Elixir test case to check if field is empty or invalid

Hi Elixir community, I am a newbie at Elixir and learning to write some effective tests for my blog application. My blog application has the following association:

Post → has_many comments
User → has_many comments
Comment → belongs to post and user
I would like to test

  1. if the post_id in my comment is available/empty and raise appropriate error
  2. if it(post_id) has invalid attribute and if yes, raise appropriate error



@required_field [
:comment_text, :string,


field :comment_text, :string,
belongs_to :post, Post
belongs_to :user, User


def changeset(comment, attrs) do
    |> foreign_key_constraint(:post_id)
    |> foreign_key_constraint(:user_id)
    |> cast(attrs, @required_field ++ @optional_field)
    |> validate_required(@required_field)

I am using factories so no fixtures.


def comment_factory do
      comment_text: "some comment_text",
      post: build(:post),
      user: build(:user)


    @valid_attrs %{
      comment_text: "some comment text"

Thanks in advance :smiley:


Could you help us by posting what you have tried and any issues you might have run into when trying that. And explaining where in the flow are your tests going to be done? (preparing the changeset, after insert attempt?)

Sure, I tried doing this
Added post with nil value to invalid_attrs

@invalid_attrs %{
      post: nil
test "when post_id is not available" do
      comment = insert(:comment, @invalid_attrs)
      assert(comment.post_id,"post doesn't exist")

This test fails when I pass @invalid_attrs and gives the “post doesn’t exist” message. And this test passes if I pass @valid_attrs. But I would like to raise an error like foreign key error.

If I’m not mistaken, when you account for a foreign constraint on the changeset, |> foreign_key_constraint(:post_id) like you did the insertion if invalid will fail and the changeset will be marked as errored, so something like

assert {:error, Ecto.Changeset{}} = insert(:comment, @invalid_attrs)

should be what you’re looking for. If you want it to raise then you shouldn’t set the foreign constraint check, and leave it to the database to error, which will make ecto throw and exception.

But I think usually it’s preferable to have no exception and deal with {:ok, _}/{:error, _}.
If then wanted the caller can raise a specific one.

You can also check that the error is in the field you expect along with its message.
You would probably have some helper in your codebase for transforming errored changesets into a map of field => errors.

With this you can build a simple helper:

def as_error_map(Ecto.Changeset{valid?: false} = changeset) do
  Ecto.Changeset.traverse_errors(changeset, fn {msg, opts} ->
    Enum.reduce(opts, msg, fn {key, value}, acc ->
      String.replace(acc, "%{#{key}}", to_string(value))

And now you can use assert %{post_id: ["post doesn't exist"]} = as_error_map(changeset)

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