Role's in Ranked

I'm still confused about how roles are choosen in ranked. I was in champ select in which I had my primary role as mid. Someone leaves so I return to que. The next game pops up and I am now top lane. The person who is mid lane does not mid lane and was auto filled into that lane even though I had mid lane as my primary role. In fact I was the only one who was a mid laner in the match. We then followed on by getting crushed by the team because our mid lane was demolished by a kassadin who plays mid all the time. Salty about the loss but more salty that even though I was the only person with mid as my primary role I was it was instead auto filled.