'安卓学习04'

2020-02-04

今天主要学习了数据库的迁移。

1、原entity的修改

原先的entity可以直接修改，例如原来的为：

package com.example.roombasic;

import androidx.room.ColumnInfo;
import androidx.room.Entity;
import androidx.room.PrimaryKey;

@Entity
public class Word {
    @PrimaryKey(autoGenerate = true)
    private int id;

    @ColumnInfo(name = "english_word")
    private String word;

    @ColumnInfo(name = "chinese_mean")
    private String chineseMean;

    public Word(String word, String chineseMean) {
        this.word = word;
        this.chineseMean = chineseMean;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getWord() {
        return word;
    }

    public void setWord(String word) {
        this.word = word;
    }

    public String getChineseMean() {
        return chineseMean;
    }

    public void setChineseMean(String chineseMean) {
        this.chineseMean = chineseMean;
    }
}

可以直接修改为：

package com.example.roombasic;

import androidx.room.ColumnInfo;
import androidx.room.Entity;
import androidx.room.PrimaryKey;

@Entity
public class Word {
    @PrimaryKey(autoGenerate = true)
    private int id;

    @ColumnInfo(name = "english_word")
    private String word;

    @ColumnInfo(name = "chinese_mean")
    private String chineseMean;
    
    @ColumnInfo(name = "is_ok")
    private boolean isOk;

    public boolean isOk() {
        return isOk;
    }

    public void setOk(boolean ok) {
        isOk = ok;
    }

    public Word(String word, String chineseMean) {
        this.word = word;
        this.chineseMean = chineseMean;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getWord() {
        return word;
    }

    public void setWord(String word) {
        this.word = word;
    }

    public String getChineseMean() {
        return chineseMean;
    }

    public void setChineseMean(String chineseMean) {
        this.chineseMean = chineseMean;
    }
}

新添加的属性记得添加get和set函数。

2、database的修改

1、不保留原有数据的迁移

不保留原有数据的修改，可以直接将数据库类的版本修改为原版本+1，然后构建数据库类的时候添加一个.fallbackToDestructiveMigration()。

package com.example.roombasic;

import android.content.Context;

import androidx.room.Database;
import androidx.room.Room;
import androidx.room.RoomDatabase;
@Database(entities = {Word.class},version = 2,exportSchema = false)
public abstract class WordDatabase extends RoomDatabase {
    private static WordDatabase INSTANCE;
    static synchronized WordDatabase getDatabase(Context context){
        if (INSTANCE == null) {
            INSTANCE = Room.databaseBuilder(context.getApplicationContext(),WordDatabase.class,"word_database")
                    .fallbackToDestructiveMigration()
                    .build();
        }
        return INSTANCE;
    }

    public abstract WordDao getWordDao();
}

2、保留数据的迁移

此时需要使用另一个方法.addMigrations()，并添加自定义的迁移方式，如：

package com.example.roombasic;

import android.content.Context;

import androidx.annotation.NonNull;
import androidx.room.Database;
import androidx.room.Room;
import androidx.room.RoomDatabase;
import androidx.room.migration.Migration;
import androidx.sqlite.db.SupportSQLiteDatabase;

@Database(entities = {Word.class},version = 2,exportSchema = false)
public abstract class WordDatabase extends RoomDatabase {
    private static WordDatabase INSTANCE;
    static synchronized WordDatabase getDatabase(Context context){
        if (INSTANCE == null) {
            INSTANCE = Room.databaseBuilder(context.getApplicationContext(),WordDatabase.class,"word_database")
                    .addMigrations()
                    .build();
        }
        return INSTANCE;
    }

    public abstract WordDao getWordDao();

    private static final Migration MIGRATION_1_2 = new Migration(1,2) {
        @Override
        public void migrate(@NonNull SupportSQLiteDatabase database) {
            database.execSQL("ALTER TABLE word ADD COLUMN is_ok INTEGER NOT NULL DEFAULT 0");
        }
    };
}

Migration(1,2) 中的两个参数是迁移的两个版本号
ALTER TABLE word ADD COLUMN is_ok INTEGER NOT NULL DEFAULT 0
- 是增加一列，名称为is_ok，不能为空，默认值为0。
- sqlite中没有boolean中，用int代替。