In my application, I have three resources: Device, SystemModelPartNumber and SystemModel. Each Device belongs to a SystemModelPartNumber via its part_number column. A SystemModelPartNumber belongs to a SystemModel via system_model_id. A SystemModel has many SystemModelPartNumbers.

This are the relevant parts of the Ash resources

defmodule Device do
  # ...
  relationships do
    belongs_to :system_model_part_number, SystemModelPartNumber do
      attribute_type :string
      source_attribute :part_number
      destination_attribute :part_number
    end
  end
  # ...
end

defmodule SystemModelPartNumber do
  attributes do
    attribute :part_number, :string, allow_nil?: false
  end

  relationships do
    belongs_to :system_model, SystemModel
  end

  identities do
    identity :part_number, [:part_number]
  end
end

# def SystemModel ...

When I was using Ecto, I was exposing SystemModel using has_one through: [:system_model_part_number, :system_model].

How can achieve a similar result using Ash?

For now I tried:

• Using a calculation

  calculations do
    calculate :system_model, :struct, expr(system_model_part_number.system_model) do
      constraints instance_of: SystemModel
    end
  end

but the DB query fails with:

* ** (Postgrex.Error) ERROR 42846 (cannot_coerce) cannot cast type bigint to jsonb

    query: SELECT d0."id", d0."device_id", d0."name", d0."online", s1."system_model_id"::bigint::jsonb::jsonb FROM "devices" AS d0 LEFT OUTER JOIN "public"."system_model_part_numbers" AS s1 ON d0."part_number" = s1."part_number" WHERE (d0."id"::bigint = $1::bigint) AND (d0."tenant_id"::bigint = $2::bigint)

• Using an aggregate (first): doesn’t work for belongs_to

The next option I’ve wanted to try was a manual has_one, but I want to check if I’m overcomplicating things before.