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.

Ecto.Changeset.traverse_errors/2
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))
    end)
  end)
end

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