向实体添加新字段

Posted 2023-03-14

【中文标题】向实体添加新字段

【英文标题】：Adding a new field to an entity

【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=Columnname='year', type='INTEGER', affinity='3', notNull=true, primaryKeyPosition=0, defaultValue='null', 

找到了

age=Columnname='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。

【参考方案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"
     );
  
;