我正在尝试在Google I/O 2017中引入的Android Room Persistence(ORM)的帮助下,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;
}
其中在其自身的ID列上有一个外键为父级。
这是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,当我制作一个文件夹对象并尝试将其插入时:
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)
有谁知道出了什么问题或为我的项目表建议了另一种设计?
最佳答案
我得到了这个工作,但没有使用int
主键。我不是那种ORM场景的忠实拥护者,仅仅因为这些问题。
因此,这是一个自引用的Category
类,该类使用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;
}
}
现在您可以拥有DAO方法,例如:
@Query("SELECT * FROM categories WHERE parentId IS NULL")
Category findRootCategory();
@Query("SELECT * FROM categories WHERE parentId=:parentId")
List<Category> findChildCategories(String parentId);