Don't hardcode model / table name unless necessary

Added a regression test because I was not sure removing the class name
from the `belongs_to` would work. It does.

app/models/mailboxer/conversation.rb

@@ -14,7 +14,7 @@ class Mailboxer::Conversation < ActiveRecord::Base
 
   scope :participant, lambda {|participant|
     where('mailboxer_notifications.type'=> Mailboxer::Message.name).
-    order("mailboxer_conversations.updated_at DESC").
+    order(updated_at: :desc).
     joins(:receipts).merge(Mailboxer::Receipt.recipient(participant)).distinct
   }
   scope :inbox, lambda {|participant|
@@ -33,10 +33,11 @@ class Mailboxer::Conversation < ActiveRecord::Base
     participant(participant).merge(Mailboxer::Receipt.not_trash)
   }
   scope :between, lambda {|participant_one, participant_two|
-    joins("INNER JOIN (#{Mailboxer::Notification.recipient(participant_two).to_sql}) participant_two_notifications ON participant_two_notifications.conversation_id = mailboxer_conversations.id AND participant_two_notifications.type IN ('Mailboxer::Message')").
+    joins("INNER JOIN (#{Mailboxer::Notification.recipient(participant_two).to_sql}) participant_two_notifications " \
+          "ON participant_two_notifications.conversation_id = #{table_name}.id AND participant_two_notifications.type IN ('Mailboxer::Message')").
         joins("INNER JOIN mailboxer_receipts ON mailboxer_receipts.notification_id = participant_two_notifications.id").
         merge(Mailboxer::Receipt.recipient(participant_one)).
-        order("mailboxer_conversations.updated_at DESC").distinct
+        order(updated_at: :desc).distinct
   }
 
   #Mark the conversation as read for one of the participants

app/models/mailboxer/message.rb

@@ -2,7 +2,7 @@ class Mailboxer::Message < Mailboxer::Notification
   attr_accessible :attachment if Mailboxer.protected_attributes?
   self.table_name = :mailboxer_notifications
 
-  belongs_to :conversation, :class_name => "Mailboxer::Conversation", :validate => true, :autosave => true
+  belongs_to :conversation, :validate => true, :autosave => true
   validates_presence_of :sender
 
   class_attribute :on_deliver_callback

spec/models/message_spec.rb

@@ -18,6 +18,10 @@
       expect(@message1.is_deleted?(@entity1)).to be true
     end
 
+    it "creates a conversation" do
+      expect(@message1.conversation).to eq(Mailboxer::Conversation.last)
+    end
+
     it "should send email only to receivers" do
       expect(ActionMailer::Base.deliveries.count).to eq 1
     end