It's a webapp, they're likely already querying all the meal information from a mysql or postgresql db. Chances are they could have made a change to their db models and written some sql to handle food requirements checking instead of handling it after the query. But that could've been premature optimization ultimately, and to speed it up now, it's probably easier to optimize the slow ruby code with some rust instead.
Exactly, I dont mean to beat a dead horse, but most bog standard web apps have the quoted items in a db anyway, and these are very common problems.
For example if we have a few tables we can determine which menu items a user could eat:
users
user_ingredient_exclusions
ingredients
menu_ingredients
menu
-- items a user can eat based on not having any items in the excluded list
select distinct m.*
from menu m
inner join menu_ingredients mi on m.id = mi.menu_id
left join user_ingredient_exclusions e on e.ingredient_id = mi.ingredient_id
inner join user u on u.id = e.user_id
where e.id is null
and u.id = @id
