Build Ash.Query dynamically from other Ash.Query structs

Ash Forum Ash Questions / Help
,
1

I’m building a module that builds up an Ash.Query from parameters. I want it to be able to “OR” two queries together, like this:

left_query = build_query(Member2, left_params)
right_query = build_query(Member2, right_params)

query = Ash.Query.filter(Member2, ^left_query or ^right_query)

But this doesn’t work. The resulting query is:

#Ash.Query<resource: Member2, filter: #Ash.Filter<true>>

This will always return all records.

How do I correctly compose a query from other queries?

Additionally, I don’t know what an Ash.Filter is. The full description in the documentation is “The representation of a filter in Ash.” I don’t know how Ash.Filter relates to Ash.Query. They seem to be related, but the documentation doesn’t appear to mention how they relate. There is no section in the Ash HQ Tutorials for Ash.Filter or Ash.Query.

The intro documentation for Ash.Filter includes this example:

Ash.Query.filter(resource, name == "Zardoz")
Ash.Query.filter(resource, first_name == "Zar" and last_name == "Doz")
Ash.Query.filter(resource, first_name == "Zar" and last_name in ["Doz", "Daz"] and high_score > 10)
Ash.Query.filter(resource, first_name == "Zar" or last_name == "Doz" or (high_score > 10 and high_score < -10))

There is another example of Ash.Query.filter() further down, but no explanation of the relation between Ash.Query and Ash.Filter.

Ash.Query.filter() returns an %Ash.Query{} struct, not an Ash.Filter. The documentation for Ash.Query.filter() contains the sentence “The filter is applied as an “and” to any filters currently on the query. For more information on writing filters, see: Ash.Filter.”. So the documentation says there’s is a link between the two modules, but doesn’t explain how, or what that link is.

2

Hey there! Sorry it took a while to get back to you. In Ash, we compose “expressions” to do this kind of building, as opposed to composing full queries. For example:

import Ash.Expr

defp build_filter(map) do
  Enum.reduce(map, true, fn {key, value}, acc -> 
    expr(^acc and ^ref(:field) == ^value)
  end)
end

Then you could or these together with

left_filter = build_filter(left_params)
right_filter = build_filter(right_params)
filter = expr(^left_query or ^right_query)

Ash.Query.filter(query, ^filter)

With that said, Ash also has a thing for this. It is missing some docs on the exact format accepted, but that is just elbow grease to be applied:

{:ok, filter} = Ash.Filter.parse(Resource, %{or: [%{a: 1}, %{b: 2}]})
# or even more succinctly
Ash.Query.filter_input(query, %{or: [%{a: 1}, %{b: 2}]})

So if you can use our format for filters, then you should do that, but if not then you’d build your own with expressions.

1 Like
3

As for the documentation for that input format, I intend to document it very soon. It needs to have a json schema because it is the same format as the one use in AshJsonApi’s filter query parameter.

2 Likes