继承表方案的有效方法

Question

我计划显示 CarServiceEntries 的列表视图。 CarServiceEntry 类包含服务的基本数据：

@Entity
abstract class CarServiceEntry(
    private int id;
    private Date date;
    private float odometer;
    
    /*
    getters and setters constructor....
    */
)

CarServiceEntry 是抽象的，因为继承它的类有更详细的信息：

@Entity
abstract class Income extends CarServiceEntry(
    @Converter(....)
    private PaymentType paymentType;
    private float totalAmount;
    
    /*
    getters and setters constructor....
    */
)

问题出现在构建 SQLScheme 时。 由于我想一起加载所有条目而不是为每个 CarServiceEntry 创建一个自己的 sqltable 并查询每个表（getAllIncomes() getAllExpenses() 等），我如何使用一个 sqltabletable 加载每个 CarServiceEntry(income,expense, service) 用于加载（如果可能的话）。 我不喜欢的当前方法如下所示：

CREATE TABLE CarServiceEntry(
    id INTEGER PRIMARY KEY,
    serviceType TEXT CHECK CONSTRAINT (....) //expense, income, service)
    date,
    odometer,
    
    /*
    A LOT of fields as each service has its own "unique" fields and I put it all together which I don' really like :( loading 30+ fields just for one sql statement is not something I like, unless I don't have any other option.
    */
)

我宁愿寻找这样的解决方案：

CREATE TABLE CarServiceEntry(
    id INTEGER PRIMARY KEY,
    date DATE,
    odometer NUMBER
)

CREATE TABLE Income INHERITS CarServiceEntry(
    paymentType TEXT,
    totalAmount NUMBER
    /*some other fields*/
)

CREATE TABLE Expense INHERITS CarServiceEntry(
    location TEXT
    totalCost NUMBER
    /*some other fields*/
)

==>

@Query("SELECT * FROM CarServiceEntry") //this should also return Income and Expense table
Flowable<List<CarServiceEntry>> getAllEntries();

有没有办法做到这一点？还是创建一个包含大量字段的表是唯一的方法？

根据：sql inheritance 为我的 subentrytype 表创建 FOREIGN KEY 是一种继承，但这并不能解决我的问题，因为我仍然需要加载每个表。我想唯一的解决方案是使用一个包含许多空值字段的表？

Answer 1

SQLite 不支持继承，我相信利用 SQLite 和 Room 支持的关系会很简单，甚至更简单。

通过房间创建多个表非常简单，创建和处理关系也是如此。所以我建议采用典型的方法。

这是一个基于我认为你正在努力完成的示例。

首先是 CarServiceEntry 表（稍后将包含与之相关的费用和收入）：-

public class CarServiceEntry {
    @PrimaryKey
    private Long id;
    private String date;
    private float odometer;

    public CarServiceEntry(){}

    .... getters and setters removed for brevity

    /* more convenient constructor (see cse3 in example) */
    /* @Ignore to supress Room warning                   */
    @Ignore
    public CarServiceEntry(String date, Float odometer) {
        this.date = date;
        this.odometer = odometer;
    }
}

• 注意简化，因此不需要 TypeConverters

接下来是收入表：-

@Entity(tableName = "income",
        foreignKeys = {
            @ForeignKey(
                    entity = CarServiceEntry.class,
                    parentColumns = "id",
                    childColumns = "incomeCarServiceEntryId",
                    onDelete = CASCADE,
                    onUpdate = CASCADE
            )
        },
        indices = {@Index(
                value = {"incomeCarServiceEntryId"}
                )}
        )
public class Income {
    @PrimaryKey
    private Long incomeId;
    private Long incomeCarServiceEntryId;
    private int paymentType;
    private float totalAmount;

    .... getters and setters
}

• 注意外键 = { .... } 或索引 = { .... } 是必需的，但建议将它们用作确保引用完整性的帮助
• 注意附加列 incomeCarServiceEntryId 这是相关 CarServiceEntry 的 ID。

接下来是费用表（和收入表差不多）：-

@Entity(tableName = "expense",
        foreignKeys = {
                @ForeignKey(
                        entity = CarServiceEntry.class,
                        parentColumns = {"id"},
                        childColumns = {"expenseCarServiceEntryId"},
                        onDelete = CASCADE,
                        onUpdate = CASCADE
                )
        },
        indices = {
                @Index(
                        value = {"expenseCarServiceEntryId"}
                        )}
)
public class Expense {
    @PrimaryKey
    private Long expenseId;
    private long expenseCarServiceEntryId;
    private String location;
    private float totalCost;
    
    .... getters and setters

}

现在是一个 POJO（不是表），用于提取名为 CarServiceEntryWithIncomeWithExpense 的相关数据（即 CarServiceEntry 以及所有相关收入和所有相关费用）：-

public class CarServiceEntryWithIncomeWithExpense {

    @Embedded
    CarServiceEntry carServiceEntry;
    @Relation(entity = Income.class,parentColumn = "id",entityColumn = "incomeCarServiceEntryId")
    List<Income> incomeList;
    @Relation(entity = Expense.class,parentColumn = "id",entityColumn = "expenseCarServiceEntryId")
    List<Expense> expenseList;
}

• 是的就是这样

Now the Dao's (All in One) AllDao :-

@Dao
interface AllDao {
    @Insert
    long insert(CarServiceEntry carServiceEntry);
    @Insert
    long insert(Expense expense);
    @Insert
    long insert(Income income);
    @Query("SELECT * FROM car_service_entry")
    List<CarServiceEntryWithIncomeWithExpense> getAllCarServiceEntriesWithIncomesAndWithExpenses();
}

