Recursive models with ecto

model.ex

defmodule Model do
  schema "models" do
    field :foo, :string
    has_many :children, Model, foreign_key: :parent_id
    belongs_to :parent, Model, foreign_key: :parent_id
  end
  
  @doc """
    Recursively loads parents into the given struct until it hits nil
  """
  def load_parents(parent) do
    load_parents(parent, 10)
  end
  
  def load_parents(_, limit) when limit < 0, do: raise "Recursion limit reached"
  
  def load_parents(%Model{parent: nil} = parent, _), do: parent
  
  def load_parents(%Model{parent: %Ecto.Association.NotLoaded{}} = parent, limit) do
    parent = parent |> Repo.preload(:parent)
    Map.update!(parent, :parent, &Model.load_parents(&1, limit - 1))
  end
  
  def load_parents(nil, _), do: nil
  
  @doc """
    Recursively loads children into the given struct until it hits []
  """
  def load_children(model), do: load_children(model, 10)
  
  def load_children(_, limit) when limit < 0, do: raise "Recursion limit reached"
  
  def load_children(%Model{children: %Ecto.Association.NotLoaded{}} = model, limit) do
    model = model |> Repo.preload(:children) # maybe include a custom query here to preserve some order
    Map.update!(model, :children, fn(list) -> 
      Enum.map(list, &Model.load_children(&1, limit - 1))
    end)
  end
end

model_controller.ex

defmodule ModelController do
  def show(id) do
    model = Repo.get(Model, id) 
      |> Model.load_parents
      |> Model.load_children
      
    # do something
  end
end