First, create umbrella application:

$ mix new --umbrella my_app
$ cd my_app/apps

Next create Phoenix app, excluding Ecto:

$ mix phoenix.new web --no-ecto

Now we create a new mix project for Ecto, with supervision tree:

$ mix new db --sup

Finally we need to set up some configuration files.

Setting Up Db Project

$ cd db

First in the db project (following the getting started docs for Ecto:

mix.exs (note that we do not need to register :ecto application)

  def application do
    [applications: [:logger, :postgrex],
     mod: {Db, []}]
  end

  defp deps do
    [{:postgrex, "~> 0.13.0"},
     {:ecto, "~> 2.1.1"}]
  end

Run configuration by running:

$ mix ecto.gen.repo -r Db.Repo

This will populate lib/db/repo.ex and config.config.exs.

lib/db.ex

children = [
  supervisor(Db.Repo, [])
]

optional

We probably want configs for multiple environments.

config/config.exs

Uncomment the last line, or replace the contents of the file with:

use Mix.Config

config :db,
  ecto_repos: [Db.Repo]

# Import environment specific config. This must remain at the bottom
# of this file so it overrides the configuration defined above.
import_config "#{Mix.env}.exs"

This will load environment-specific configs.

config/dev.exs

use Mix.Config

config :db, Db.Repo,
  adapter: Ecto.Adapters.Postgres,
  username: "foo",
  database: "myproject_dev",
  hostname: "localhost",
  pool_size: 10

config/test.exs

use Mix.Config

config :db, Db.Repo,
  adapter: Ecto.Adapters.Postgres,
  username: "foo",
  password: "12345",
  database: "myproject_test",
  hostname: "localhost",
  pool: Ecto.Adapters.SQL.Sandbox

config :logger,
  backends: [:console],
  compile_time_purge_level: :debug

Set Up Phoenix App

$ cd ../web

mix.exs

  def application do
    [mod: {Web, []},
     applications: [:phoenix_ecto, :db]]
  end

  defp deps do
    [{:phoenix_ecto, "~> 3.2.1"},
     {:db, in_umbrella: true}]
  end

config/config.exs

config :web,
  ecto_repos: [Db.Repo]

# Configure phoenix generators
config :phoenix, :generators,
  migration: false

Notes

• From now on, when running mix phoenix.gen.html or mix phoenix.gen.json, pass the --no-model option. (update: see question below.)

• Since app is umbrella app now, remember to cd into phoenix app directory before running generators, or files will be generated at umbrella’s top level.

Questions

• Is it there a way to make it so that there’s no need for --no-model option every time?

config :phoenix, :generators, model: false

Answer provided by the man himself :) https://elixirforum.com/t/should-we-separate-ecto-from-phoenix-in-umbrella-app/1515/22