> %User{}
%Top5.Accounts.User{
  __meta__: #Ecto.Schema.Metadata<:built, "users">,
  email: nil,
  id: nil,
  inserted_at: nil,
  password: nil,
  updated_at: nil,
  username: nil
}

> User.changeset(%User{}, %{})
#Ecto.Changeset<
  action: nil,
  changes: %{},
  errors: [
    username: {"can't be blank", [validation: :required]},
    email: {"can't be blank", [validation: :required]},
    password: {"can't be blank", [validation: :required]}
  ],
  data: #Top5.Accounts.User<>,
  valid?: false

> User.changeset(%User{}, %{:username => "Richard"})
#Ecto.Changeset<
  action: nil,
  changes: %{username: "Richard"},
  errors: [
    email: {"can't be blank", [validation: :required]},
    password: {"can't be blank", [validation: :required]}
  ],
  data: #Top5.Accounts.User<>,
  valid?: false