Android Room FOREIGN KEY约束失败

Question

I'm trying to design and implement a folder tree structure in Android SQLite with help of Android Room Persistence (an ORM) which Google introduced in I/O 2017. In my design, a folder can contain another folders and files. Here are my codes for folder and file:

File Model:

@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
}

And Here is Folder Code:

@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;
}

Which have a foreign key on ID column of it self for parent.

and here is 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);
}

But when I make a folder object and try to insert it:

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

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

        }
    });

get this error:

FOREIGN KEY constraint failed

         --------- 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)

Does anyone know what's wrong or suggest another design for my project table?

Answer 1

I got this to work, but not using int primary keys. I'm not a huge fan of those for this sort of ORM scenario, just because of these sorts of problems.

So, here's a self-referential Category class that uses a UUID for its primary key:

/***
 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;
  }
}

Now you can have DAO methods like:

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

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

Answer 2

It took me a loong time to figure out that having a parentColumns element which is an autogenerated primary key does not seem to be allowed at all, even if it is a uuid using a Kotlin data class with a default value like this:

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

Always make sure the parent column is specified directly by you.