SimpleForm (1.4.x) input for mongoid_taggable -- place in app/inputs

tags_input.rb

# Input builder to render tags from an Array field, for use with my fork
# of mongoid_taggable:
#
#   https://github.com/ches/mongoid_taggable
#
class TagsInput < SimpleForm::Inputs::StringInput

  map_type :tags, :to => :text_field

protected

  def html_options_for(namespace, extra)
    formatted_tags = object.send(attribute_name).
                            try(:join, "#{object.send(:tags_separator)} ")
    super.merge(:value => formatted_tags)
  end

end

Sorry, this was written for SimpleForm 1.4.x. I haven't updated yet, so I'm not sure how the API has changed.