Android Room FOREIGN KEY 约束失败

Question

我正在尝试借助 Google 在 I/O 2017 中引入的 Android Room Persistence（一种 ORM）在 Android SQLite 中设计和实现文件夹树结构。 在我的设计中，一个文件夹可以包含另一个文件夹和文件。 这是我的文件夹和文件代码：

文件模型：

@Entity(tableName = "files", foreignKeys = @ForeignKey(entity = Folder.class,
    parentColumns = "id",
    childColumns = "parent_id",
    onDelete = CASCADE))
public class File {
    @PrimaryKey(autoGenerate = true)
    private int id;

    private String title;
    private Date creationDate;

    @ColumnInfo(name = "parent_id")
    public int parentId;

    //here setters and getters skipped but exist in original code
}

这是文件夹代码：

@Entity(tableName = "folders", foreignKeys = @ForeignKey(entity = Folder.class,
    parentColumns = "id",
    childColumns = "parent_id",
    onDelete = CASCADE,onUpdate = SET_NULL))
public class Folder {
    @PrimaryKey(autoGenerate = true)
    private int id;

    private String name;

    @ColumnInfo(name = "parent_id")
    private int parentId;
}

在它自己的 ID 列上有一个外键作为父级。

这里是 FolderDAO：

@Dao
public interface FolderDAO {
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    public void insertFolder(Folder... folders);

    @Update
    public void updateFolder(Folder... folders);


    @Delete
    public void deleteFolders(Folder... folders);

    @Query("SELECT * FROM folders")
    List<Folder> getAll();

    @Query("SELECT * FROM folders WHERE id IN (:folderIds)")
    List<Folder> loadAllByIds(int[] folderIds);
}

但是当我创建一个文件夹对象并尝试插入它时：

AsyncTask.execute(new Runnable() {
        @Override
        public void run() {
            Folder folder = new Folder();
            folder.setName("All");

            DatabaseInstance.getInstance(getApplicationContext()).folderDAO().insertFolder(folder);

        }
    });

得到这个错误：

外键约束失败

         --------- beginning of crash
E/AndroidRuntime: FATAL EXCEPTION: AsyncTask #1
              Process: sahraei.hamidreza.com.note, PID: 1835
              android.database.sqlite.SQLiteConstraintException: FOREIGN KEY constraint failed (code 787)
                  at android.database.sqlite.SQLiteConnection.nativeExecuteForLastInsertedRowId(Native Method)
                  at android.database.sqlite.SQLiteConnection.executeForLastInsertedRowId(SQLiteConnection.java:782)
                  at android.database.sqlite.SQLiteSession.executeForLastInsertedRowId(SQLiteSession.java:788)
                  at android.database.sqlite.SQLiteStatement.executeInsert(SQLiteStatement.java:86)
                  at android.arch.persistence.db.framework.FrameworkSQLiteStatement.executeInsert(FrameworkSQLiteStatement.java:80)
                  at android.arch.persistence.room.EntityInsertionAdapter.insert(EntityInsertionAdapter.java:80)
                  at sahraei.hamidreza.com.note.DAO.FolderDAO_Impl.insertFolder(FolderDAO_Impl.java:80)
                  at sahraei.hamidreza.com.note.ItemListActivity$1.run(ItemListActivity.java:62)
                  at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
                  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
                  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
                  at java.lang.Thread.run(Thread.java:818)

有人知道我的项目表有什么问题或建议另一种设计吗？

Answer 1

我得到了这个工作，但没有使用 int 主键。我不是那种 ORM 场景的超级粉丝，只是因为这些问题。

所以，这是一个自引用的 Category 类，它使用 UUID 作为其主键：

/***
 Copyright (c) 2017 CommonsWare, LLC
 Licensed under the Apache License, Version 2.0 (the "License"); you may not
 use this file except in compliance with the License. You may obtain  a copy
 of the License at http://www.apache.org/licenses/LICENSE-2.0. Unless required
 by applicable law or agreed to in writing, software distributed under the
 License is distributed on an "AS IS" BASIS,  WITHOUT WARRANTIES OR CONDITIONS
 OF ANY KIND, either express or implied. See the License for the specific
 language governing permissions and limitations under the License.
 */

package com.commonsware.android.room.dao;

import android.arch.persistence.room.Entity;
import android.arch.persistence.room.ForeignKey;
import android.arch.persistence.room.Ignore;
import android.arch.persistence.room.Index;
import android.arch.persistence.room.PrimaryKey;
import java.util.UUID;
import static android.arch.persistence.room.ForeignKey.CASCADE;

@Entity(
  tableName="categories",
  foreignKeys=@ForeignKey(
    entity=Category.class,
    parentColumns="id",
    childColumns="parentId",
    onDelete=CASCADE),
  indices=@Index(value="parentId"))
public class Category {
  @PrimaryKey
  public final String id;
  public final String title;
  public final String parentId;

  @Ignore
  public Category(String title) {
    this(title, null);
  }

  @Ignore
  public Category(String title, String parentId) {
    this(UUID.randomUUID().toString(), title, parentId);
  }

  public Category(String id, String title, String parentId) {
    this.id=id;
    this.title=title;
    this.parentId=parentId;
  }
}

现在您可以使用以下 DAO 方法：

@Query("SELECT * FROM categories WHERE parentId IS NULL")
Category findRootCategory();

@Query("SELECT * FROM categories WHERE parentId=:parentId")
List<Category> findChildCategories(String parentId);

Answer 2

我花了很长时间才弄清楚，似乎根本不允许使用自动生成的主键 parentColumns 元素，即使它是使用 Kotlin 数据类的 uuid，默认值如下这个：

class MyClass {
    // Don't do this
    @PrimaryKey(autoGenerate = true)
    var uuid: String = UUID.randomUUID()
}

始终确保父列由您直接指定。