添加一对多关系android Room数据库？
添加一对多关系android Room数据库？
提问于 2021-08-06 01:55:48
我很难弄清楚如何在我的数据库中添加一对多关系。Book实体需要保存许多计数。当我尝试它时，我得到了一个错误，它说：
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.counter, PID: 11949
java.lang.RuntimeException: cannot find implementation for com.example.counter.db.AppDatabase. AppDatabase_Impl does not exist
at androidx.room.Room.getGeneratedImplementation(Room.java:97)
at androidx.room.RoomDatabase$Builder.build(RoomDatabase.java:1358)
at com.example.counter.db.AppDatabase.getInstance(AppDatabase.java:18)下面是我的AppDatabase的样子：
package com.example.counter.db;
import android.content.Context;
import androidx.room.Database;
import androidx.room.Room;
import androidx.room.RoomDatabase;
@Database(entities = {Count.class, Book.class}, version = 2, exportSchema = false)
public abstract class AppDatabase extends RoomDatabase {
private static AppDatabase instance;
public static AppDatabase getInstance(Context context){
if(instance != null){
return instance;
}else{
instance = Room.databaseBuilder(context, AppDatabase.class, "AppDatabase_database")
.build();
return instance;
}
}
public abstract CountDAO countDAO();
public abstract BookDAO bookDAO();
}计数实体：
package com.example.counter.db;
import androidx.room.Embedded;
import androidx.room.Entity;
import androidx.room.ForeignKey;
import androidx.room.PrimaryKey;
import java.text.SimpleDateFormat;
import java.util.Date;
import static androidx.room.ForeignKey.CASCADE;
@Entity
public class Count {
@PrimaryKey(autoGenerate = true)
private long count_id;
private long book_id;
private String title;
private Date date;
private int total;
public Count( long book_id, String title, Date date, int total) {
this.book_id = book_id;
this.title = title;
this.date = date;
this.total = total;
}
public long getCount_id() {
return count_id;
}
public void setCount_id(long id) {
this.count_id = id;
}
public long getBook_id() {
return book_id;
}
public void setBook_id(long book_id) {
this.book_id = book_id;
}
public String getCountTitle() {
return title;
}
public void setCountTitle(String title) {
this.title = title;
}
public Date getDate() {
return date;
}
public void setDate(Date date) {
this.date = date;
}
public int getTotal() {
return total;
}
public void setTotal(int total) {
this.total = total;
}
@Override
public String toString() {
return "Count{" +
"id=" + count_id +
", book_id=" + book_id +
", title='" + title + '\'' +
", date=" + date +
", total=" + total +
'}';
}
}账簿实体：
package com.example.counter.db;
import androidx.room.Entity;
import androidx.room.PrimaryKey;
import java.util.Date;
@Entity
public class Book {
@PrimaryKey(autoGenerate = true)
private long book_id;
private String title;
private Date date;
public Book(String title, Date date) {
this.title = title;
this.date = date;
}
public long getBook_id() {
return book_id;
}
public void setBook_id(long id) {
this.book_id = id;
}
public String getBookTitle() {
return title;
}
public void setBookTitle(String title) {
this.title = title;
}
public Date getDate() {
return date;
}
public void setDate(Date date) {
this.date = date;
}
@Override
public String toString() {
return "Book{" +
"id=" + book_id +
", title=" + title +
", date=" + date +
'}';
}
}BookWithCounts实体
package com.example.counter.db;
import androidx.room.Embedded;
import androidx.room.Relation;
import java.util.List;
public class BookWithCounts {
@Embedded
public Book book;
@Relation(
parentColumn = "book_id",
entityColumn = "book_id"
)
public List<Count> counts;
public BookWithCounts(Book book, List<Count> counts){
this.book = book;
this.counts = counts;
}
}CountDAO
package com.example.counter.db;
import androidx.room.Embedded;
import androidx.room.Relation;
import java.util.List;
public class BookWithCounts {
@Embedded
public Book book;
@Relation(
parentColumn = "book_id",
entityColumn = "book_id"
)
public List<Count> counts;
public BookWithCounts(Book book, List<Count> counts){
this.book = book;
this.counts = counts;
}
}BookDAO
import androidx.lifecycle.LiveData;
import androidx.room.Dao;
import androidx.room.Delete;
import androidx.room.Insert;
import androidx.room.Query;
import androidx.room.Transaction;
import androidx.room.Update;
import java.util.List;
@Dao
public interface BookDAO {
@Query("select * from Book")
LiveData<List<Book>> getAllBooks();
@Query("select * from Book where book_id=:id")
List<Book> getBookByID(long id);
@Query("select title from Book")
LiveData<List<String>> getAllBookTitles();
@Insert
long insertBook(Book book);
@Insert
long insertCount(Count count);
@Update
void updateBook(Book book);
@Delete
void deleteBook(Book book);
}BooksWithCountsDAO
package com.example.counter.db;
import androidx.room.Dao;
import androidx.room.Insert;
import androidx.room.Transaction;
import java.util.List;
@Dao
public interface BookWithCountsDAO {
@Transaction
@Insert
long insertBook(Book book);
@Insert
void insertCounts(List<Count> counts);
}我也有用于Books和Counts的视图模型，用于填充回收器视图。但是，当我尝试查看回收器视图时出现错误。
回答 1
Stack Overflow用户
发布于 2021-08-07 03:46:30
更新:我发现了问题所在。这是多种因素的综合作用。首先，我没有批注处理器依赖项。这些是您需要的依赖项。
def room_version = "2.3.0"
implementation "androidx.room:room-runtime:$room_version"
annotationProcessor "androidx.room:room-compiler:$room_version"我在使用不带日期转换器的Date类型时也遇到了问题。我不想弄乱日期转换器。因此，我只是简单地将类型更改为字符串。在我能让它工作之后。
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/68675242
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