has_many problem :)

gistfile1.rb

Models

class User < ActiveRecord::Base
  has_many :dnshospitals
  has_many :hospitals, :through => :dnshospitals
end

class Hospital < ActiveRecord::Base
  has_many :dnshospitals
  has_many :users, :through => :dnshospitals
  
  validates_presence_of :name, :abbreviation, :city
end

class Dnshospital < ActiveRecord::Base
  belongs_to :user
  belongs_to :hospital
end

Controller

class DnshospitalsController < ApplicationController
  def new
    @dns = current_user.dnshospitals.build
  end
  
  def create
    @dns = current_user.dnshospitals.build(params[:dnshospital])
    if @dns.save
      flash[:notice] = "DNS added successfully"
      redirect_to user_profile_path + "#tabs-DNS"
    else
      flash[:alert] = "Please fill up the required fields"
      render :action => 'new'
    end
  end

Views
_form.html.haml
= form_for([current_user, @dns])  do |f|
  = render 'shared/flash_messages'
  %p
    %br/
    = hidden_field_tag "dnshospital[hospital_ids][]", nil
    - Hospital.all.each do |h|
      = check_box_tag "dnshospital[hospital_ids][]", h.id, current_user.hospital_ids.include?(h.id), id: dom_id(h)
      = label_tag dom_id(h), h.name
      %br/
  %p  
    = f.submit 'Add DNS', :class => "btn btn-green"
    .clear