** (RuntimeError) could not lookup Ecto repo MySQL because it was not started or it does not exist

I realize there are multiple topics with the same title, but the questions and answers in those threads either don’t make sense to me or are not addressing my elephant.

I have followed the instructions on Elixir School: Ecto Basics and I am stuck.

I created a new mix mix new cc. It did not generate an application.ex.

I then set up a repository by running:

$ mix ecto.gen.repo -r Cc.MySQL
* creating lib/cc
* creating lib/cc/my_sql.ex
* creating config/config.exs
Don't forget to add your new repo to your supervision tree
(typically in lib/cc/application.ex):
  • I edited config.exs to add config :cc, ecto_repos: [Cc.MySQL], correct the MySQL creds and add protocol: :tcp:
    import Config
config :cc, Cc.MySQL,
  database: "ecto",
  username: "ecto",
  password: "ecto",
  hostname: "localhost",
  protocol: :tcp

config :cc, ecto_repos: [Cc.MySQL]
  • I edited my_sql.ex to use MyXQL instead of the generated Postgres.
defmodule Cc.MySQL do
  use Ecto.Repo,
    otp_app: :cc,
    adapter: Ecto.Adapters.MyXQL

According to the Ecto documentation, I need an application.ex. So I created one in lib/cc/application.ex:

defmodule Cc.Application do
  use Application

  def start(_type, _args) do
    children = [

    opts = [strategy: :one_for_one, name: Cc.Supervisor]
    Supervisor.start_link(children, opts)

after compiling the project and running, I use an existing Schema to try insert data:

$ iex -S mix
Compiling 1 file (.ex)
Interactive Elixir (1.16.1) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> cars = %CC.Cars.Model{agent: "localhost"}
  __meta__: #Ecto.Schema.Metadata<:built, "cars">,
  id: nil,
  date: nil,
  car: nil,
  agent: "localhost",
iex(2)> Cc.MySQL.insert!(cars)
** (RuntimeError) could not lookup Ecto repo Cc.MySQL because it was not started or it does not exist

However if I run {:ok, pid} = Cc.MySQL.start_link() in iex directly, I will get a PID. If I then run the command Cc.MySQL.insert!(cars) I will start getting MySQL errors instead.

What am I missing that with allow my app to generate a PID for Cc.MySQL?

Just creating application.ex doesn’t magically have the callbacks in that module be called. You also need to adjust mix.exs for that to happen.

You can run mix new --sup cc for it to create the mix.exs as needed and also have application.ex already be generated by the command.

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Thanks @LostKobrakai. Running that command inside my project, generated a new project within my project.

On comparing the new project’s application.ex with my own, I saw a tiny difference:

def application do
    extra_applications: [:logger],
    mod: {Cc.Application, []}

my app did not have mod: {Cc.Application, []}.

So I deleted the new project inside my project and added that line to my mix.exs which was created with just mix new cc. That line allows my project to run the custom application children.

You are supposed to replace the previous project.

Just start over.

Having figured out the difference between the two it doesn’t really matter.

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