Fix protocol Ecto.Queryable not implemented error

Question

I'm new to using Ecto and Elixir and I've come across an error that I can't explain. My code looks just like the example in the Ecto README.

Here are my modules for the Ecto Model and Query

defmodule Registration do
  use Ecto.Model

  schema "registrations" do
    field :user_id, :string
    field :created_at, :datetime, default: Ecto.DateTime.local
    field :updated_at, :datetime, default: Ecto.DateTime.local
  end
end

defmodule RegistrationQuery do
  import Ecto.Query

  def by_user(user_id) do
    query = from r in Registration,
          where: r.user_id == ^user_id,
         select: r
    Repo.all(query)
  end
end

Here is how I call the query function

registrations = Repo.all RegistrationQuery.by_user("underwater")

This all seems exactly in line with the Ecto documentation, and I can't find anything saying otherwise. But I get the following error.

protocol Ecto.Queryable not implemented for [%Ensalutilo.Registration{user_id: "underwater"}]

José Valim José Valim · Accepted Answer · 2014-12-27T22:09:45

Your by_user/1 function is already calling Repo.all, so when you call registrations = Repo.all(...) later on, you are passing the result of the first Repo.all as argument, which is a list as you see in the error message!

To be clear, you get this error message because you can pass anything that implements the Ecto.Queryable protocol into Repo.all.

Fix protocol Ecto.Queryable not implemented error

1 Answers