Try plucking instead of mapping.

Use pluck instead of map for upvoted/downvoted IDs.

Mapping involves doing the projection locally, whereas plucking will ask
MongoDB to do the projection for us.  This is the difference between
sending the whole document back and sending back the ObjectId itself.

Multipled against hundreds or thousands of documents, mapping can really
add up, so plucking should really help these 99%'ile cases.

models/user.rb

@@ -111,11 +111,11 @@ class User
   end
 
   def upvoted_ids
-    Content.up_voted_by(self).map(&:id)
+    Content.up_voted_by(self).pluck(:_id)
   end
 
   def downvoted_ids
-    Content.down_voted_by(self).map(&:id)
+    Content.down_voted_by(self).pluck(:_id)
   end
 
   def followers

spec/api/user_spec.rb

@@ -42,10 +42,15 @@ describe "app" do
       end
       it "returns error if new information has conflict with other users" do
         put "/api/v1/users/1", username: "user2"
-        last_response.status.should == 400 
+        last_response.status.should == 400
       end
     end
+
     describe "GET /api/v1/users/:user_id" do
+      let(:author) { User.find_by(external_id: "1") }
+      let(:reader) { User.find_by(external_id: "2") }
+      let(:thread) { make_standalone_thread(author) }
+
       it "returns user information" do
         get "/api/v1/users/1"
         last_response.status.should == 200
@@ -54,10 +59,28 @@ describe "app" do
         res["external_id"].should == user1.external_id
         res["username"].should == user1.username
       end
+
       it "returns 404 if user does not exist" do
         get "/api/v1/users/3"
         last_response.status.should == 404
       end
+
+      it "returns no threads when user hasn't voted" do
+        get "/api/v1/users/1", complete: "true"
+        last_response.status.should == 200
+        res = parse(last_response.body)
+        res["upvoted_ids"].should == []
+      end
+
+      it "returns threads when user votes" do
+        reader.vote(thread, :up)
+
+        get "/api/v1/users/2", complete: "true"
+        last_response.status.should == 200
+        res = parse(last_response.body)
+        res["upvoted_ids"].should == [thread.id.to_s]
+      end
+
       describe "Returns threads_count and comments_count" do
           before(:each) { setup_10_threads }
 

spec/models/user_spec.rb

+require 'spec_helper'
+require 'unicode_shared_examples'
+
+describe User do
+  let(:author) { create_test_user(666) }
+  let(:reader) { create_test_user(667) }
+  let(:thread) { make_standalone_thread(author) }
+
+  before(:each) do
+    [Comment, CommentThread, User].each(&:delete_all).each(&:remove_indexes).each(&:create_indexes)
+  end
+
+  it "should have no votes if it never voted" do
+    reader.upvoted_ids.should == []
+  end
+
+  it "should have one vote if it voted once" do
+    reader.upvoted_ids.should == []
+    reader.vote(thread, :up)
+    reader.upvoted_ids.should == [thread._id]
+  end
+end

spec/spec_helper.rb

@@ -342,6 +342,17 @@ def make_comment(author, parent, text)
   comment
 end
 
+def make_standalone_thread(author)
+  make_thread(
+      author,
+      "standalone thread 0",
+      DFLT_COURSE_ID,
+      "pdq",
+      :discussion,
+      :standalone
+  )
+end
+
 # add standalone threads and comments to the @threads and @comments hashes
 # using the namespace "standalone t#{index}" for threads and "standalone t#{index} c#{i}" for comments
 # takes an index param if used within an iterator, otherwise will namespace using 0 for thread index