Function overriding causes warning

Hi, I have a “base” module with this

defmodule DbModel do
  @callback schema()  :: Map.t

  defmacro __using__(_opts) do
    quote do
      @behaviour DbModel

      ...boiler plates with actual implementation...

      def schema(),       do: %{}
      defoverridable schema: 0
    end
  end

  ...stuffs implemented for this base module...
end

Then I have a module

defmodule CustomerM do
  defstruct id:            nil,
                 ...other props...
  use ExConstructor

  @impl DbModel
  def schema(), do: %{
     ...specific schema validation and conversion...
  }

  use DbModel
end

It works well for my application, but one thing bugs me is this:

warning: this clause cannot match because a previous clause at line xx always matches
  lib/db/model/customer_m.ex:31

Yes, I know about Ecto etc, but I choose this way of handling db model for a reason.

My question is more general: What is the correct way to achieve this kind of function overriding?

Thanks a lot!