use helper method to simplify

app.rb

@@ -20,134 +20,141 @@ Mongoid.logger.level = Logger::INFO
 Dir[File.dirname(__FILE__) + '/models/*.rb'].each {|file| require file}
 Dir[File.dirname(__FILE__) + '/lib/**/*.rb'].each {|file| require file}
 
+
+helpers do
+  def commentable
+    @commentable ||= Commentable.find(params[:commentable_id])
+  end
+
+  def user # TODO handle 404 if integrated user service
+    @user ||= (User.find_or_create_by(external_id: params[:user_id]) if params[:user_id])
+  end
+
+  def thread
+    @thread ||= CommentThread.find(params[:thread_id])
+  end
+
+  def comment
+    @comment ||= Comment.find(params[:comment_id])
+  end
+
+  def source
+    @source ||= case params["source_type"]
+      when "user"
+        User.find_or_create_by(external_id: params["source_id"])
+      when "thread"
+        CommentThread.find(params["source_id"])
+      when "other"
+        Commentable.find(params["source_id"])
+    end
+  end
+
+  def vote_for(obj)
+    user.vote(obj, params["value"].to_sym)
+    obj.reload.to_hash.to_json
+  end
+
+  def undo_vote_for(obj)
+    user.unvote(obj)
+    obj.reload.to_hash.to_json
+  end
+
+end
+
 delete '/api/v1/:commentable_id/threads' do |commentable_id|
-  Commentable.find(commentable_id).comment_threads.destroy_all
+  commentable.comment_threads.destroy_all
   {}.to_json
 end
 
 get '/api/v1/:commentable_id/threads' do |commentable_id|
-  Commentable.find(commentable_id).comment_threads.map{|t| t.to_hash(recursive: params["recursive"])}.to_json
+  commentable.comment_threads.map{|t| t.to_hash(recursive: params["recursive"])}.to_json
 end
 
 post '/api/v1/:commentable_id/threads' do |commentable_id|
   thread = CommentThread.new(params.slice(*%w[title body course_id]).merge(commentable_id: commentable_id))
-  author = User.find_or_create_by(external_id: params["user_id"]) if params["user_id"]
-  thread.author = author
+  thread.author = user
   thread.save!
-  if params["auto_subscribe"]
+  if params["auto_subscribe"] and author
     author.subscribe(thread)
   end
   thread.to_hash.to_json
 end
 
 get '/api/v1/threads/:thread_id' do |thread_id|
-  thread = CommentThread.find(thread_id)
   thread.to_hash(recursive: params["recursive"]).to_json
 end
 
 put '/api/v1/threads/:thread_id' do |thread_id|
-  thread = CommentThread.find(thread_id)
   thread.update_attributes!(params.slice(*%w[title body]))
   thread.to_hash.to_json
 end
 
 post '/api/v1/threads/:thread_id/comments' do |thread_id|
-  thread = CommentThread.find(thread_id)
   comment = thread.comments.new(params.slice(*%w[body course_id]))
-  author = User.find_or_create_by(external_id: params["user_id"]) if params["user_id"]
-  comment.author = author
+  comment.author = user 
   comment.save!
-  if params["auto_subscribe"]
+  if params["auto_subscribe"] and author
     author.subscribe(thread)
   end
   comment.to_hash.to_json
 end
 
 delete '/api/v1/threads/:thread_id' do |thread_id|
-  thread = CommentThread.find(thread_id)
   thread.destroy
   thread.to_hash.to_json
 end
 
 get '/api/v1/comments/:comment_id' do |comment_id|
-  comment = Comment.find(comment_id)
   comment.to_hash(recursive: params["recursive"]).to_json
 end
 
 put '/api/v1/comments/:comment_id' do |comment_id|
-  comment = Comment.find(comment_id)
   comment.update_attributes!(params.slice(*%w[body endorsed]))
   comment.to_hash.to_json
 end
 
 post '/api/v1/comments/:comment_id' do |comment_id|
-  comment = Comment.find(comment_id)
   sub_comment = comment.children.new(params.slice(*%w[body course_id]))
-  sub_comment.author = User.find_or_create_by(external_id: params["user_id"])
+  sub_comment.author = user
   sub_comment.comment_thread = comment.comment_thread
   sub_comment.save!
   sub_comment.to_hash.to_json
 end
 
 delete '/api/v1/comments/:comment_id' do |comment_id|
-  comment = Comment.find(comment_id)
   comment.destroy
   comment.to_hash.to_json
 end
 
 put '/api/v1/comments/:comment_id/votes' do |comment_id|
-  comment = Comment.find(comment_id)
   vote_for comment
 end
 
 delete '/api/v1/comments/:comment_id/votes' do |comment_id|
-  comment = Comment.find(comment_id)
   undo_vote_for comment
 end
 
 put '/api/v1/threads/:thread_id/votes' do |thread_id|
-  thread = CommentThread.find(thread_id)
   vote_for thread
 end
 
 delete '/api/v1/threads/:thread_id/votes' do |thread_id|
-  thread = CommentThread.find(thread_id)
   undo_vote_for thread
 end
 
 get '/api/v1/users/:user_id' do |user_id|
-  user = User.find_or_create_by(external_id: user_id)
   user.to_hash(complete: params["complete"]).to_json
 end
 
 get '/api/v1/users/:user_id/notifications' do |user_id|
-  user = User.find_or_create_by(external_id: user_id)
   user.notifications.map(&:to_hash).to_json
 end
 
 post '/api/v1/users/:user_id/subscriptions' do |user_id|
-  user = User.find_or_create_by(external_id: user_id)
-  source = case params["source_type"]
-    when "user"
-      User.find_or_create_by(external_id: params["source_id"])
-    when "thread"
-      CommentThread.find(params["source_id"])
-    when "other"
-      Commentable.find(params["source_id"])
-  end
   user.subscribe(source).to_hash.to_json
 end
 
 delete '/api/v1/users/:user_id/subscriptions' do |user_id|
-  user = User.find_or_create_by(external_id: user_id)
-  source = case params["source_type"]
-    when "user"
-      User.find_or_create_by(external_id: params["source_id"])
-    when "thread"
-      CommentThread.find(params["source_id"])
-    when "other"
-      Commentable.find(params["source_id"])
-  end
   user.unsubscribe(source).to_hash.to_json
 end
 
@@ -180,14 +187,6 @@ if env.to_s == "development"
   end
 end
 
-def vote_for(obj)
-  user = User.find_or_create_by(external_id: params["user_id"])
-  user.vote(obj, params["value"].to_sym)
-  obj.reload.to_hash.to_json
-end
-
-def undo_vote_for(obj)
-  user = User.find_or_create_by(external_id: params["user_id"])
-  user.unvote(obj)
-  obj.reload.to_hash.to_json
+error BSON::InvalidObjectId do
+  error 404
 end