can add relation between co-author

app/models/user/co_author.rb

@@ -8,6 +8,7 @@ class CoAuthor
   field :email
   field :type
 
+  has_and_belongs_to_many :co_author_relations
 
   VALID_EMAIL_REGEX = /\A[\w+\-.]+@[a-z\d\-.]+\.[a-z]+\z/
   validates :email, format: { with: VALID_EMAIL_REGEX },

app/models/user/co_author_relation.rb

@@ -0,0 +1,8 @@
+class CoAuthorRelation
+  include Mongoid::Document
+  include Mongoid::Timestamps
+
+  field :relation, localize: true
+
+  has_and_belongs_to_many :co_authors
+end