向实体添加新字段答案

【问题标题】：Adding a new field to an entity向实体添加新字段
【发布时间】：2020-07-01 14:07:30
【问题描述】：

我创建了一个director 类

@Entity(tableName = "director")
public class Director {

    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(name = "did")
    public int id;

    @ColumnInfo(name = "full_name")
    @NonNull
    public String fullName;


    public Director(@NonNull String fullName) {
        this.fullName = fullName;
    }
}

我决定通过添加迁移方法和向类添加字段来添加新列

@Entity(tableName = "director")
public class Director {

    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(name = "did")
    public int id;

    @ColumnInfo(name = "full_name")
    @NonNull
    public String fullName;

    @ColumnInfo(name = "age")
    public int age;

    public Director(@NonNull String fullName) {
        this.fullName = fullName;
    }
}

方法

static final Migration MIGRATION_1_2 = new Migration(1, 2) {
    @Override
    public void migrate(SupportSQLiteDatabase database) {

        String query = "ALTER TABLE 'director' ADD COLUMN 'age' INTEGER ";

        database.execSQL(query);

    }
};

当我执行代码时出现此错误

Caused by: java.lang.IllegalStateException: Migration didn't properly handle

它说它是预期的

age=Column{name='year', type='INTEGER', affinity='3', notNull=true, primaryKeyPosition=0, defaultValue='null'}},

找到了

age=Column{name='year', type='INTEGER', affinity='3', notNull=false, primaryKeyPosition=0, defaultValue='null'},

我可以看到问题出在“notnull”部分，但如何解决这个问题？我希望它为空。

【问题讨论】：

对于非空值，您必须提供默认值。

标签： java android android-room database-migration

【解决方案1】：

尝试将默认年龄值设置为 0

 static final Migration MIGRATION_1_2 = new Migration(1, 2) {

  @Override
  public void migrate(SupportSQLiteDatabase database) {
    database.execSQL(
      "ALTER TABLE 'director' ADD COLUMN 'age' INTEGER NOT NULL DEFAULT 0"
     );
  }
};

【讨论】：