Validation on array checkbox in Model

Hi,

I am using array ‘sub_id[]’ for multiple checkbox,
i want to place validations on all array ‘sub_ids[]’ checbox in model just like other validations.

<% for e in @subjects do -%>
<input type=“checkbox” name=“sub_ids[]” id="<%= e.id %>" value="<%= e.id %>"<% if @book.subjects.include? e %> checked=“checked”<%end%>>

<%= e.name %>
<% end %>

MODEL

class Book < ActiveRecord::Base

has_and_belongs_to_many :subjects
 validates_presence_of:title
validates_numericality_of:

price
validate :at_least_one_checkbox_was_ticked

protected

def at_least_one_checkbox_was_ticked

unless params[:sub_ids[]].nil?
    end

end

end

I’m using only two models book and subject.

thanx in adv.