Config from `mix run --config` invocations doesn't get deeply merged

[pdobacz]

Environment

$ elixir --version
Erlang/OTP 21 [erts-10.2.2] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe]

Elixir 1.8.0 (compiled with Erlang/OTP 20)

• Operating system: Ubuntu 18.04

Current behavior

When I put, e.g.:

config :my_app, MyApp.Endpoint,
  http: [port: 7435]

into a some_config.exs file and then mix run --config some_config.exs, I'll get:

IO.inspect Application.get_env(:my_app, MyApp.Endpoint)

# [http: [port: 7435]]

despite having entries in the "default" config files loaded in a rather standard manner (i.e. config/config.exs doing import_config config/dev.exs etc.).

Expected behavior

I'd expect the config from some_config.exs be deeply merged as are the config/config.exs and config/dev.exs loaded by default.

That is, if without the override in --config some_config.exs I have:

IO.inspect Application.get_env(:my_app, MyApp.Endpoint)

# [
#   secret_key_base: ...
# <snip>,
#   http: [port: 7434],
# <snip>
# ]

with the override --config some_config.exs, I'd expect to have

# [
#   secret_key_base: ...
# <snip>,
#   http: [port: 7435],
# <snip>
# ]

which would effectively be config/config.exs with config/dev.exs merged in with some_config.exs merged in, deeply

[josevalim]

Those happen at two very distinct steps and therefore we cannot properly deep merge them since the original config has been long gone. I think we have two options here:

1. Deprecate the --config flag and push developers to handle those conflicts in different ways (for example, by adding extra environments)

2. Improve the documentation to say it does not deep merge

I am more inclined to go with 1 because I beleive most would expect deep merge to work and documenting it is not enough.

[pdobacz]

1 does sound like more along the pattern here.

We're using the --config files to sneak bulky runtime configuration into Application's env, which is probably what is against the pattern. However, despite being non-standard, it worked well, and then it's easy to run into this "no deep merge" issue.

Actually, what would be the correct and convenient way of populating Application's env based on runtime information contained in a file, if --config is removed? I did some quick searches and found nothing relevant which would provide an end-to-end, drop-in replacement.

[josevalim]

You can use Mix.Config.load and Mix.Config.persist to achieve a similar result or just set Application.get_env directly from a script file. So you would do something like mix run -r sets_config.exs or similar. But if you want merging, doing it inside config/config.exs is the best way to go. You can even do something like:

if extra = System.get_env("EXTRA_CONFIG") do
  import_config "#{extra}.exs"
end

[pdobacz]

Thank you very much! I'll give these a try when I have the chance, for now continuing with --config and anticipating it might be deprecated.

Not sure what should I do with this issue - close as deep-merging for --config is not an option or keep open to cover taking either 1 (or 2) path?

[josevalim]

We will keep it open until it is deprecated and then we will close it. Thank you!

[josevalim]

Closed in favor of #8932.