I flew YYZ to YUL to PVG a short time ago.

I had trouble boarding in YYZ because the agent wanted to see my visa for China (which, of course, I didn't have). I had to pull up the Wiki and plead my case. Eventually he called for help and let me aboard. It was far simpler after that boarding in YUL. The point is: Be prepared with the material in case you need it. It was also very easy to enter China and get the visa once there.