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 uses ActiveRecord objecs keyed by their ids Common way:

ary = [collection of ActiveRecord objects]
hash = ary.inject({}) {|hash, obj| hash[obj.id] = obj }

Another Way:

ary = [collection of ActiveRecord objects]
hash = Hash[*(ary.map {|obj| [obj.id, obj]}).flatten]

Dream Way: I could and might create this myself, but is there anything in Ruby or Rails that will this?

ary = [collection of ActiveRecord objects]
hash = ary.to_hash &:id
#or at least
hash = ary.to_hash {|obj| obj.id}

August Lilleaas · Accepted Answer · 2009-01-20T10:24:00.153

67

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

edited Jan 20 '09 at 10:24

answered Jan 05 '09 at 12:17

August Lilleaas

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1

I think if you change the merge to merge! you'll avoid creating a bunch of intermediate hashes you don't need. – Scott Jan 05 '09 at 15:26
If you are going to use this many times in the critical path of you app, you might want to consider using ary.index_by{|o| o.id} instead of using symbol_to_proc. – krusty.ar Jan 05 '09 at 19:08
`index_by` seems to be a duplication of Ruby's `group_by`. Am I missing anything? – JMH Jan 26 '15 at 22:54
2

group_by has an array as the value, whereas index_by assumes there's only one item per key so the value is that single item, not an array. – August Lilleaas Jan 27 '15 at 08:21
`['an array', 'of active record', 'objects'].index_by(&:id)` fails with error NoMethodError: undefined method `id' for "an array":String using Rails 4.2.5 and Ruby 2.3.0 – Sven R. Feb 24 '16 at 16:33
The example is not self contained, the strings are representing, as their contents suggest, active record objects, which has an id method :) – August Lilleaas Feb 25 '16 at 09:11

score 10 · Answer 2 · answered Apr 14 '13 at 22:44

10

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

answered Apr 14 '13 at 22:44

zed_0xff

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score 9 · Answer 3 · answered Mar 10 '17 at 14:15

9

In case someone got plain array

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

answered Mar 10 '17 at 14:15

Fedcomp

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score 5 · Answer 4 · answered Jan 05 '09 at 11:52

5

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

answered Jan 05 '09 at 11:52

ewalshe

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score 0 · Answer 5 · answered Jan 05 '09 at 17:14

0

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

answered Jan 05 '09 at 17:14

Lolindrath

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2

I wouldn't recommend adding a dependency for a simple method – ricks Aug 15 '19 at 20:55

Cleanest way to create a Hash from an Array

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