Been pulling my hair out on this issue for a couple hours now. I'm not sure what causes it but 2.4.10 introduces a bug where duplicate embedded documents are created on one of my embeds_many relations.

class Post
  include Mongoid::Document

  cache

  embeds_many :topic_mentions, as: :topic_mentionable, :class_name => "TopicMention"

  def save_topic_mention(topic)
    existing = topic_mentions.detect{|mention| mention.id == topic.id}
    unless existing
      mention = self.topic_mentions.build({name: topic.name})
      mention.id = topic.id
    end
  end
end

class TopicMention
  include Mongoid::Document

  field :name

  embedded_in :topic_mentionable, polymorphic: true
end

Calling save_topic_mention with just one topic inserts the embedded document just fine. There is only one document on the post when I check in the debugger but after the entire request is complete the resulting database structure has two of the same topic_mention.

2.4.9 works fine, 2.4.10 introduced this problem.

Please let me know if I can provide more information.