user.ex

defmodule MyApp.User do

alias MyApp.Repo

 ## Stuff omitted

  def changeset(model, params) do
    model
    |> cast(params, @required_fields, @optional_fields)
    |> validate_unique(:username, on: Repo)
  end
end

defmodule MyApp.UserTest do
  # Stuff omitted

  @valid_attrs %{username: "thedude"}

  test "validates uniqueness of username" do
    changeset = User.changeset(%User{}, @valid_attrs)                                                                                                                                                 
    Repo.insert(changeset)                                                                                                                                                                            
    refute changeset.valid?        
  end
end