Ambrose of Milan

And it came to pass that He ordained many others, whom the Jews sent back to him disgraced and empty, for they could reap nothing from them; as it follows, And again he sent another servant.

Catena Aurea by Aquinas ↗

Bede

And he added the sending of another servant. But they also beat him and treated him shamefully, and sent him away empty-handed. The other servant signifies David the prophet and king. Who was sent after Moses, to urge the tenants of the vineyard, through the modulation of psalmody and the sweetness of the lyre, to the practice of good work after the legal decrees. …

On the Gospel of Luke ↗

Bonaventure

Second, regarding the perversity with respect to King David, the chief worshiper of God, he adds: And he proceeded to send another servant; The Gloss says: "The other servant is David the prophet, who after Moses invited the tenants of the vineyard to the exercise of good works by the melody of psalmody and the sweetness of the harp." The Lord sent him when he chose him from the pastures to rule and feed his people, according to what the prophet David himself says in the Psalm: "He chose David his servant and took him from the …

Commentary on Luke, Chapter 20 ↗