getting the sub-document with the document in mongoose.findOne

问题

First, my code is working well, but i don t know if doing the right thing. I have a parent model Units with this schema

{
  info: {...},
  chapters: {_id: string, ...}
}

the chapter schema is like this:

{
  info: {...},
  content:  string
}

now in the client if i am editing the a chapter, and want to put the changes to the database, i do it like this

Units.findOne({'chapters._id': chapter_id}) //this query will return the parent unit
.then(unit=>{ //is there a way to have the chapter with unit here??
  const chapter = unit.chapters.find(chapter=> chapter._id === chapter_id)
 //seems like duplicated. i already found the unit based on the chapter_id
  //after that i update the chapter
})

that's it, I wish there is a way to return the chapter in the first query

来源：https://stackoverflow.com/questions/57745539/getting-the-sub-document-with-the-document-in-mongoose-findone