如何添加到mongoose嵌套文档，而不是覆盖它？

This is my mongoose model:

这是我的猫鼬模型：

...
images: new Schema({
 poster: new Schema({
   small: String,
   medium: String,
   big: String
 }, { _id: false }),
 backdrop: new Schema({
   small: String,
   medium: String,
   big: String
 }, { _id: false })
}, { _id: false }),
...
...
im