mongoid building an embedded many document with a references_many in it saves when it shouldn't

mongoid embedded build fail

class Formula
  include Mongoid::Document
  
  referenced_in :formula_category
  
  field :quantities, :type => Array, :default => []
  field :directions, :type => String
  
  embeds_many :formula_medications
end

class FormulaMedication
  include Mongoid::Document
  
  embedded_in :formula, :inverse_of => :formula_medications
  referenced_in :medication, :inverse_of => :formula_medications
  
  field :concentration
end

class Medication
  include Mongoid::Document
  
  field :name, :type => String
  field :concentrations, :type => Array, :default => []
  field :description, :type => String
  
  references_many :formula_medications
end