Now that `Instructeur.email` is merely an alias to `instructeur.user.email`,
and we changed every occurence of `instructeurs.pluck(:email)` to
`instructeurs.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 Instructeur:
- Instructeur and User have a strongly coupled relationship
- It avoids N+1 queries everywhere in the app
Of course fetching an instructeur 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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