一对多 / 多对一
以下以 Kotlin、MySQL 为演示标准
一对多几乎称得上是最普遍的关联模式，只要存在一般性的“从属关系”，基本就可以视为一条一对多关联。
在数据模型层面，这条关联会表现为由子实体的外键列指向主实体的主键列的外键约束。而在模型层面，这条关联自然可以反向存在。
下面就对多对一和一对多这对反转的关联进行简单的建模演示。
可直接复制 copy-data.json 至模型设计器中。
订单与订单明细（多对一，子指向主）
copy-data.json
json
{
"tables": [
{
"name": "order_detail",
"comment": "订单明细",
"remark": "",
"type": "TABLE",
"superTables": [],
"columns": [
{
"orderKey": 1,
"name": "id",
"comment": "ID",
"typeCode": -5,
"overwriteByRaw": false,
"rawType": "BIGINT",
"typeNotNull": true,
"dataSize": 0,
"numericPrecision": 0,
"partOfPk": true,
"autoIncrement": true,
"remark": "",
"logicalDelete": false,
"businessKey": false
},
{
"orderKey": 2,
"name": "order_id",
"comment": "订单 ID",
"typeCode": -5,
"overwriteByRaw": false,
"rawType": "BIGINT",
"typeNotNull": true,
"dataSize": 0,
"numericPrecision": 0,
"partOfPk": false,
"autoIncrement": false,
"remark": "",
"logicalDelete": false,
"businessKey": false
},
{
"orderKey": 3,
"name": "commodity_name",
"comment": "商品名",
"typeCode": 12,
"overwriteByRaw": false,
"rawType": "VARCHAR",
"typeNotNull": true,
"dataSize": 0,
"numericPrecision": 0,
"partOfPk": false,
"autoIncrement": false,
"remark": "",
"logicalDelete": false,
"businessKey": false
},
{
"orderKey": 4,
"name": "commodity_quantity",
"comment": "商品数量",
"typeCode": 4,
"overwriteByRaw": false,
"rawType": "INTEGER",
"typeNotNull": true,
"dataSize": 0,
"numericPrecision": 0,
"partOfPk": false,
"autoIncrement": false,
"remark": "",
"logicalDelete": false,
"businessKey": false
}
],
"indexes": []
},
{
"name": "sale_order",
"comment": "订单",
"remark": "",
"type": "TABLE",
"superTables": [],
"columns": [
{
"orderKey": 1,
"name": "id",
"comment": "ID",
"typeCode": -5,
"overwriteByRaw": false,
"rawType": "BIGINT",
"typeNotNull": true,
"dataSize": 0,
"numericPrecision": 0,
"partOfPk": true,
"autoIncrement": true,
"remark": "",
"logicalDelete": false,
"businessKey": false
},
{
"orderKey": 2,
"name": "serial_no",
"comment": "流水号",
"typeCode": 12,
"overwriteByRaw": false,
"rawType": "VARCHAR",
"typeNotNull": true,
"dataSize": 0,
"numericPrecision": 0,
"partOfPk": false,
"autoIncrement": false,
"remark": "",
"logicalDelete": false,
"businessKey": false
},
{
"orderKey": 3,
"name": "create_time",
"comment": "创建时间",
"typeCode": 93,
"overwriteByRaw": false,
"rawType": "TIMESTAMP",
"typeNotNull": true,
"dataSize": 0,
"numericPrecision": 0,
"partOfPk": false,
"autoIncrement": false,
"remark": "",
"logicalDelete": false,
"businessKey": false
}
],
"indexes": []
}
],
"associations": [
{
"type": "MANY_TO_ONE",
"fake": false,
"name": "fk_detail_to_order",
"updateAction": "",
"deleteAction": "",
"sourceTableName": "order_detail",
"targetTableName": "sale_order",
"columnReferences": [
{
"sourceColumnName": "order_id",
"targetColumnName": "id"
}
],
"dissociateAction": "DELETE"
}
],
"enums": [],
"optionsList": [
{
"x": 482,
"y": 0
},
{
"x": 0,
"y": 0
}
]
}以下就是关联属性的映射结果，就是一对 @ManyToOne 和 @OneToMany 属性：
kotlin
/**
* 订单
*/
@ManyToOne
@JoinColumn(
name = "order_id",
referencedColumnName = "id"
)
@OnDissociate(DissociateAction.DELETE)
val saleOrder: SaleOrderkotlin
/**
* 订单明细
*/
@OneToMany(mappedBy = "saleOrder")
val orderDetails: List<OrderDetail>而生成的外键即与模型一致，由 detail 指向 order：
