Rails 3. How to get the difference between two arrays?

a = [2, 4, 6, 8]
b = [1, 2, 3, 4]

a - b | b - a # => [6, 8, 1, 3]

First you would get a list of shipping_id's that appear in invoices:

ids = i.map{|x| x.shipment_id}

Then 'reject' them from your original array:

s.reject{|x| ids.include? x.id}

Note: remember that reject returns a new array, use reject! if you want to change the original array

Use substitute sign

irb(main):001:0> [1, 2, 3, 2, 6, 7] - [2, 1]
=> [3, 6, 7]

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