relationships is a RxJS pipe operator which is useful when we already have a stream of existing entities
and would like to extend them with relationships.
For that we need:
- a root selector with relationships we want to apply
- an observable stream of entities
Let's pretend that we have a
user$ stream which emits a user entity time to time.
Then, we could extend the emitted entity in the next way in order to get it with relationships.
The same can be done for a stream which emits an array of users. In this case, the root selector for arrays should be used.