Ash Elixir•5d ago

select fields from nested associations

I have two tables: users and user_profiles, with a one-to-one relationship. I know it might sound a bit silly, but I don't always need the information from the profile. Back to the point: I created a read action for searching that looks like this:

read :search_instructors_by_name do
  argument :query, :ci_string do
    constraints allow_empty?: true
    default ""
  end

  require Ash.Query

  prepare fn %{arguments: %{query: full_name}} = query, _context ->
    query
    |> Ash.Query.load(user_profile: :full_name)
    |> Ash.Query.filter(expr(role == :instructor))
    |> Ash.Query.filter(contains(user_profile.full_name, ^full_name))
  end
end

read :search_instructors_by_name do
  argument :query, :ci_string do
    constraints allow_empty?: true
    default ""
  end

  require Ash.Query

  prepare fn %{arguments: %{query: full_name}} = query, _context ->
    query
    |> Ash.Query.load(user_profile: :full_name)
    |> Ash.Query.filter(expr(role == :instructor))
    |> Ash.Query.filter(contains(user_profile.full_name, ^full_name))
  end
end

The full_name is a calculation field based on other fields in user_profiles. For the search functionality, I only need the user's id and full_name — nothing more. However, when I trigger the search, it returns a whole bunch of fields, including full_name and all other user_profile fields. Is there a way to modify the search so that it only returns a simple map with just the :id and :full_name fields? In Ecto terms, would be something like:

Comment
|> join(:inner, [c], p in Post, on: c.post_id == p.id)
|> select([c, p], {p.title, c.text})

Comment
|> join(:inner, [c], p in Post, on: c.post_id == p.id)
|> select([c, p], {p.title, c.text})

I’ve tried many things without success — such as adding Ash.Query.select(:id) or using modify_query, but I’m not sure how to get an alias. Another option is to do some post-processing with Ecto.map, but I’m wondering if there’s a more elegant solution.

7 Replies

ZachDaniel•5d ago

When loading, if you add the strict?: true option, then all attributes are not selected by default you'll still get structs back but you won't be selecting things you don't need

  prepare fn %{arguments: %{query: full_name}} = query, _context ->
    query
    |> Ash.Query.load([user_profile: :full_name], strict?: true)
    |> Ash.Query.filter(expr(role == :instructor))
    |> Ash.Query.filter(contains(user_profile.full_name, ^full_name))
  end

  prepare fn %{arguments: %{query: full_name}} = query, _context ->
    query
    |> Ash.Query.load([user_profile: :full_name], strict?: true)
    |> Ash.Query.filter(expr(role == :instructor))
    |> Ash.Query.filter(contains(user_profile.full_name, ^full_name))
  end

is that sufficient for what you're looking for? getting an actual map back is doable, but doesn't really have much tangible benefit in my experience

catrapatoOP•5d ago

Added another tool to my toolbox => strict?: true . Thanks! Out of curiosity, what do you recommend for returning a map?

ZachDaniel•5d ago

There are a few ways to do it One way, using declarative calculations:

calculate :profile_with_less_data, :map, expr(%{
  full_name: user_profile.full_name
})

calculate :profile_with_less_data, :map, expr(%{
  full_name: user_profile.full_name
})

One way, using dynamic calculations

  prepare fn %{arguments: %{query: full_name}} = query, _context ->
    query
    |> Ash.Query.calculate(:user_profile_with_less_data, :map, expr(%{full_name: user_profile.full_name}))
    |> Ash.Query.filter(expr(role == :instructor))
    |> Ash.Query.filter(contains(user_profile.full_name, ^full_name))
  end

  prepare fn %{arguments: %{query: full_name}} = query, _context ->
    query
    |> Ash.Query.calculate(:user_profile_with_less_data, :map, expr(%{full_name: user_profile.full_name}))
    |> Ash.Query.filter(expr(role == :instructor))
    |> Ash.Query.filter(contains(user_profile.full_name, ^full_name))
  end

catrapatoOP•5d ago

Interesting!!

ZachDaniel•5d ago

In the second one, it would be under user.calculations.user_profile_with_less_data If you just want to make full_name available as a field on user you can also easily do that

calculate :full_name, :string, expr(user_profile.full_name)

calculate :full_name, :string, expr(user_profile.full_name)

Which would give you

  prepare fn %{arguments: %{query: full_name}} = query, _context ->
    query
    |> Ash.Query.load(:full_name)
    |> Ash.Query.filter(expr(role == :instructor))
    |> Ash.Query.filter(contains(full_name, ^full_name))
  end

  prepare fn %{arguments: %{query: full_name}} = query, _context ->
    query
    |> Ash.Query.load(:full_name)
    |> Ash.Query.filter(expr(role == :instructor))
    |> Ash.Query.filter(contains(full_name, ^full_name))
  end

catrapatoOP•5d ago

OMG! This is just bananas — in a good way! Thanks, Zach. I’m all set. 🙏 This is great! After applying your suggestion, I can see that I'm now making only one query instead of two. 🚀

forest•4d ago

That last way is awesome.

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select fields from nested associations

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