OK, I've written a solution, it might not be the nicest one, but it works to me! It parses the input token value to JSON format (when validation fails), which is used when loading the page. Under page load it just loads tags from DB.
def tag_tokens
if @tag_tokens
#if there is user info, parse it to json format. create an array
array = @tag_tokens.split(",")
tokens_json = []
#loop through each tag and check if it's new or existing
array.each do |tag|
if tag.to_s.match(/^CREATE_/)
#if new generate json part like this:
tag.gsub!(/CREATE_(.+?)_END/) do
tokens_json << "{\"id\":\"CREATE_#{$1.strip.downcase}_END\",\"name\":\"Add: #{$1.strip.downcase}\"}"
end
else
#if tag is already in db, generate json part like this:
tokens_json << "{\"id\":#{tag},\"name\":\"#{Tag.find_by_id(tag).name}\"}"
end
end
#encapsulate the value for token input with [] and add all tags from array
"[#{tokens_json.to_sentence(:last_word_connector => ',', :words_connector => ',', :two_words_connector => ',')}]"
else
#if there is no user input already load from DB
tags.to_json(:only => [:id, :name])
end
end