名为 Database 的 @Database（包括单例方法）（最好使用其他名称）：-

@Database(entities = {CarServiceEntry.class,Income.class,Expense.class},version = 1)
public abstract class Database extends RoomDatabase {
    abstract AllDao getAllDao();

    private static volatile Database instance;

    public static Database getInstance(Context context) {
        if (instance == null) {
            instance = Room.databaseBuilder(context,Database.class,"carservice.db")
                    .allowMainThreadQueries()
                    .build();
        }
        return instance;
    }
}

• 注意，为了演示的简洁和方便，使用了主线程。

最后将它们放在一起并演示 MainActivity。

该演示添加了 3 个 carServiceEntries，包括收入和支出（第 3 个都没有）。然后它提取 3 个 carServiceEntries 以及所有收入和支出。遍历提取的内容并将提取的内容输出到日志中。

:-

public class MainActivity extends AppCompatActivity {

    Database  db;
    AllDao dao;
    private static final String TAG = "CSEINFO";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        db = Database.getInstance(this);
        dao = db.getAllDao();
        /* Add a Service Entry noting it's ID */
        CarServiceEntry cse1 = new CarServiceEntry();
        cse1.setDate("2021-06-01");
        cse1.setOdometer(5120.78F);
        long cse1Id = dao.insert(cse1);
        /* Add another Service Entry noting it's id */
        CarServiceEntry cse2 = cse1;
        cse2.setDate("2021-06-02");
        cse2.setOdometer(7065.83F);
        long cse2Id = dao.insert(cse2);

        /* Use a single Income to add 3 Income Entries for (linked to) the 1st Service Entry */
        Income incomeEntry = new Income();
        incomeEntry.setIncomeCarServiceEntryId(cse1Id);
        incomeEntry.setPaymentType(10);
        incomeEntry.setTotalAmount(120.00F);
        dao.insert(incomeEntry);
        incomeEntry.setPaymentType(20);
        incomeEntry.setTotalAmount(230.00F);
        dao.insert(incomeEntry);
        incomeEntry.setPaymentType(15);
        incomeEntry.setTotalAmount(75.55F);
        dao.insert(incomeEntry);

        /* Use the same Income Entry to add 1 Entry for the 2nd Service Entry */
        incomeEntry.setIncomeCarServiceEntryId(cse2Id);
        incomeEntry.setPaymentType(25);
        incomeEntry.setTotalAmount(134.56F);
        dao.insert(incomeEntry);

        /* Add some Expense Entries */
        Expense expenseEntry = new Expense();
        expenseEntry.setExpenseCarServiceEntryId(cse1Id);
        expenseEntry.setLocation("London");
        expenseEntry.setTotalCost(500.00F);
        dao.insert(expenseEntry);
        expenseEntry.setLocation("New York");
        expenseEntry.setTotalCost(60.66F);
        dao.insert(expenseEntry);
        expenseEntry.setExpenseCarServiceEntryId(cse2Id);
        expenseEntry.setLocation("Paris");
        dao.insert(expenseEntry);
        expenseEntry.setLocation("Hamburg");
        dao.insert(expenseEntry);
        expenseEntry.setLocation("Madrid");
        dao.insert(expenseEntry);

        dao.insert(new CarServiceEntry("2021-06-03",1765.34F));

        for (CarServiceEntryWithIncomeWithExpense cse: dao.getAllCarServiceEntriesWithIncomesAndWithExpenses()) {
            Log.d(
                    TAG,
                    "CSE ID = " + cse.carServiceEntry.getId() +
                            " Date = " + cse.carServiceEntry.getDate() +
                            " ODO = " + cse.carServiceEntry.getOdometer()
            );
            for (Income i: cse.incomeList) {
                Log.d(
                        TAG,
                        "\tIncome Payment Type is " + i.getPaymentType() + " Total is " + i.getTotalAmount()
                        );
            }
            for(Expense e: cse.expenseList) {
                Log.d(
                        TAG,
                        "\tExpense Location is " + e.getLocation() + " Total is " + e.getTotalCost()
                );
            }
        }
    }
}

结果

以下输出到日志：-

2021-06-11 13:01:35.116 D/CSEINFO: CSE ID = 1 Date = 2021-06-01 ODO = 5120.78
2021-06-11 13:01:35.116 D/CSEINFO:  Income Payment Type is 10 Total is 120.0
2021-06-11 13:01:35.116 D/CSEINFO:  Income Payment Type is 20 Total is 230.0
2021-06-11 13:01:35.116 D/CSEINFO:  Income Payment Type is 15 Total is 75.55
2021-06-11 13:01:35.116 D/CSEINFO:  Expense Location is London Total is 500.0
2021-06-11 13:01:35.116 D/CSEINFO:  Expense Location is New York Total is 60.66
2021-06-11 13:01:35.116 D/CSEINFO: CSE ID = 2 Date = 2021-06-02 ODO = 7065.83
2021-06-11 13:01:35.117 D/CSEINFO:  Income Payment Type is 25 Total is 134.56
2021-06-11 13:01:35.117 D/CSEINFO:  Expense Location is Paris Total is 60.66
2021-06-11 13:01:35.117 D/CSEINFO:  Expense Location is Hamburg Total is 60.66
2021-06-11 13:01:35.117 D/CSEINFO:  Expense Location is Madrid Total is 60.66
2021-06-11 13:01:35.117 D/CSEINFO: CSE ID = 3 Date = 2021-06-03 ODO = 1765.34