sql
ALTER TABLE `order_detail`
ADD CONSTRAINT `fk_detail_to_order`
FOREIGN KEY (`order_id`)
REFERENCES `sale_order` (`id`);班级和学生（一对多，主指向子）
copy-data.json
json
{
"tables": [
{
"name": "edu_class",
"comment": "班级",
"remark": "",
"type": "TABLE",
"superTables": [],
"columns": [
{
"orderKey": 1,
"name": "id",
"comment": "ID",
"typeCode": -5,
"overwriteByRaw": false,
"rawType": "BIGINT",
"typeNotNull": true,
"dataSize": 19,
"numericPrecision": 0,
"partOfPk": true,
"autoIncrement": true,
"remark": "",
"logicalDelete": false,
"businessKey": false
},
{
"orderKey": 2,
"name": "class_code",
"comment": "班号",
"typeCode": 12,
"overwriteByRaw": false,
"rawType": "VARCHAR",
"typeNotNull": true,
"dataSize": 500,
"numericPrecision": 0,
"partOfPk": false,
"autoIncrement": false,
"remark": "",
"logicalDelete": false,
"businessKey": false
},
{
"orderKey": 3,
"name": "name",
"comment": "名称",
"typeCode": 12,
"overwriteByRaw": false,
"rawType": "VARCHAR",
"typeNotNull": true,
"dataSize": 500,
"numericPrecision": 0,
"partOfPk": false,
"autoIncrement": false,
"remark": "",
"logicalDelete": false,
"businessKey": false
}
],
"indexes": []
},
{
"name": "edu_student",
"comment": "学生",
"remark": "",
"type": "TABLE",
"superTables": [],
"columns": [
{
"orderKey": 1,
"name": "id",
"comment": "ID",
"typeCode": -5,
"overwriteByRaw": false,
"rawType": "BIGINT",
"typeNotNull": true,
"dataSize": 19,
"numericPrecision": 0,
"partOfPk": true,
"autoIncrement": true,
"remark": "",
"logicalDelete": false,
"businessKey": false
},
{
"orderKey": 2,
"name": "student_code",
"comment": "学号",
"typeCode": 12,
"overwriteByRaw": false,
"rawType": "VARCHAR",
"typeNotNull": true,
"dataSize": 500,
"numericPrecision": 0,
"partOfPk": false,
"autoIncrement": false,
"remark": "",
"logicalDelete": false,
"businessKey": false
},
{
"orderKey": 3,
"name": "name",
"comment": "名称",
"typeCode": 12,
"overwriteByRaw": false,
"rawType": "VARCHAR",
"typeNotNull": true,
"dataSize": 500,
"numericPrecision": 0,
"partOfPk": false,
"autoIncrement": false,
"remark": "",
"logicalDelete": false,
"businessKey": false
},
{
"orderKey": 4,
"name": "class_id",
"comment": "班级",
"typeCode": -5,
"overwriteByRaw": false,
"rawType": "BIGINT",
"typeNotNull": true,
"dataSize": 19,
"numericPrecision": 0,
"partOfPk": false,
"autoIncrement": false,
"remark": "",
"logicalDelete": false,
"businessKey": false
}
],
"indexes": []
}
],
"associations": [
{
"type": "ONE_TO_MANY",
"fake": false,
"name": "fk_edu_student_edu_class",
"updateAction": "",
"deleteAction": "",
"sourceTableName": "edu_class",
"targetTableName": "edu_student",
"columnReferences": [
{
"sourceColumnName": "id",
"targetColumnName": "class_id"
}
]
}
],
"enums": [],
"optionsList": [
{
"x": 455,
"y": 0
},
{
"x": 0,
"y": 0
}
]
}此时关联类型为一对多，但是本质还是多对一映射，因此生成的外键为 student 指向 class：
sql
ALTER TABLE `edu_student`
ADD CONSTRAINT `fk_edu_student_edu_class`
FOREIGN KEY (`class_id`)
REFERENCES `edu_class` (`id`);生成的属性也和上面的多对一没有本质差异
kotlin
/**
* 学生
*/
@OneToMany(mappedBy = "eduClass")
val eduStudents: List<EduStudent>kotlin
/**
* 班级
*/
@ManyToOne
@JoinColumn(
name = "class_id",
referencedColumnName = "id"
)
val eduClass: EduClass