How to merge two arrays of hashes

[a, b].map { |arr| arr.group_by { |e| e[:key] } }
      .reduce(&:merge)
      .flat_map(&:last)

Here we use hash[:key] as a key to build the new hash, then we merge them overriding everything with the last value and return values.

uniq would work if you concatenate the arrays in reverse order:

(b + a).uniq { |h| h[:key] }
#=> [
#     {:key=>1, :value=>"bar"},
#     {:key=>1000, :value=>"something"},
#     {:key=>2, :value=>"baz"}
#   ]

It doesn't however preserve the order.