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.
Real sociedad really played very well and I think they deserve a draw in that match but they were very unlucky and of course, Madrid is not there match in every standard. Real sociedad don't have a pressing striker that could probably frustrate the Madrid defense and on the contrary Madrid have much more quality players in every angle from defense to midfield and forward. Although, the refereeing wasn't too fair for Sociadad but what matters now is that Real madrid won and that's all. We would probably going to witness another El-classico final if Barcelona managed to win against Athletico in the returning leg which I really doubt but as for Real Madrid, I don't think Real sociedad would stand a chance in Santiago being handicapped already.