Who knows what the allusion about Sima Yan driving a sheep cart to choose his concubine for the night is called?

Emperor Wu of the Western Jin Dynasty had nearly 10,000 beauties in his harem, and he couldn't choose from them, so he came up with a way. He would ride a goat cart around the harem. Wherever the goat cart was parked, he would go to the concubine's residence to rest. Therefore, the concubines all looked forward to the sheep cart stopping in front of their door. They inserted bamboo leaves in front of the door, sprinkled salt water on the aisle leading to the door, and induced the sheep pulling the cart to lick the salt water. Come to the door and eat bamboo leaves in order to gain the emperor's favor.