Adjacency data structure to nested hash - ruby-on-rails

Question

I have the following model in rails:
class Model < ActiveRecord::Base
# id — integer
# name — string
# model_id — integer
belongs_to :parent, class_name: 'Model', foreign_key: 'model_id'
has_many :children, class_name: 'Model', foreign_key: 'model_id'
end
I am using adjacency structure, which can have infinite depth. I am on a Postgres database using recursive selects.
What will be the most sane way to get a nested hash of objects? I tried to select instances of Model and sort them, yet could not bring this to any usable result.
Lets say I have four Model instances saved in my database: Model_1, Model_2, Model_3 and Model_4. Model_3 is a child of Model_2 and Model_4 is a child of Model_3.
Here is an output I am trying to achieve (a nested hash of Model instances):
{
#<Model_1...> => {},
#<Model_2...> => {
#<Model_3...> => {
#<Model_4...> => {}
}
}
}
Any ideas?
Update: Tree is already recovered — either as a CollectionProxy, Relation or any other array-ish data structure. I wan't to sort that tree into the hash of nested hashes.

Answer 1

I would name it parent_id field.
belongs_to :parent, class_name: "Model"
has_many :children, class_name: "Model", foreign_key: "parent_id"
When you have the hash, you would use sort or sort_by:
http://www.ruby-doc.org/core-2.1.0/Enumerable.html#method-i-sort_by
def sort(hash)
hash.sort { |m1, m2| m1.id <=> m2.id }
sort(hash.children)
end

Answer 2

First, define the ff method inside your Model class:
def to_hash
if children.empty?
{self => {}}
else
{self => children.inject({}) {|hash, model| hash.merge(model.to_hash) } }
end
end
Then do the ff to get the output you want:
top_level_models = #code that queries top-level models while eager-loading all nested children
hash_of_nested_models = top_level_models.inject({}) {|hash, ancestor| hash.merge(ancestor.to_hash) }
The hash argument that you pass to includes should cover the depth of your nesting. The argument passed to includes above will be the nesting for children with a depth of 3 descendants. For as long as you includes all the nested children in your where query, generating the hash will not do any more db queries.
Hope that helps!

Answer 3

Getting AR to do this without N+1 queries would be difficult. Will have to write something that works with the data in memory.
You would have to write a custom function which looks something like:
def to_hash
root_hash = {}
# Load all the model into memory and into a hash to allow faster access when we have the id
models = Hash[Model.all.collect {|m| [m.id, m]}]
# The resultant hash for each child
models_with_hash = Hash[map.values.collect {|m| [m.id, {}]} ]
# Stitch them together
models.each do |id, m|
if m.model_id.nil?
root_hash.merge! m => models_with_hash[m.id]
else
# should name model_id to parent_id
models_with_hash[m.model_id].merge! m => models_with_hash[m.id]
end
end
root_hash
end