QN typeguard for relation loading

[ondratra]

Sometimes we encounter a hardly spotted bug in the mappings when we rely on some relation being loaded that is actually not loaded.

// invalid case
const video = await store.get(Video, { where: { id: SomeId } })
console.log(video.channel) // undefined; 

// valid case
const video = await store.get(Video, { where: { id: SomeId }, relations: ['channel'] })
console.log(video.channel)

As shown in the example, if a developer forgets to add relations: ['myEntity'] and then tries to access it, the error occurs. Currently, this is not caught by a compiler.

Let's try to improve our Typescript type guards for DatabaseManager so that .get and other methods return type that signals what relations are actually loaded and triggers compilation errors when not-loaded relations are used.