Ash Framework•2mo ago

"Second Newest" in has_many

I'm trying to set up a relationship or calculation that can let me express the "second newest" item in a has_many relationship. Grabbing the most recent is easy peasy:

elixir 
    has_one :most_recent_log, MyLog do
      public? true
      from_many? true
      sort inserted_at: :desc
    end

elixir 
    has_one :most_recent_log, MyLog do
      public? true
      from_many? true
      sort inserted_at: :desc
    end

Ideally I'd be able to do something like:

elixir 
    has_one :second_most_recent_log, MyLog do
      public? true
      from_many? true
      sort inserted_at: :desc
      offset 1
    end

elixir 
    has_one :second_most_recent_log, MyLog do
      public? true
      from_many? true
      sort inserted_at: :desc
      offset 1
    end

But offset isn't available to a has_one relationship. What's the proper way to do this?My use case is to just display the difference between the most_recent_log.score and second_most_recent_log.score. I've tried a handful of different approaches at this point and none of them have panned out. Can't help but think there's a simple way I'm just not seeing.

5 Replies

Rise•2mo ago

This is also something I am trying to figure out right now but what could be the solution is to just use a mix of loading and sorting like shown in the docs

posts = MyApp.Blog.list_posts!(
  query: [
    filter: [
      author: [verified: true],
      comments_count: [greater_than: 5]
    ]
  ]
)

posts = MyApp.Blog.list_posts!(
  query: [
    filter: [
      author: [verified: true],
      comments_count: [greater_than: 5]
    ]
  ]
)

https://hexdocs.pm/ash/read-actions.html and put the logic into a calculation Also probably has_many is more suitable in this case even if it’s only two?

⿻ eileenOP•2mo ago

Yeah I'm putting the logic into a calculation now, but I was hoping there was a way to do it purely in expressions so I could push it to the data layer Probably not an important optimization though, tbh I have tried many variations involving has_many. None of them allow me to get the difference of score1 - score2, so far

Rise•2mo ago

Also for the latest you can use aggregate do First : name, : field End Yeah having some kind of calculation between different object is difficult without custom calculations from what I have seen, but I am new to ash The question is how would the SQL statement look like. If it’s not trivial then ash is probably also not that easy. Just my feeling

⿻ eileenOP•2mo ago

That makes sense! Seems simple in concept. Calculate the difference between result1.score and result2.score, when ordering by [inserted_at: desc]. But in practice, not so much

ZachDaniel•2mo ago

make a first relationship and filter where the id of the second does not equal the id of the first and sort by the thing you want to sort by

has_one :second_biggest_thing, Thing do
  filter expr(id != parent(first_biggest_thing.id))
  sort [number: :desc]
end

has_one :second_biggest_thing, Thing do
  filter expr(id != parent(first_biggest_thing.id))
  sort [number: :desc]
end

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"Second Newest" in has_many

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