Unknown In my app I use has_and_belongs_to_many associations in embedded models – behaviour which used to work in Mongoid 7, but no longer works in Mongoid 8. I tried going through the changelog and docs, but could not find anything that seemed to indicate this behaviour should no longer work or has been changed. This seems like a bug.
class Page include Mongoid::Document embeds_many :blocks, class_name: "Block" end class Block include Mongoid::Document embedded_in :page end class ImageBlock < Block has_and_belongs_to_many :attachments, inverse_of: nil accepts_nested_attributes_for :attachments end class Attachment include Mongoid::Document field :file, type: String end page = Page.create! image_block = page.blocks.build({ _type: "ImageBlock", attachments_attributes: [{ file: "foo.jpg" }] }) image_block.save! # => "Mongo::Error::OperationFailure: An empty update path is not valid. (on 127.0.0.1:27017, legacy retry, attempt 1)" page.save! # => "Mongoid::Errors::InvalidPath: message: Having a root path assigned for ImageBlock is invalid. summary: Mongoid has two different path objects for determining the location of a document in the database, Root and Embedded. This error is raised when an embedded document somehow gets a root path assigned. resolution: Most likely your embedded model, ImageBlock is also referenced via a has_many from a root document in another collection. Double check the association definitions and fix any instances where embedded documents are improperly referenced from other collections."