reduce params to filtered updates based on active record accept nested attributes with deletion

# ruby
res = params.require(:table).permit(:field1, 
      table_sub_table_attributes: [:key_field_id, :id, :_destroy])
res[:table_sub_table_attributes] = reduce_params(res[:table_sub_table_attributes], :key_field_id)

# reduce delete to delete existing which is not selected
#   and insert only when is not an existing
#   does not handle updating existing (which is not used)
def reduce_params(attributes, key_field)
  res = []
  retain = []
  attributes.each do |row|
    # only append destroy if no new record found
    if row[:_destroy]
      # existing destroy
      del = true
      attributes.each do |inner_row|
        if !inner_row[:_destroy] && row[key_field] == inner_row[key_field]
          # found insert, means don't delete
          del = false
          break
        end
      end # inner loop
      if !del
        retain << row[key_field] # ignoring
      else
        res << row # append for deletion
      end
    elsif !row[:id]
      # only append new record if not an existing
      if !retain.include?(row[key_field])
        res << row
      end
      # insert or existing
    end
  end # outer loop
  # Rails.logger.info "in: #{attributes.inspect}, \n result: #{res.inspect}"
  res
end