#Implements nested sets

      ## In your Ecto model(ExShop.Category):

          defmodule ExShop.Category do
            use ExShop.Web, :model

            # Specify 
            order_by_field: field by which we'll order result set
            changeset: changeset method for updating tree
            children_association: association name to get children
            parent_id_column: column name storing the parent_id of the node
            left_col: column name for storing node's left value
            right_col: column name for storing node's right value

            use NestedSet, order_by_field: :name, changeset: :nested_set_changeset, children_association: :children, parent_id_column: :parent_id, left_col: :lft, right_col: :rgt

            schema "categories" do
              field :lft, :integer, default: 0
              field :rgt, :integer, default: 0

              has_many :children, ExShop.Category, [foreign_key: :parent_id, on_delete: :delete_all]
              belongs_to :parent, ExShop.Category, [foreign_key: :parent_id]
            end

            @required_fields ~w(name)
            @optional_fields ~w(parent_id lft rgt)


            # Nested Set expect a changeset like:
            def nested_set_changeset(model, params ) do
              model
              |> cast(params, @required_fields, @optional_fields)
            end

          end



      # First get the model by:
      model = ExShop.Repo.get_by(ExShop.Category, id: 35)

      # Get descendent's count by:
      ExShop.Category.descendents_count(model) #=> 3

      # Get the descendants ordered by default field by:
      ExShop.Category.descendants(model)

      # Get the descendants ordered by default field by other field like id:
      ExShop.Category.descendants(model, %{order_by_field: :id})

      # Get the unordered descendants 
      ExShop.Category.descendants(model, %{ordered: :false})


      # Get the ancestors ordered by default field by:
      ExShop.Category.ancestors(model)

      # Get the ancestors ordered by default field by other field like id:
      ExShop.Category.ancestors(model, %{order_by_field: :id})

      # Get the unordered ancestors 
      ExShop.Category.ancestors(model, %{ordered: :false})            

      # Get the self_and_descendants by:
      ExShop.Category.self_and_descendants(model)


      # Get the self_and_ancestors by:
      ExShop.Category.self_and_ancestors(model)

      # Get the ancestors_count by:
      ExShop.Category.ancestors_count(model, ExShop.Repo)

      # Get the self_and_siblings by:
      ExShop.Category.self_and_siblings(model)

      # Get the leaves by:
      ExShop.Category.leaves(model)

      # Recalculate lft and rgt values for tree, or sub tree by passing node and the lft value(default 1) of the node
      ExShop.Category.recalculate_lft_rgt(model, ExShop.Repo)
      ExShop.Category.recalculate_lft_rgt(model, ExShop.Repo, %{order_by_field: :name, lft: 5})

      #Get root node:
      ExShop.Category.get_root_node()