ecto_error

** (Mix) Could not start application hello_world_phoenix: HelloWorldPhoenix.start(:normal, []) returned an error: shutdown: failed to start child: HelloWorldPhoenix.Repo
    ** (EXIT) shutdown: failed to start child: Ecto.Adapters.Postgres
        ** (EXIT) an exception was raised:
            ** (RuntimeError) could not find Ecto.Adapters.Postgres.Connection.

Please verify you have added :postgrex as a dependency:

    {:postgrex, ">= 0.0.0"}

And remember to recompile Ecto afterwards by cleaning the current build:

    mix deps.clean ecto

                (ecto) lib/ecto/adapters/sql.ex:420: Ecto.Adapters.SQL.start_link/4
                (stdlib) supervisor.erl:343: :supervisor.do_start_child/2
                (stdlib) supervisor.erl:326: :supervisor.start_children/3
                (stdlib) supervisor.erl:292: :supervisor.init_children/2
                (stdlib) gen_server.erl:328: :gen_server.init_it/6
                (stdlib) proc_lib.erl:240: :proc_lib.init_p_do_apply/3