","lastname001","address001","city001","country001","postcode001",(int) phone01ProductId));
dao.insert(new Customer("c002","password002","firstname002","lastname002","address002","city002","country002","postcode002",(int) phone02ProductId));
for(CustomerWithPhoneDetails cwpd: dao.getAllCustomersWithPhoneDeytails()) {
Log.d("DBINFO","Customer is " + cwpd.customer.getUserName() + " etc. Phone is " + cwpd.phoneDetails.getProductId() + " etc." );
}
}
}
请注意,合适的构造函数已在 Phone 和 Customer class 中编码(默认/空构造函数和一个,使用@Ignore
注释进行注释,允许所有值禁止传递 id,如下例中所用)
请注意,最好将 long 而不是 int 用于 id 列。
结果
日志:-
D/DBINFO: Customer is c001 etc. Phone is 1 etc.
D/DBINFO: Customer is c002 etc. Phone is 2 etc.
应用检查:-
和:-
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