My current use case is that I want to create a tree-like data structure where all trees are documents in a Tree collection, and all nodes are embedded into that Tree document instead of belonging to a separate association, for performance reasons. The code at its simplest could look like this:

class Tree
  include Mongoid::Document
    embeds_many :nodes
    field :name, type: String
end

class Node
  include Mongoid::Document
    embedded_in :trees
    has_many :subnodes, class_name: "Node"
    belongs_to :parent_node, class_name: "Node"
    field :name, type: String
end

Trying to create a node will raise a `Mongoid::Errors::MixedRelations` error. Which is fine, I can see why this wouldn't be currently possible.

The feature request is to add a way to pass to the `belongs_to` a new argument, maybe name it `:through` or `:collection` or `:scope` or something, that lets it know that it shouldn't search or create new Nodes into a main Node collection, but within the embedded nodes of its own parent Tree class. Similarly for the has_many relation.

The issue of having access to the root document of Node would be solved trivially as all Nodes will have direct access to their parent Tree