reduceFlat action helps to add data from a flat response to the store.
Imagine that a backend returns the next flat shape:
There is a selector that fetches this data from the store like that: a user with its company and the address of the company.
We only need to specify related bucket names from response as
flatKey in selector's meta:
Then the store can be updated by dispatching the
ngrxEntityRelationshipReducer will catch this action, parse the data and put it into the store like that: