How to save multiple checkbox values to string before create?

in view

<input id="checkbox-1"  type="checkbox" value="drama" name="user[book][]">    
<input id="checkbox-2"  type="checkbox" value="poem" name="user[book][]">    
<input id="checkbox-3" type="checkbox" value="fantasy" name="user[book][]"> 

if checked 1 and 3 checkbox, it is expected that something like "drama, fantasy" save in field book

in controller

private
 def user_params
  params[:user][:book] = params[:user][:book].join(',') 
  params.require(:user).permit(title, email, :book, message)
 end 

When I try to save the form input to database i get the following error:

Template is missing