How to get data from Ecto in a custom mix task

As addition to Jason Harrelson's answer: it's also necessary to start Postgrex and Ecto.

[:postgrex, :ecto]
|> Enum.each(&Application.ensure_all_started/1)

MyApp.Repo.start_link

UPDATE:

Another approach is to use mix task to start application:

Mix.Task.run "app.start", []

Ecto 3.x:

ensure_started has since been removed from Ecto. There has been a lot of confusion around this topic. See here https://github.com/elixir-ecto/ecto/pull/2829#issuecomment-456313417 for more. José suggests to either start the app using Mix.Task.run "app.start" or run the repo using MyApp.Repo.start_link(...).

Ecto 2.x:

This used to work in 2.x, but apparently Mix.Ecto was not considered part of the public API.

There is actually a helper module Mix.Ecto (https://github.com/elixir-ecto/ecto/blob/master/lib/mix/ecto.ex) that makes it easier to write mix tasks that use ecto:

defmodule Mix.Tasks.Users.List do
  use Mix.Task
  import Mix.Ecto

  def run(args) do
    repos = parse_repo(args)

    Enum.each repos, fn repo ->
      Mix.shell.info "=== Active users ==="

      ensure_repo(repo, args)
      ensure_started(repo, [])
      users = repo.all(Ectotask.User)

      Enum.each(users, fn(s) -> IO.puts(s.name) end)
    end
  end
end

This helper gives you access to parse_repo/1, ensure_repo/2, ensure_started/1. parse_repo will let your task fit in nicely with other ecto mix tasks, for example it will let you pass -r to specify a different repo.

➤ mix users.list
=== Active users ===
Adam
➤ mix users.list -r Ectotask.Repo22
=== Active users ===
** (Mix) could not load Ectotask.Repo22, error: :nofile. Please pass a repo with the -r option.

ensure_started makes sure the repo is running, which you were lacking.

For guidance and inspiration, you can look at how other ecto mix tasks are implemented at https://github.com/elixir-ecto/ecto/tree/master/lib/mix/tasks