Cleanest way to create a Hash from an Array

Question

I seem to run into this very often. I need to build a Hash from an array using an attribute of each object in the array as the key.

Lets say I need a hash of example us

Answer 1

Install the Ruby Facets Gem and use their Array.to_h.

Answer 2

a shortest one?

# 'Region' is a sample class here
# you can put 'self.to_hash' method into any class you like 

class Region < ActiveRecord::Base
  def self.to_hash
    Hash[*all.map{ |x| [x.id, x] }.flatten]
  end
end

Answer 3

There is already a method in ActiveSupport that does this.

['an array', 'of active record', 'objects'].index_by(&:id)

And just for the record, here's the implementation:

def index_by
  inject({}) do |accum, elem|
    accum[yield(elem)] = elem
    accum
  end
end

Which could have been refactored into (if you're desperate for one-liners):

def index_by
  inject({}) {|hash, elem| hash.merge!(yield(elem) => elem) }
end

Answer 4

You can add to_hash to Array yourself.

class Array
  def to_hash(&block)
    Hash[*self.map {|e| [block.call(e), e] }.flatten]
  end
end

ary = [collection of ActiveRecord objects]
ary.to_hash do |element|
  element.id
end

Answer 5

In case someone got plain array

arr = ["banana", "apple"]
Hash[arr.map.with_index.to_a]
 => {"banana"=>0, "apple"=>1}