I’m using an adjacency list to store a hierarchy of categories. There can be many root categories each with many children/descendants.

defmodule WordSolverDal.Categories.Category do
  use Ecto.Schema
  import Ecto.Changeset

  use Arbor.Tree,
    foreign_key: :parent_id,
    foreign_key_type: :integer

  schema "categories" do
    field(:name, :string)
    field(:parent_id, :integer)
    many_to_many(:answers, WordSolverDal.Answers.Answer, join_through: "answers_categories")
    timestamps()
  end

  @required [:name]
  @optional [:parent_id]

  def changeset(category, attrs) do
    category
    |> cast(attrs, @required ++ @optional)
    |> validate_required(@required)
    |> validate_length(:name, min: 3)
    |> unique_constraint(:name, name: :unique_categories_name_parent_id_null)
    |> unique_constraint(:name, name: :unique_categories_name_parent_id)
  end
end

The db throws a constraint error if a user tries to delete a record that has children due to the foreign key constraint on :parent_id.

constraint error when attempting to delete struct:

    * "categories_parent_id_fkey" (foreign_key_constraint)

If you would like to stop this constraint violation from raising an
exception and instead add it as an error to your changeset, please
call `foreign_key_constraint/3` on your changeset with the constraint
`:name` as an option.

This error gives advice on how to handle the error without raising an exception but I don’t see how this could work. As I understand it, foreign_key_constraint/3 is typically used for insert/update change sets.

How can I attempt a delete and not raise an exception as described? Or should I just rescue the error?