load all the latest messages for conversations at once

Change-Id: I5df13199af77c52a17f172e24b87089a7f15775b
Reviewed-on: https://gerrit.instructure.com/5143
Reviewed-by: Jon Jensen <jon@instructure.com>
Tested-by: Hudson <hudson@instructure.com>

Author: zwily

app/controllers/conversations_controller.rb

@@ -27,7 +27,7 @@ class ConversationsController < ApplicationController
   add_crumb(lambda { I18n.t 'crumbs.messages', "Conversations" }) { |c| c.send :conversations_url }
 
   def index
-    @conversations = case params[:scope]
+    conversations = case params[:scope]
       when 'unread'
         @view_name = I18n.t('index.inbox_views.unread', 'Unread')
         @no_messages = I18n.t('no_unread_messages', 'You have no unread messages')
@@ -55,8 +55,17 @@ def index
         @view_name = I18n.t('index.inbox_views.inbox', 'Inbox')
         @no_messages = I18n.t('no_messages', 'You have no messages')
         @current_user.conversations.default
-    end.map{ |c| jsonify_conversation(c) }
+    end
     @scope ||= params[:scope].to_sym
+    # optimize loading the most recent messages for each conversation into a single query
+    last_messages = ConversationMessage.latest_for_conversations(conversations).human.
+                      inject({}) { |hash, message|
+                        if !hash[message.conversation_id] || hash[message.conversation_id].id < message.id
+                          hash[message.conversation_id] = message
+                        end
+                        hash
+                      }
+    @conversations_json = conversations.map{ |c| jsonify_conversation(c, last_messages[c.conversation_id]) }
     @user_cache = Hash[*jsonify_users([@current_user]).map{|u| [u[:id], u] }.flatten]
   end
 
@@ -277,10 +286,10 @@ def create_message_on_conversation(conversation=@conversation, update_for_sender
     message
   end
 
-  def jsonify_conversation(conversation)
+  def jsonify_conversation(conversation, last_message = nil)
     hash = {:audience => formatted_audience(conversation, 3)}
     hash[:avatar_url] = avatar_url_for(conversation)
-    conversation.as_json.merge(hash)
+    conversation.as_json(:last_message => last_message).merge(hash)
   end
 
   def jsonify_users(users, blank_avatar_fallback = false)

app/models/conversation_message.rb

@@ -29,6 +29,14 @@ class ConversationMessage < ActiveRecord::Base
 
   named_scope :human, :conditions => "NOT generated"
   named_scope :with_media_comments, :conditions => "media_comment_id IS NOT NULL"
+  named_scope :latest_for_conversations, lambda{ |conversation_participants| {
+    :select => 'conversation_messages.*',
+    :joins => 'JOIN conversation_message_participants ON conversation_messages.id = conversation_message_id',
+    :conditions => [
+        "conversation_participant_id in (?) AND conversation_messages.created_at = (SELECT MAX(created_at) FROM conversation_messages cm2 WHERE cm2.conversation_id = conversation_messages.conversation_id)",
+        conversation_participants.map(&:id)
+      ]
+  }}
 
   validates_length_of :body, :maximum => maximum_text_length
 

app/models/conversation_participant.rb

@@ -56,7 +56,7 @@ def self.labels
   validates_inclusion_of :label, :in => labels.map(&:first), :allow_nil => true
 
   def as_json(options = {})
-    latest = messages.human.first
+    latest = options[:last_message] || messages.human.first
     {
       :id => conversation_id,
       :participants => participants(private?),
@@ -67,7 +67,7 @@ def as_json(options = {})
       :subscribed => subscribed?,
       :private => private?,
       :label => label,
-      :properties => properties
+      :properties => properties(latest)
     }
   end
 
@@ -114,8 +114,8 @@ def infer_defaults
     self.has_media_objects = conversation.has_media_objects?
   end
 
-  def properties
-    latest = messages.human.first
+  def properties(latest = nil)
+    latest ||= messages.human.first
     properties = []
     properties << :last_author if latest && latest.author_id == user_id
     properties << :attachments if has_attachments?

app/views/conversations/index.html.erb

@@ -216,7 +216,7 @@
     MessageInbox.user_id = <%= @current_user.id %>;
     MessageInbox.user_cache = <%= raw @user_cache.to_json %>;
     MessageInbox.contexts = <%= raw @contexts.to_json %>;
-    MessageInbox.initial_conversations = <%= raw @conversations.to_json %>;
+    MessageInbox.initial_conversations = <%= raw @conversations_json.to_json %>;
     MessageInbox.scope = <%= raw @scope.to_json %>;
     MessageInbox.label_scope = <%= raw @label.to_json %>;
   </script>

spec/controllers/conversations_controller_spec.rb

@@ -45,7 +45,7 @@ def conversation(num_other_users = 1)
 
       get 'index'
       response.should be_success
-      assigns[:conversations].map{|c|c[:id]}.should == @user.conversations.map(&:conversation_id)
+      assigns[:conversations_json].map{|c|c[:id]}.should == @user.conversations.map(&:conversation_id)
     end
 
     it "should work for an admin as well" do
@@ -56,7 +56,7 @@ def conversation(num_other_users = 1)
 
       get 'index'
       response.should be_success
-      assigns[:conversations].map{|c|c[:id]}.should == @user.conversations.map(&:conversation_id)
+      assigns[:conversations_json].map{|c|c[:id]}.should == @user.conversations.map(&:conversation_id)
     end
   end