Pastable version of Grouping an array of hashes

<pre class='prettyprint' language='ruby'># given this structure:
array_of_hashes = [
  {:callid =&gt; '1', :foo =&gt; '1', :created_at =&gt; '2007-09-21 11:45:15'}, 
  {:callid =&gt; '1', :foo =&gt; '2', :created_at =&gt; '2007-09-21 11:45:16'},
  {:callid =&gt; '2', :foo =&gt; '1', :created_at =&gt; '2007-09-21 11:45:19'}, 
  {:callid =&gt; '2', :foo =&gt; '2', :created_at =&gt; '2007-09-21 11:45:20'},
  {:callid =&gt; '3', :foo =&gt; '1', :created_at =&gt; '2007-09-21 11:45:21'}
]

# group using this:
array_of_arrays_of_hashes = returning([]) do |grouped_array|
  array_of_hashes.map {|x| x[:callid]}.uniq.each do |callid|
    grouped_array[callid.to_i] = array_of_hashes.select {|y| y[:callid] == callid}
  end
end.compact

# and this is the result:
array_of_arrays_of_hashes == [
  [{:callid=&gt;&quot;1&quot;, :foo=&gt;&quot;1&quot;, :created_at=&gt;&quot;2007-09-21 11:45:15&quot;}, 
   {:callid=&gt;&quot;1&quot;, :foo=&gt;&quot;2&quot;, :created_at=&gt;&quot;2007-09-21 11:45:16&quot;}],

[{:callid=&gt;&quot;2&quot;, :foo=&gt;&quot;1&quot;, :created_at=&gt;&quot;2007-09-21 11:45:19&quot;}, 
   {:callid=&gt;&quot;2&quot;, :foo=&gt;&quot;2&quot;, :created_at=&gt;&quot;2007-09-21 11:45:20&quot;}],

[{:callid=&gt;&quot;3&quot;, :foo=&gt;&quot;1&quot;, :created_at=&gt;&quot;2007-09-21 11:45:21&quot;}]
]</pre>
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Pastable version of Grouping an array of hashes