Detect cycles in a collection of `acts_as_tree` objects

gistfile1.txt

def has_cycle?(items)  # hash: { id => parent_id, id => nil }
  return false if items.size < 2

  items = items.dup

  items.keys.each do |starting_point|
    visited_items = []
    item = starting_point
    while item.present?
      return true if visited_items.include?(item)
      visited_items << item
      item = items[item]
    end
  end

  return false
end

# Usage:
graph = Category.where(...).pluck(:id, :parent_id).to_h
has_cycle?(graph)

# Note: this works in-memory, so it's not suitable for huge collections.