undefined method errors for before_save in post model

Question

I have a post model where I'm trying to implement a before_save call so I can manipulate input from a form before saving it to the database. Anything I try throws an error, whether it's downcase, parameterize, gsub, or split. I get an undefined method 'downcase' for error every time. Just replace downcase with whatever I'm trying to do. All I want to do is take whatever the user inputs and replace the spaces with an underscore.

Here is one version of the Post model.

class Post < ApplicationRecord
  belongs_to :user
  has_many :comments, dependent: :destroy
  before_save :permalink_tag
  strip_attributes

  acts_as_taggable

  private
  def permalink_tag 
    self.tag_list = self.tag_list.split(' ').join('_') unless self.tag_list.nil?
  end
end

Now you can simply replace the code in the permalink_tag method with any of these alternatives and it still results in an error.

self.tag_list.gsub(/\s+/, "_")
before_save { |post| post.tag_list = post.tag_list.downcase }
self.tag_list.parameterize.underscore

The error in more detail:

NoMethodError in PostsController#update
undefined method `downcase' for ["sample-tag"]:ActsAsTaggableOn::TagList

Application Trace:

app/models/post.rb:4:in `block in <class:Post>'
app/controllers/posts_controller.rb:45:in `block in update'
app/controllers/posts_controller.rb:44:in `update'

Answer 1

Knowing that this was an issue with the gem, I was able to find the answer in their docs.

ActsAsTaggableOn.force_lowercase = true
ActsAsTaggableOn.force_parameterize = true