Question

我正在使用会议室数据库，但是我有一个表，忘了添加onDelete = CASCADE

以前的版本

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

我现在想要的是

@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

Ok finally I found the solution

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)");

Also change the schema in entity class to

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

The sequence is important.

Answer 2

SQLite不完全支持SQL ALTER TABLE标准：

仅RENAME TABLE，ADD COLUMN和RENAME COLUMN变体支持ALTER TABLE命令。其他种类的ALTER TABLE DROP COLUMN，ALTER COLUMN，ADD CONSTRAINT等操作省略。

Source

所以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)")

如何更改表并添加onDelete级联约束sqlite

2 个答案: