What is the relationship between the legend of the white snake and the gourd doll in Yun-peng Yue's classic jokes?

The joke said that one day, Xu Xian picked up a snake and took it home. When the snake woke up, he was with Xu Xian. After a long time, when Xu Xian was old, the snake spirit went to the mountains and became a demon. Xu Xian planted a gourd, and when he collected seven gourd dolls, he took revenge.