Model Schemas Not Generated Properly When Using Type Alases For Property Types

[taicho]

Steps to reproduce

A common pattern in TypeScript is to pull types from properties of other objects ensuring type safety and reducing redundancy...

1. Create a Model or Entity:

@model({})
export class User extends Model {
    @property()
    public email : string;
    @property()
    public name : string;
    @property()
    public streetAddress : string;
    @property()
    public passwordHashedWithAPinchOfSalt : string;
}

2. Create another with only a subset properties (maybe for use as a basic DTO for another endpoint):

@model({})
export class UserLogin extends Model {
    @property()
    public email : User["email"]; // --> When JSONSchema is generated this will resolve to an empty object type instead of string.
    @property()
    public password : string;
}

Current Behavior

As shown in the repro steps above, when pulling property types from other object types an incorrect JSON Schema is formed and validation on that property is no longer enforced due to the type not being inferred correctly.

Expected Behavior

In the example above, since User.email is of type string, it would be expected that the resolved type of UserLogin.email should be the same e.g. type string.

Sidenote. A "workaround" for this is to manually specify the type in the @property decorator options but this is less than ideal, defeats the purpose of eliminating redundancy, and I would argue unexpected.

Additional information

• darwin x64 10.16.0

├── @loopback/authentication@2.1.8
├── @loopback/boot@1.4.3
├── @loopback/context@1.20.1
├── @loopback/core@1.9.0
├── @loopback/openapi-v3@1.6.4
├── @loopback/repository@1.10.1
├── @loopback/rest@1.16.2
├── @loopback/rest-explorer@1.2.4
├── @loopback/service-proxy@1.2.4
├── loopback-connector-mongodb@4.2.0

Related Issues

None found upon basic search.

[bajtos]

I am afraid this is a limitation of TypeScript.

Here is the code emitted for your UserLogin class:

let UserLogin = class UserLogin extends __1.Model {
};
__decorate([
    __1.property(),
    __metadata("design:type", Object)
], UserLogin.prototype, "email", void 0);
__decorate([
    __1.property(),
    __metadata("design:type", String)
], UserLogin.prototype, "password", void 0);
UserLogin = __decorate([
    __1.model()
], UserLogin);

As you can see on the line 3, TypeScript is providing the following design:type metadata for the email property: Object. LoopBack is using this metadata to infer the property type, because that's the only information available at runtime. And because TypeScript thinks the type is Object, LoopBack defines the property as object too.

Feel free to search TypeScript issue tracker to see if this is a known problem and perhaps open a new issue if it's not.

As for LoopBack, I think our best option is to improve the documentation to make this fact easier to find by users. Do you have any suggestions where would you look for such information yourself?

Would you like to contribute the doc changes yourself? See our Contributing guide and Submitting a pull request to LoopBack 4 to get started.

[taicho]

Ah. I'm relatively new to LB4 - I assumed the OpenAPI spec was getting generated via TS API by parsing the AST rather than reflection metadata, that makes sense. Also interesting that the injected metadata doesn't handle this situation on the TypeScript side, definitely seems like a gap in the compiler.

In any case, @bajtos thanks for the quick reply and breakdown. My best guess to provide footnote/warning/caveat would be in Key Concepts->Models. That being said, I'll look at contributing the change to the documentation on this side and the issue on the TypeScript side. Thanks!

[dhmlau]

@taicho, i've assigned this issue to you because you have a PR #3871 in progress