How to merge array of hashes to get hash of arrays of values

Take your pick:

hs.reduce({}) {|h,pairs| pairs.each {|k,v| (h[k] ||= []) << v}; h}

hs.map(&:to_a).flatten(1).reduce({}) {|h,(k,v)| (h[k] ||= []) << v; h}

I’m strongly against messing with the defaults for hashes, as the other suggestions do, because then checking for a value modifies the hash, which seems very wrong to me.