Error with Enum.sort_by/3 (ArgumentError) you attempted to apply :field_name on)

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Hi everyone,

I’m prototyping some queries that I would need at work should we decide to go with Elixir and I wanted to see how Elixir, Ecto, Plug, and Cowboy would feel to me in real world context.

I’m getting this error when doing a Enum.sort_by/3 and I’m not understanding what I’m doing wrong. Here’s my Ecto schema (abbreviated):

@primary_key {:id, :integer, source: :RefID}
  schema "Supplier" do
    field :company_id, :integer, source: :CompanyID
    field :supplier_id, :integer, source: :SupplierID
    # ...
    field :last_modified, :naive_datetime, source: :LastModified

I’m auto-generating all Schema from a C# Linq-To-SQL DBML (but that’s a story for a later post). I’d like to migrate to Elixir from outdated 15-20 years old C# legacy business web application.

This is the function I created that I’m running from IEx:

defmodule Data.Test do
  import Ecto.Query

  alias DB.HiddenName.Supplier
  alias DB.HiddenName.Repo

  def report(id) do
    sup = Supplier
      |> where([s], s.company_id == ^id)
      |> order_by([s], desc: s.last_modified)
      |> select([s], s)
      |> Repo.all()

    sup
    |> Enum.group_by(fn x -> x.supplier_id end)
    |> Enum.map(fn {_k, v} -> Enum.take(v, 4) end)
    |> Enum.sort_by(&(&1.last_modified), {:desc, NaiveDateTime})
  end
end

The error I’m getting:

** (ArgumentError) you attempted to apply :last_modified on [%DB.HiddenName.Supplier{supplier_id: 123, company_id: 6, last_modified: ~N[2019-06-18 15:32:00]], :last_modified, [])

When I comment out the last Enum.sort_by there’s no error. I tried replacing the &1.last_modified with the primary key id and just use :desc as the 3 argument, but I’m getting the same error with :id instead of the NaiveDateTime :last_modified.

In short, I just want to sort by that field, and I’ll be completely honest I’m not very sure what &(&1.last_modified) does exactly (I’m in my day 2 with Elixir),

Probably basics question but I’m not phrasing that properly to get any answer myself.

Thanks for your time and help.