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  • 安卓学习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(MIGRATION_1_2)
                        .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。

     

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  • 原文地址:https://www.cnblogs.com/wuren-best/p/12260895.html
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