Improving FindOneOptions.relations typing

[luisgurmendezMLabs]

Feature Description

Improve Typescript definition

The Problem

The relations type of the FindOneOptions interface can be improved. Now it's represented by string[] and that is really annoying and prone to errors.

The Solution

Now that typescript 4.1 is out we can now define types as template literal string type https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-1.html

So this gives us the advantage of making the nested objects properties as types!

I built up a quick example of how we can implement this:
https://www.typescriptlang.org/play?ts=4.1.2#code/JYOwLgpgTgZghgYwgAgIIBN1QgZxwbwFgAoZM5HMbCMALgqtAHMBuE85AGzjE5CfoAZHoP5tiAXxIlQkWIhTCwopkVJcewMAFd0EeiG0BbAEbRxZTgHt+W3fuSHT5klOIzw0eEmQBVHNBqHCBwRg6UUMwW5HCY2Hj0GFi4OOJuJGAAngAOKAAKkUZawABuKAC87MgAPgyR-FW1TmZQjcgmwEyybSZWVpwQcCBtOJmm-W2GnJxt2iB6MKAQ6OLSxFm5yABqcJzaEADyMAA8ACoAfMjlyKcA2gDWEJlWMDcAuqvrOSioCEh4ADlcJB0AUrLkoGBgLgtsA4AARKxgAFIzQ2M6Xa47PaHE74W55ZCgZCPZ6vU5veiEiAAD0g8xwdWYAH5kAAKO55N7IWn09CMgrAIpQsrIVmE+gAAwAJPg8hIAHSy37-HBAyjLMEQqEwuGI5GoqHozlvc4SSXIACUBggZSgEnOnw2KH80HVIK10B1jOuKpS7s1UHBXuhOFhCKRKLAaJAx1dUEdyAA9EnkAAiEJhNM1dOxZJ4BURCA0bO1NN5+I4BXcXj8auaHR6Uu5uIpeu1pjVmxdRsQNMkIA

If this is added as a type we could then change the relations type with string[] AccessNestedPropertiesViaDotNotation<Entity>

Considered Alternatives

An alternative can be a combination of what we remove here: 8cd8710#diff-91e84474846af2b18f59cff2ae65ec2c38001069b382885e40307aef68435658L21

so: relations?: ((keyof Entity) | string)[];

Are you willing to resolve this issue by submitting a Pull Request?

• [x ] Yes, I have the time, and I know how to start.
  (If it's considered a good idea).

[nebkat]

I've been doing some tests with this syntax, it will be very useful in future for all locations where property paths are used!

We will have to use something like downlevel-dts if we intend on maintaining backwards compatibility to users with older versions of TypeScript, since the template literal types won't be supported. I have mentioned it here #6810 (comment). Currently template literals aren't converted by downlevel-dts so that will also have to be implemented first.

[luisgurmendezMLabs]

I didn't thought of the backwards compatibility, that is surely a problem. I guess till your comment #6810 (comment) isn't answered this will get blocked. Should I close this pr?