Create mix tasks that invoke existing modules which use Ecto

I’m still curious about this but having hacked at this a bit it seems like using mix run to run scripts is a more felicitous approach for my use case.

You need to make sure to start Repo before using it in a Mix task:

def run(_) do
 Mix.Ecto.ensure_started(App.Repo, [])
 App.Adapter.list_all(App.Thing)
end

WARNING: In Ecto 2.X you should not reference Mix.Ecto because it is a private API: Preload fails to checkout connection from poolboy · Issue #1586 · elixir-ecto/ecto · GitHub

And in Ecto 3.X, while Mix.Ecto is now considered public (i.e. no longer has @moduledoc false), the ensure_started function has been removed. Off the top of my head I’m not sure what you should use instead (but maybe it is Mix.Task.run).

Running into this issue myself. Currently creating a mix task to generate a shell of my graphql schema based on my ecto schema. Need to get info about indexed columns/fields as well as nullability of fields, so attempting to execute:

Ecto.Adapters.SQL.query!(MyApp.Repo, "select * from pg_indexes where schemaname='public'")

for example to get a list of indexes. I encounter the same error:

** (RuntimeError) could not lookup MyApp.Repo because it was not started or it does not exist
    lib/ecto/repo/registry.ex:18: Ecto.Repo.Registry.lookup/1
    lib/ecto/adapter.ex:127: Ecto.Adapter.lookup_meta/1

Mix.Ecto.ensure_started... No longer exists. How else can we start the repo in a mix task?

"ecto": {:hex, :ecto, "3.1.7",},

The Phoenix docs suggest using:

Mix.Task.run("app.start")

in your task’s run/1 function to boot the application and its dependencies.

Alternatively, if you just need a repo, there’s Ecto.Migrator.with_repo/2: ecto_sql/lib/ecto/migrator.ex at master · elixir-ecto/ecto_sql · GitHub

Thanks @al2o3cr, Ecto.Migrator.with_repo/2 was just what the doctor ordered. Works! Wish I could select as answered.