Real Sociedad relied on their defense, but against an attack like Real's, it is very difficult to do. After the goal conceded, it was difficult to change strategy, although they had enough time, but they could not do anything, so Real gets an important victory. Ancelotti made a small rotation for this match, but this did not help Sociedad either, although they had several good attacks, Real's defense played this match perfectly.
Defending against Real Madrid is not a good tactic, because Real Madrid has the best attacking line in the world. it will be hard to mark them, so its better you play attacking game with them, show some strength. Real Sociedad tried to concede just a goal, because they just wanted a draw in the first leg. so they relied on packing bus. unfortunately they could not stop the Real Madrid attackers. But their defense was solid through out the match.
Real Madrid will host Real Sociedad at home in their second leg, Real Sociedad has no chance to qualify to the next round. Real Madrid will defeat them at home. Real Madrid target is to win the triple this season.