Given those classes :

class Article
  include Mongoid::Document
  has_many :comments
end

class Comment
  include Mongoid::Document
  belongs_to :article
end 

When I create a comment with an article, the number of comments in the article isn't updated after destroy_all. The clear does the job.

    article = Article.create
    comment = Comment.create(article: article)

    pp article.comments.length # => 1
    article.comments.destroy_all
    pp article.comments.length # => Continues to give 1 and should give 0
    article.comments.clear
    pp article.comments.length # => 0

If I add article.reload after the creation of the comment the behavior is correct. This problem isn't in Mongoid 6.2.

delete_all also currently does not clear the association - this may be the correct/desired behavior and when this ticket is worked on tests should be added to this effect if there aren't already.