typescript - 复杂的泛型类型约束和类型推断

Question

我正在尝试在 TypeScript 中构建一个微型 ORM，但我遇到了这个编译问题：

Type 'TObject' does not satisfy the constraint '{ [k in k]: object; }'.
  Type 'object' is not assignable to type '{ [k in k]: object; }'.

有谁知道为什么这个代码示例会引发编译错误？以及如何解决？

type NavigationProperties<T, TKey extends keyof T> = T[TKey] extends object ? TKey : never;
 type Relationship<T extends { [k in TMember]: TTarget }, TMember extends keyof T, TTarget extends object = object> = {
    name: TMember,
    isComposite?: boolean,
};
class A<TObject extends object>
{
    public readonly relationships: Partial<{ [k in NavigationProperties<TObject, keyof TObject>]: Relationship<TObject, k> }> = { }
}

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编辑

根据 Shivam 的要求，我正在为其添加更多上下文：当我有混合类型或不匹配类型时，我也想让它工作： 操场

标签： typescripttypescript-genericsmapped-types

Answer 1

问题

在类型Relationship中，T被约束到{ [k in TMember]: TTarget }，同时声明属性relationships，TObject 被传递的类型object是 的超类型{ [k in TMember]: TTarget }。因此不兼容。

解决方案

type NavigationProperties<T, TKey extends keyof T> = T[TKey] extends object ? TKey : never;

type TObject<T extends object, TType extends object> = { [K in keyof T]: TType }

type Relationship<T extends TObject<T, TType>, TKeys extends keyof T, TType extends object> = {
    name: TKeys,
    isComposite?: boolean,
};

type Relationships<T extends TObject<T, TType>, TType extends object> = {
    [K in NavigationProperties<T, keyof T>]?: Relationship<T, K, TType>
}

class A<T extends TObject<T, TType>, TType extends object = object> {
    public readonly relationships: Relationships<T, TType> = { }
}

// example usage
type MyType = {
    prop1: number
    porp2: string
}

type X = {
    a: MyType
    b: MyType
}

declare const obj: A<X>

obj.relationships // type is Relationships<X, object>

// type of obj1.relationships.a is Relationship<X, "a", object>
obj.relationships.a?.name // "a"
obj.relationships.b?.name // "b"

declare const obj1: A<X, MyType>

obj1.relationships // type is Relationships<X, MyType>

// type of obj1.relationships.a is Relationship<X, "a", MyType>
obj1.relationships.a?.name // "a"
obj1.relationships.b?.name // "b"

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