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PON+1 while enumerating self-referencing records
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copied!<p>I'm doing a pretty basic thing - displaying a tree of categories in topological order and ActiveRecord issues extra query for enumerating each category's children.</p> <pre><code>class Category < ActiveRecord::Base attr_accessible :name, :parent_id belongs_to :parent, :class_name => 'Category' has_many :children, :class_name => 'Category', :foreign_key => 'parent_id' def self.in_order all = Category.includes(:parent, :children).all # Three queries as it should be root = all.find{|c| c.parent_id == nil} queue = [root] result = [] while queue.any? current = queue.shift result << current current.children.each do |child| # SELECT * FROM categories WHERE parent_id = ? queue << child end end result end end </code></pre> <p>UPD. As far as I understand what's going here is that when a category is referred as a children of some category it's not the same object as the one in the initial list and so it hasn't it's children loaded. Is there a way to implement desired behavior without resorting to creating extra adjacency list?</p> <p>UPD2: Here's the manual adjacency list solution. It uses only one query but I'd really like to use something more idiomatic</p> <pre><code> def self.in_order_manual cache = {} adj = {} root = nil all.each do |c| cache[c.id] = c if c.parent_id != nil (adj[c.parent_id] ||= []) << c.id else root = c.id end end queue = [root] result = [] while queue.any? current = queue.shift result << current (adj[current] || []).each{|child| queue << child} end result.map{|id| cache[id]} end </code></pre>

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