How can I group this array of hashes?

Question

I have this array of hashes:

- :name: Ben
  :age: 18
- :name: David
  :age: 19
- :name: Sam
  :age: 18

I need to group them by age, so they end up like this:

18:
- :name: Ben
  :age: 18
- :name: Sam
  :age: 18
19:
- :name: David
  :age: 19

I tried doing it this way:

array = array.group_by &:age

but I get this error:

NoMethodError (undefined method `age' for {:name=>"Ben", :age=>18}:Hash):

What am I doing wrong? I'm using Rails 3.0.1 and Ruby 1.9.2

Solution 1

The &:age means that the group_by method should call the age method on the array items to get the group by data. This age method is not defined on the items which are Hashes in your case.

This should work:

array.group_by { |d| d[:age] }

Solution 2

out = {}
array_of_hashes.each do |a_hash|
  out[a_hash[:age]] ||= []
  out[a_hash[:age]] << a_hash
end

or

array.group_by {|item| item[:age]}

Solution 3

As others have pointed out ruby's Symbol#to_proc method is invoked and calls the age method on each hash in the array. The problem here is that the hashes do not respond to an age method.

Now we could define one for the Hash class, but we probably don't want it for every hash instance in the program. Instead we can simply define the age method on each hash in the array like so:

array.each do |hash|
  class << hash
    def age
      self[:age]
    end
  end
end

And then we can use group_by just as you were before:

array = array.group_by &:age