Iterating in ruby using an if else statement ...(along w. ancestry gem) - ruby-on-rails

Question

Recently started withrails along with ancestry gem for the first time.
I am attempting to iterate through an array of Category objects which are stored in a tree structure via the ancestry gem, in order to display buttons, wired to link_to to other category objects, a given category is related to.
The first 3 categories in the database are the roots, and so I will eventually not have enough space in the view to display buttons for all descendants. So, if the category.parent == 'nil', I would like to display only the children of given category.
However, for all other category objects, I would like to display all descendants of a given category.
I was thinking something along these lines, as to iterate through
<%= current_user.folders.each do |folder| %>
<% if folder.is_root? %>
<% folder.children.each do |child| %>
<%= link_to child, folder_path(child), class: 'btn' %>
<% end %>
<% else %>
<% folder.descendants.each do |desc| %>
<%= link_to desc, folder_path(desc), class: 'btn' %>
<% end %>
<% end %>
Would anyone be able to advise?

Answer 1

How are your child categories related to their parents? I'll assume the categories table has a parent_id column.
Given these categories:
parent_category = Category.create!
child_category1 = Category.create! parent_id: parent_category.id
child_category2 = Category.create! parent_id: parent_category.id
Now you can check if the parent_id is nil to see if it is a parent category or not, we'll add a self-referential association and scope to the Category model:
class Category < ActiveRecord::Base
scope :parents, -> { where(parent_id: nil).joins :children }
has_many :children, class_name: "Category", foreign_key: :parent_id
end
Now you can iterate over the parents and iterate over each parent's children:
Category.parents.each do |parent|
parent.children.each do |child|
# code
end
end