Now that `Administrateur.email` is merely an alias to
`administrateur.user.email`,
and we changed every occurence of `administrateurs.pluck(:email)` to
`administrateurs.map(&:email)`, the new version using `map` may cause N+1
queries if the users have not been preloaded.
It makes sense to always preload the user when fetching an
Administrateur:
- Administrateur and User have a strongly coupled relationship
- It avoids N+1 queries everywhere in the app
Of course fetching an administrateur without needing its user will now do
an unecessary fetch of the associated user. But it seems better than
leaving a risk of N+1 queries in many places.
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