def subtract(other, num_partitions=nil)
      mapping_function = 'lambda{|x| [x,nil]}'

      self.map(mapping_function)
          .subtract_by_key(other.map(mapping_function), num_partitions)
          .keys
    end
def subtract(other, num_partitions=nil)
      mapping_function = 'lambda{|x| [x,nil]}'

      self.map(mapping_function)
          .subtract_by_key(other.map(mapping_function), num_partitions)
          .keys
    end
Return an RDD with the elements from self that are not in other.

 == Example:
   rdd1 = $sc.parallelize([["a", 1], ["a", 2], ["b", 3], ["c", 4]])
   rdd2 = $sc.parallelize([["a", 2], ["c", 6]])
   rdd1.subtract(rdd2).collect
   # => [["a", 1], ["b", 3], ["c", 4]]