Ruby nested hash fetching dynamically

Question

I have an issue trying to update a nested Hash using a key.

The nested hash I have is like this:

main_hash = {   
    "Energy"=>
      {"name"=>"Energy", "uri"=>"energy", 
      "children"=>
        {"Renewable Energy"=>{"name"=>"Renewable Energy", "uri"=>"energy/renewable_energy"}}}
    , 
    "Farming"=>
      {"name"=>"Farming", "uri"=>"farming", 
        "children"=>
        {"Organic Gas"=>
            {"name"=>"Organic Gas", "uri"=>"farming/organic_gas"
              "children" =>
                {"Gas Oil"=>{"name"=>"Gas Oil", "uri"=>"farming/organic_gas/gas_oil"}}
              }}}}

What I would like to do is to update an item from the hash (e.g., I want to add another child to "Organic Gas"). I know I can do this:

  main_hash["Farming"]["children"]["Organic Gas"]["children"].merge!(another_hash)

The problem is I need to get that dynamically as it can get quite deeply nested.

So to get to the desired level, I would do this (which does work as above).

main_hash.send(:fetch, "Farming").send(:fetch, "children").send(:fetch, "Organic Gas").send(:fetch, "children")

It would really be great if I could call "send" method dynamically like below (obviously it won't work).

main_hash.send(:try, 'send(:fetch, "Farming").send(:fetch, "children").send(:fetch, "Organic Gas").send(:fetch, "children")')

I hope it makes it clear what I want to achieve. I have gone through all Ruby Hash's built in function and I can't get the one suited for my need.

Any help would be greatly appreciated.

Cheers.

Why not just write a little wrapper that accepts a collection of keys and drills iteratively or recursively into a hash? — Dave Newton
– Dave Newton, Commented Oct 5, 2011 at 21:11
I can do just that but the issue with that is I need to update that nested deep item and still retain the whole hash as it is. I have got to a point where I can update that item (recursively) but I couldn't keep the origian main_hash. — Phyo Wai Win
– Phyo Wai Win, Commented Oct 5, 2011 at 23:05
I'm not sure what you mean by that; updating an item inside the hash won't change the rest of it. — Dave Newton
– Dave Newton, Commented Oct 5, 2011 at 23:11

Carl Suster · Accepted Answer · 2011-10-06 03:11:04Z

I'm not sure a Hash is really the best data structure here. You're trying to use it to represent a tree, which is fine and all, but it might be a bit clearer if you just explicitly made it a tree:

class Tree
  attr_reader :name, :uri, :children, :parent

  def initialize(name, uri, *children)
    @children = children
    @name, @uri = name, uri
  end

  def <<(child)
    @children << child
  end

  def find(name)
    each_branch.detect {|branch| branch.name == name }
  end

  def leaf?
    @children.empty?
  end

  # The parameter `i` just decides whether or not to include self.
  def each_branch( i=true, &blk )
    enum = Enumerator.new do |y|
      y.yield self if i
      @children.each do |c|
        next unless c.is_a? Tree
        y.yield c
        c.each_branch( false ).each {|b| y.yield b }
      end
    end
    block_given? ? enum.each( &blk ) : enum
  end

  # This yields each leaf and its parent.
  def each_leaf( parent=self, &blk )
    enum = Enumerator.new do |y|
      @children.each do |c|
        if !c.leaf?
          c.each_leaf( c ).each do |l,p|
            y.yield l, p
          end
        else y.yield c, parent
        end
      end
    end
    block_given? ? enum.each( &blk ) : enum
  end

end

(I just borrowed those enumerators from a tree structure I'd made before - the each_leaf method might be helpful too, and you can check for the class not being Tree instead of leaf? returning true if you had a tree structure that could contain other objects, like strings).

Then you could do:

root_tree = Tree.new "Farming", "farming"
root_tree << Tree.new( "Organic Gas", "organic_gas" )

gas = root_tree.find "Organic gas"
gas << Tree.new(...)

I think this is just a case of finding the right data structure for the job. Even if the tree method were a bit less efficient, it's clearer what's going on, and will probably lead to fewer bugs in your dynamic code down the track.

If the problem is that you don't want the original tree to be modified, only copied, then just redefine:

class Tree
  attr_accessor :children

  def <<(child)
    new_tree = self.dup
    new_tree.children = @children + [child]
    new_tree
  end
end

That way, you retain the original tree but return a new tree with the extra child added.

Brian K · Accepted Answer · 2013-07-26 03:48:52Z

2

To answer the original question, you can use the XKeys gem to traverse a dynamic path as follows:

require 'xkeys' # on rubygems.org

main_hash.extend XKeys::Hash
path = ['Farming', 'children', 'Organic Gas', 'children']
main_hash[*path].merge!(another_hash)

I would also recommend using :children instead of 'children' to avoid having huge numbers of duplicate strings.

answered Jul 26, 2013 at 3:48

Brian K

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