Ruby sort by multiple values?

I have an array of hashes:

a=[{ 'foo'=>0,'bar'=>1 },
   { 'foo'=>0,'bar'=>2 },
   ... ]

I want to sort the array first by each hash's 'foo', then by 'bar'. Google tells me this is how it's done:

a.sort_by {|h| [ h['foo'],h['bar'] ]}

But this gives me the ArgumentError "comparison of Array with Array failed". What does this mean?

Solution 1:

a.sort { |a, b| [a['foo'], a['bar']] <=> [b['foo'], b['bar']] }

Solution 2:

It probably means you're missing one of the fields 'foo' or 'bar' in one of your objects.

The comparison is coming down to something like nil <=> 2, which returns nil (instead of -1, 0 or 1) and #sort_by doesn't know how to handle nil.

Try this:

a.sort_by {|h| [ h['foo'].to_i, h['bar'].to_i ]}