如何解决EdgeDB的‘constraints cannot contain paths with more than one hop’错误

问题 #

我在.esdl文件中编写了极简示例代码，试图通过约束限制Combo类型中obj1与obj2的类型组合（例如不支持A与A组合，支持A与B组合），但触发错误：constraints cannot contain paths with more than one hop。我参考了官方文档，且未使用multi属性，仍无法解决问题。使用的是EdgeDB v5.6。

ESDL代码示例 #

abstract type Base;
type A extending Base;
type B extending Base;
type C extending Base;
type D extending Base;

type Combo {
    obj1: Base;
    obj2: Base;

    # 我的目标是允许某些类型组合，禁止其他组合。
    # 例如：不支持A+A，但支持A+B。
    constraint expression on (
        not (.obj1 is A and .obj2 is A) or
        not (.obj1 is A and .obj2 is D) or
        not (.obj1 is B and .obj2 is C)
    );

    # 我尝试过的另一种写法，同样报错
    # constraint expression on (
    #     not (__subject__.obj1 is typeof A and __subject__.obj2 is typeof A) or
    #     not (__subject__.obj1 is typeof A and __subject__.obj2 is typeof D) or
    #     not (__subject__.obj1 is typeof B and __subject__.obj2 is typeof C)
    # );
}

错误信息 #

error: constraints cannot contain paths with more than one hop
    ┌─ /<path>/default.esdl:11:9
    │
11  │ ╭         constraint expression on (
12  │ │             not (.obj1 is A and .obj2 is A) or
13  │ │             not (.obj1 is A and .obj2 is D) or
14  │ │             not (.obj1 is B and .obj2 is C)
15  │ │         );
    │ ╰──────────^ error

解决方案 #

1. 调整约束为全局约束 #

EdgeDB v5.x中，类型内部的constraint expression仅允许引用单个属性路径，无法同时关联多个属性（如obj1和obj2）。要实现跨属性的组合约束，需要将约束定义为全局约束，放在类型定义外部：

abstract type Base;
type A extending Base;
type B extending Base;
type C extending Base;
type D extending Base;

type Combo {
    obj1: Base;
    obj2: Base;
}

# 全局约束：禁止指定的类型组合
constraint expression on Combo (
    not (
        (.obj1 is A and .obj2 is A) or
        (.obj1 is A and .obj2 is D) or
        (.obj1 is B and .obj2 is C)
    )
);

2. 修正逻辑表达式 #

原约束逻辑存在错误：not X or not Y or not Z等价于not (X and Y and Z)，这意味着只有当三个禁止组合同时满足时才会触发约束（实际不可能）。正确逻辑应为禁止任意一个指定组合，即使用not (X or Y or Z)（等价于not X and not Y and not Z），只要匹配任意一个禁止组合就会被拦截。

额外优化 #

如果需要更严谨的类型校验，可使用typeof操作符明确检查类型：

constraint expression on Combo (
    not (
        (typeof .obj1 = typeof A and typeof .obj2 = typeof A) or
        (typeof .obj1 = typeof A and typeof .obj2 = typeof D) or
        (typeof .obj1 = typeof B and typeof .obj2 = typeof C)
    )
);

内容的提问来源于stack exchange，提问作者Friendly King