如何更改表并添加删除级联约束 sqlite

Question

我正在使用房间数据库，我有一个表，我忘了在其中添加onDelete = CASCADE

以前的版本

@Entity(indices = {@Index(value = {"info_id"})}, foreignKeys = @ForeignKey(entity = StudentClass.class, parentColumns = "id", childColumns = "info_id"))

我现在想要什么

@Entity(indices = {@Index(value = {"info_id"})}, foreignKeys = @ForeignKey(entity = StudentClass.class, parentColumns = "id", childColumns = "info_id", onDelete = CASCADE))

我正在尝试迁移数据库

static final Migration MIGRATION_1_2 = new Migration(1, 2) {
@Override
public void migrate(SupportSQLiteDatabase database) {
database.execSQL("What should I do here to add onDelete = CASCADE");
}
};

Answer 1

SQLite 并不完全支持 SQL ALTER TABLE 标准：

只有重命名表、添加列和重命名列变体 支持更改表命令。其他种类的更改表 诸如"删除列"、"更改列"、"添加约束"等操作 第四个被省略。

源

因此，添加约束的唯一方法是重新创建表并将旧表的数据复制到新表：

// 1. Rename your table
database.execSQL("ALTER TABLE Entity RENAME TO Entity_old")
// 2. Recreate the same entity, but now with the constraint
database.execSQL("""
CREATE TABLE Entity(
id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, 
parentId INTEGER, 
someField STRING, 
CONSTRAINT fk_Entity_entityId_EntityParent_id FOREIGN KEY (entityId) REFERENCES EntityParent(id) ON DELETE CASCADE
)
""")
// 3. Copy the data from the old table
database.execSQL("INSERT INTO Entity SELECT * FROM Entity_old")
// 4. Delete old table
database.execSQL("DROP TABLE Entity_old")
// 5. Recreate the indices
database.execSQL("CREATE INDEX index_Entity_id ON Entity(id)")

Answer 2

好的，我终于找到了解决方案

database.execSQL("CREATE TABLE attendance_temp(attendanceId INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, time INTEGER NOT NULL, student_id INTEGER, student_name TEXT , roll_number INTEGER, image_path TEXT, info_id INTEGER, present INTEGER , CONSTRAINT fk_class_info FOREIGN KEY (info_id) REFERENCES class_info(id) ON DELETE CASCADE)");
database.execSQL("INSERT INTO attendance_temp SELECT * FROM Attendance");
database.execSQL("DROP TABLE Attendance");
database.execSQL("ALTER TABLE attendance_temp RENAME TO Attendance");
database.execSQL("CREATE INDEX index_Attendance_info_id ON Attendance(info_id)");

同时将实体类中的架构更改为

@Entity(indices = {@Index(value = {"info_id"})}, foreignKeys = @ForeignKey(entity = StudentClass.class, parentColumns = "id", childColumns = "info_id", onDelete = CASCADE))

顺序很重要。