Adding if validates in rails

Question

I search a solution to require several selections of response (3), whitout "check_must_be_3" I do not have problem. If I add this solution, I have a rollback. This is my idea but it seems not to be working!

model.rb:

validates :check, presence: true
validate :check_must_be_3

  private
    def checks_must_be_3
      if !check != 3
        errors[:base] << "You must select exactly 3 checks"
      end
    end

html:

<%= simple_form_for @answer do |f| %>
  <h3>Choose 3 answers</h3>
    <ul>
    <% (1..5).each do |x| %>
      <div class="checkbox">
        <label>
          <input type="checkbox" name="answer[check][]" id="optionsCheckbox<%= x %>" value="<%= x %>" />
          <%= x %>
        </label>
      </div>
    <% end %>
    <%= f.button :submit, "Submit", class: "btn btn-primary" %>
<% end %>

controller:

private

def answer_params
    params.require(:answer).permit(check:[])
end

Answer 1

#try this
    private
    def checks_must_be_3
      unless check.count == 3
        errors.add(:base , "You must select exactly 3 checks")
      end
    end