Mongoid doesn't provide typed Array declarations, so you have to to convert them by hand when pushing params from request to your model instance. This one helps by defining the conversion callback automatically

field_converters.rb

# class YoClass
#   include Mongoid::Document
#   include FieldConverters
#   
#   field :an_array, type => Array
#   convert_array [:an_array], :to_array_of => Integer,  :before => :validate
# end

module FieldConverters
  def self.included(base)
    base.send :extend, ClassMethods
  end

  module ClassMethods
    @@CONVERTERS = { Integer => :to_i, String => :to_s, Float => :to_f}.freeze
    def convert_array(attributes, opts)
      class_eval do
        set_callback(opts[:before], :before) do |model|
          attributes.each do |attribute|
            val = self.send(attribute)
            self.send((attribute.to_s + '='), []) if val.nil?
            val.map!(&@@CONVERTERS[opts[:to_array_of]])
          end
        end
      end
    end
  end
end