ecto_models.ex

defmodule Node do
  use Ecto.Model

  schema "nodes" do
    has_many :children, NodeToNode, foreign_key: :parent_id
    has_many :parents, NodeToNode, foreign_key: :child_id
  end
end

defmodule NodeToNode do
  use Ecto.Model

  schema "node_to_node", primary_key: false do
    belongs_to :parent, Node, foreign_key: :parent_id
    belongs_to :child, Node, foreign_key: :child_id
  end
end