My setup

These are the files I think are useful, please let me know if you need to see other setup. The repo itself is private so I can’t link to it.

lib/site/user/user.ex

  schema "user" do
    field :first_name, :string
    field :last_name, :string
    field :full_name, :string
    belongs_to :countries, Site.Countries.Country, foreign_key: :countries_id, on_replace: :nilify
    belongs_to :currencies, Site.Countries.Currency, foreign_key: :currencies_id, on_replace: :nilify

    timestamps()
  end

  def changeset(employee, attrs) do
    employee
    |> cast(attrs, [:first_name, :last_name, :full_name])
    |> validate_required([:first_name, :last_name])
    |> put_assoc(:countries,
    Site.Countries.get_country_by_name!(attrs["country_name"] || attrs[:country_name]))
    |> put_assoc(:currencies,
    Site.Countries.get_currency_by_code!(attrs["currency_code"] || attrs[:currency_code]))
    |> generate_full_name()
  end

  defp generate_full_name(changeset) do
    case changeset do
      %Ecto.Changeset{valid?: true, changes: %{first_name: first_name, last_name: last_name}} ->
        put_change(changeset, :full_name, "#{first_name} #{last_name}")
      _ -> changeset
    end
  end

lib/be_exercise/countries.ex

...
  def get_currency_by_code!(code) do
    query =
      from(c in Currency,
        where: c.code == ^code
      )

    Repo.one!(query)
  end

  def get_country_by_name!(name) do
    query =
      from(c in Country,
        where: (c.name == ^name)
      )

    Repo.one!(query)
  end

lib/site/countries/country.ex

  schema "countries" do
    field :code, :string
    field :name, :string
    field :currency_id, :id
    has_many :users, Site.Users.User, foreign_key: :countries_id

    timestamps()
  end

  @doc false
  def changeset(country, attrs) do
    country
    |> cast(attrs, [:name, :code, :currency_id])
    |> validate_required([:name, :code])
    |> unique_constraint(:name)
    |> unique_constraint(:code)
  end

lib/site/countries/currency.ex

  schema "currencies" do
    field :code, :string
    field :name, :string
    field :symbol, :string
    has_many :users, Site.User.Users, foreign_key: :currencies_id

    timestamps()
  end

  @doc false
  def changeset(currency, attrs) do
    currency
    |> cast(attrs, [:code, :name, :symbol])
    |> validate_required([:code, :name, :symbol])
    |> unique_constraint(:name)
    |> unique_constraint(:code)
  end

test/support/fixtures/users_fixtures.ex

  import Site.CountriesFixtures

  def user_fixture(attrs \\ %{}) do
    currency = currency_fixture()
    country = country_fixture()

    {:ok, user} =
      attrs
      |> Enum.into(%{
        first_name: Faker.Person.En.first_name(),
        last_name: Faker.Person.En.last_name(),
        country_name: country.name,
        currency_code: currency.code
      })
      |>Site.User.create_employee()

    user
  end

The problem

I originally used mix phx.gen context to generate the User context, at the time I did not create any foreign keys on the table. After I added in the country and currency associations, and updated the schema to use put_assoc to generate the associations, the unit tests started to fail. I’ve been working my through them and have fixed all except the last one:

    test "change_user/1 returns a user changeset" do
      user = user_fixture()
      assert %Ecto.Changeset{} = Users.change_user(user)
    end

this is change_user/1

  def change_user(%User{} = user, attrs \\ %{}) do
    User.changeset(user, attrs)
  end

The error:

  1) test user change_user/1 returns a user changeset (Site.UsersTest)
     test/site/userss_test.exs:122
     ** (ArgumentError) comparison with nil is forbidden as it is unsafe. If you want to check if a value is nil, use is_nil/1 instead
     code: assert %Ecto.Changeset{} = Users.change_user(user)
     stacktrace:
       (ecto 3.7.2) lib/ecto/query/builder.ex:958: Ecto.Query.Builder.not_nil!/1
       (site 0.1.0) lib/site/countries.ex:176: Site.Countries.get_country_by_name!/1
       (site0.1.0) lib/site/users/user.ex:26: Site.Users.User.changeset/2
       test/site/users_test.exs:124: (test)

I have tried

• Using IEx.pry to inspect the attrs in the changeset and in get_country_by_name(). When I stop use the breakpoint in the changeset this line:

put_assoc(:countries, Site.Countries.get_country_by_name!(attrs["country_name"] || attrs[:country_name]))

does NOT error.

Also when I investigate get_country_by_name!/1 the name parameter is NOT nil and the query returns a country.

• IO.inspect(user) from the unit test gives me:

%Site.Users.User{
  __meta__: #Ecto.Schema.Metadata<:loaded, "euser">,
  countries: %Site.Countries.Country{
    __meta__: #Ecto.Schema.Metadata<:loaded, "countries">,
    code: "BS",
    currency_id: nil,
    users: #Ecto.Association.NotLoaded<association :users is not loaded>,
    id: 566,
    inserted_at: ~N[2022-03-24 15:53:51],
    name: "Montenegro",
    updated_at: ~N[2022-03-24 15:53:51]
  },
  countries_id: 566,
  country_name: "Montenegro",
  currencies: %Site.Countries.Currency{
    __meta__: #Ecto.Schema.Metadata<:loaded, "currencies">,
    code: "EUR",
    users: #Ecto.Association.NotLoaded<association :users is not loaded>,
    id: 560,
    inserted_at: ~N[2022-03-24 15:53:51],
    name: "Norfolk Island currency",
    symbol: "¢",
    updated_at: ~N[2022-03-24 15:53:51]
  },
  currencies_id: 560,
  currency_code: "EUR",
  first_name: "Margaretta",
  full_name: "Margaretta Padberg",
  id: 165,
  inserted_at: ~N[2022-03-24 15:53:51],
  last_name: "Padberg",
  updated_at: ~N[2022-03-24 15:53:51]
}

• I also tried manually building an %User{} object to pass to change_user/1. When I do this from an iex session I successfully get back a changeset but when I do it from the test I get the same ArgumentError.

• The countries_name and currencies_code fields did not initially exist in the Users table, I was just using them to find a given Country and Currency and did not need to persist them. One of my attempts to fix this error was to create those fields in the User table and to update the schema accordingly but that didn’t help.

I know this is a very verbose post but I have really tried a lot of things and any advice this community can provide would be much appreciated!