szajbus
Pattern match to check if association is preloaded
Hi, I’m wondering if the following is a viable approach.
def followed_by?(%{followers: followers}, user) when is_list(followers) do
followers
|> Enum.any?(fn follower ->
follower.id == user.id
)
end
def followed_by?(offer, user) do
offer
|> assoc(:followers)
|> where(user_id: ^user.id)
|> Repo.exists?()
end
Depending on the context, an offer may or may not have the followers association preloaded.
Pattern matching is used to check this, so that I can avoid unneeded database query when it actually is preloaded.
My doubt is about the guard: is is_list a good approach here?
It obviously works, because %Ecto.Association.NotLoaded{} is not a list and since it’s a has_many assoctiation, it produces a list when loaded.
But how about the readability/understandability?
Marked As Solved
blatyo
I’d probably reverse it and match on %Ecto.Association.NotLoaded{}, because it would be clearer to me.
Also Liked
thomasbrus
Another thing to think about.. I think the first function knows a bit too much.
It reaches into the %Offer{} which happens to have followers preloaded, and thats why it works. Imagine %Offer{} had other associations that may or may not be preloaded but are needed by followed_by?. Then the pattern matching would have to take into account all these different scenarios.
Perhaps you can accomplish this in a different way (while still efficient). Let’s say you have a show page on which you show 1 offer. Then you can simply use the followed_by?(offer, user) you already have (2nd function).
But then perhaps there’s an index page with a list of offers which preloads followers (I assume). In that case I would perform an additional query: offers_followed_by_user(user). It can just return the ids of the offers for example. Then it is straightforward and efficient to check if a certain offer on the index page is followed by the user (offer.id in offers_followed_by_user).
thomasbrus
There is also Ecto.assoc_loaded?/1. But you can’t use it as a guard.
Also in terms of readability def followed_by?(%Offer{followers: .. } is a bit easer to follow I think.
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