Test that the correct message is assigned for auto-scrolling, depending on the viewing history.
user
```ruby
require 'spec_helper'

describe MessageThreadsController do
  context "thread views" do
    let(:thread) { FactoryGirl.create(:message_thread) }
    let!(:message_a) { FactoryGirl.create(:message, thread: thread, created_at: Time.now - 4.days) }
    let!(:message_b) { FactoryGirl.create(:message, thread: thread, created_at: Time.now - 3.days) }
    let!(:message_c) { FactoryGirl.create(:message, thread: thread, created_at: Time.now - 2.days) }

    context "as a guest" do
      it "should not assign a message to view from" do
        get :show, id: thread.id
        assigns(:view_from).should be_nil
      end
    end

    context "as a site user" do
      let(:user) { FactoryGirl.create(:user) }

      before do
        sign_in user
      end

      context "who hasn't viewed the thread before" do
        it "should not assign a message to view from" do
          get :show, id: thread
          assigns(:view_from).should be_nil
        end
      end

      context "who viewed the thread and no messages have been posted since" do
        it "should assign the final message" do
          tv = FactoryGirl.create(:thread_view, thread: thread, user: user, viewed_at: Time.now - 1.day)
          get :show, id: thread
          assigns(:view_from).should eql(thread.messages.last)
        end
      end

      context "who viewed the thread and two messages have been posted since" do
        it "should assign the first of the new messages" do
          tv = FactoryGirl.create(:thread_view, thread: thread, user: user, viewed_at: Time.now - 3.5.days)
          get :show, id: thread
          assigns(:view_from).should eql(message_b)
        end
      end
    end
  end
end
```
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