can select relection from relation db

app/models/user/co_author.rb

@@ -13,7 +13,7 @@ class CoAuthor
   VALID_EMAIL_REGEX = /\A[\w+\-.]+@[a-z\d\-.]+\.[a-z]+\z/
   validates :email, format: { with: VALID_EMAIL_REGEX },
             allow_blank: true,
-             uniqueness: { case_sensitive: false }
+            uniqueness: { case_sensitive: false }
 
   validates :co_author, presence: true
   before_save { |coauthor| coauthor.email = email.downcase if not coauthor.email.nil?}

app/models/user/co_author_relation.rb

@@ -5,4 +5,6 @@ class CoAuthorRelation
   field :relation, localize: true
 
   has_and_belongs_to_many :co_authors
+
+  validates :relation, presence: true
 end

app/models/user/user.rb

@@ -3,7 +3,7 @@ class User
   include Mongoid::Document
   include Mongoid::Timestamps
 
-  devise :database_authenticatable, :recoverable, :rememberable, :trackable, :registerable, :timeoutable
+  devise :database_authenticatable, :recoverable, :rememberable, :trackable, :registerable, :validatable, :timeoutable
   
   mount_uploader :avatar, AvatarUploader
   
@@ -194,4 +194,4 @@ class User
     end
   end
 
-end
+end