Ecto config in dependencies

Question

I have two mix projects P1 and P2.

P1 depends on ecto and provides a genserver interface. It is also added as a dependency in P2 only to have access to some client functions. So P2 does not try to start P1. It is started independently.

The problem is that P2 cannot be compiled. mix compile says that the the adapter configuration for P1.Repo is missing.

I do not want to include any repo configuration for P1 inside P2. Is there any way to use functionality provided by P1's modules inside P2 under these circumstances?

anthonator · Answer 1 · 2018-06-05T16:04:11.097

This is Ecto specific, but you can provide compile-time config when defining a module as a repo.

defmodule MyApp.Repo do
  use Ecto.Repo, otp_app: :my_app,
                 adapter: Ecto.Adapters.Postgres
end

You can then add runtime config in the repo's init/2 callback.

defmodule MyApp.Repo do
  use Ecto.Repo, otp_app: :my_app,
                 adapter: Ecto.Adapters.Postgres

  def init(_type, config) do
    new_config = Keyword.put(config, :url, System.get_env("DATABASE_URL"))

    {:ok, new_config}
  end
end

A more generic solution would be to provide application config as part of mix.exs's :env option. I believe this is runtime only though (someone please correct me if I'm wrong).

Here's what mix help compile.app provides on this.

:env - default values for the application environment. The application environment is one of the most common ways to configure applications. See the Application module for mechanisms to read and write to the application environment.

Here's an example of application/0 in mix.exs using :env.

def application do
  [extra_applications: [:logger, :crypto],
   env: [key: :value],
   registered: [MyServer]]
end

score 0 · Answer 2 · answered Apr 21 '16 at 07:30

If P1 is dependency of P2, P2 is responsible for configuration and nothing can change that, but you have couple of other options:

use umbrella project The umbrella project can have its own config with P1.Repo. This way you keep P1 and P2 configs cleaner.
additionally you can tear out meaningful chunk from P1 that is used by P2 and move it to P3. Both P1 and P2 depend on P3.

Ecto config in dependencies

2 Answers2