Controlling Vin output with relay

I need to turn on and off a small 12v lock, I'm powering my Arduino Uno via the power jack with 12V and wondered if I could use this.

According to the Arduino UNO documentation "You can supply voltage through the vin pin, or, if supplying voltage via the power jack, access it through this pin."

Since the 12v is available through the vin pin I decided to use this to power the lock via a relay, I've tried it and everything works fine. The questions I have are:-

1 Are there any issues with using the vin pin to power an external device

2 My relay has optoissolation, do I need to use this since I'm only connecting 12v DC

3 Will the optoisolation even work since there is a common ground between the vin pin powering the relay coil and the arduino output pin powering the diode that triggers the relay.

Hi, 1: Only the amount of current the PC board and pins can provide. Should be no problem with less than 2 amps.

2,3: Optoisolation will not work with common ground.

If the power is going thru the barrel jack and out the Vin pin, the current needs to be less than 1A, or you risk blowing the reverse polarity diode.

Thanks for the replies, since opto-isolation wont work is it likely to be an issue not having opto-isolation in my setup? I'm doing it this way as it's physically neater and I only need one power supply to power the arduino and the lock. I could use a separate power supply and have it isolated but was wondering if this is overkill considering the relay is driving such a small load.