John Chrysostom

David in truth was both the Father and the servant of Christ, the former indeed according to the flesh, the latter in the Spirit.

Catena Aurea by Aquinas ↗

Theophylact of Ohrid

David, He says, calls Him Lord (Ps. 109:1): how then is He merely his Son? He was indeed a son of David according to the flesh, but at the same time also his God, yet they considered Him only the son of David. Therefore He refutes this opinion of theirs, that Christ is merely a son of David, and declares that He is not opposed to the Father, but has great unity of mind with Him, for the Father vanquishes His enemies. This He said to the scribes.

Commentary on Luke ↗

Bonaventure

Third, as regards the apparent controversy of both, he adds: David therefore calls him Lord, and how is he his son? For it is impossible to say that according to his human nature Christ was the lord of David, because Christ was not yet born nor conceived; and moreover, a son is never called the lord of his father; and again, since David was an exalted king, it does not seem that he had any mere man as his lord. …

Commentary on Luke, Chapter 20 ↗