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Parameterized ExUnit tests

In ExUnit, it is not immediately obvious how to do the same “test” using different parameters.

It can be tedious to write individual tests for each required field asserting the validation. It’s also difficult for future-you to determine if you have complete coverage.

The cheating way

Remove the all the required fields from the source map before calling changeset and make one massive assert

The better way

The solution I use here is to set the @tag test attribute as two properties of the test. The first @tag field: field_name is the property I’m testing against The second @tag message_attr: %{attr_name => nil} is the value to assign that field before running the test.

You’ll see that these @tag values are available in the test context by the given tag name

      {:entity_name, :entity_name},
      {:entity_uuid, :team_uuid}
    |> Enum.each(fn {field_name, attr_name} ->
      @tag field: field_name
      @tag message_attr: %{attr_name => nil}
      test "when `#{field_name}` missing, invalid ... required", context do
        message = TestMessageHelpers.market_message(context.message_attr)

        %Changeset{valid?: false} = changeset = Subject.changeset(%Subject{}, message)

        assert changeset.errors == [{context.field, {"can't be blank", [validation: :required]}}]