Protocol to determine what repo to use for a struct defined by Ecto.Schema.

It defines a single callback, repo/1 which should return an Ecto.Repo module.

Examples

In its most simple form, the protocol implementation might just return a Repo.

defmodule Project do
  use Ecto.Schema

  defimpl DAL.Repo.Discoverable do
    def repo(_), do: Project.Repo
  end

  # ...schema definition
end

A Repo may be determined based on pattern matching of the target. For example, we may have projects that are for a company that has a separate repo to the main database.

defmodule Project do
  use Ecto.Schema

  defimpl DAL.Repo.Discoverable do
    # In practice this would probably not be hard coded
    @cba_id 100

    def repo(%{company_id: @cba_id}), do: Repo.Private.CBA
    def repo(_), do: Project.Repo
  end

  # ...schema definition
end

Use in Schemas

The DAL defines schemas as macros so that they can be shared across lots of contexts while minimising repetition. Consequently, the Discoverable protocol implementation should be defined in these macros rather than in the contexts themselves.

defmodule DAL.Types.Project do
  defmacro __using__(_) do
    quote do
      use Ecto.Schema

      defimpl DAL.Repo.Discoverable do
        def repo(_), do: Project.Repo
      end

      # ...schema definition
    end
  end
end