MongoDB update field with another field from another collection

Question

I have two collections: books and categories. Categories collections represent a tree structure which make them nested categories using parents and children.

The book can have multiple categories and stores them in an array.

Example: Book has category and I want to retire it and set it to the parent category.

This is how categories collection is populated.

db.categories.insertMany([
  {
    _id: "Space Opera",
    ancestors: ["Science Fiction", "Fiction", "Science Fiction & Fantasy"],
    parent: ["Science Fiction"],
  },
  {
    _id: "Dystopian",
    ancestors: ["Science Fiction", "Fiction", "Science Fiction & Fantasy"],
    parent: ["Science Fiction"],
  },
  {
    _id: "Cyberpunk",
    ancestors: ["Science Fiction", "Fiction", "Science Fiction & Fantasy"],
    parent: ["Science Fiction"],
  },
  {
    _id: "Science Fiction",
    ancestors: ["Fiction", "Science Fiction & Fantasy"],
    parent: ["Fiction", "Science Fiction & Fantasy"],
  },
  {
    _id: "Fantasy",
    ancestors: ["Science Fiction & Fantasy"],
    parent: ["Science Fiction & Fantasy"],
  },
  {
    _id: "Science Fiction & Fantasy",
    ancestors: [],
    parent: [],
  },
  {
    _id: "Fiction",
    ancestors: [],
    parent: [],
  },
]);

Also, how do I query this one and get only the value "Science Fiction" (Note that it is stored in an array)?

db.categories.find({_id : "Space Opera"}, {_id : 0, parent : 1})[0].parent // Did not work  

db.categories.find({_id : "Space Opera"}, {_id : 0, parent : 1}) // find parent

// result 

[
  {
    "parent": [
      "Science Fiction"
    ]
  }
]

db.books.update(
    {title : "Book1"}, 
    {$set : {category : [**PARENT CATEGORY**]}}
)

I believe I can use code above inside books.update()

I could store this in a separate variable but in vscode it gives me undefined. And the inner query does not give me the right value as stated before, but I think you got the idea.

db.books.update(
    {title : "Book1"}, 
    {$set : {category : [db.categories.find({_id : "Space Opera"}, {_id : 0, parent : 1})]}}
)

Answer 1

The parent you can get with this aggregation pipeline:

db.categories.aggregate([
   { $match: { _id: "Space Opera" } },
   { $project: { _id: 0, parent: { $first: "$parent" } } }
])

or even

db.categories.aggregate([
   { $match: { _id: "Space Opera" } },
   { $project: { _id: 0, parent: { $first: "$parent" } } }
]).toArray().shift().parent

In order to join collections you have to use the $lookup operator. Bear in mind, NoSQL databases like MongoDB are not optimized for join/lookup. In real life you should have a look for a better design.

db.books.aggregate([
   { $match: { title: "Book1" } },
   {
      $lookup:
         {
            from: "categories",
            pipeline: [
               { $match: { _id: "Space Opera" } },
               { $project: { _id: 0, parent: { $first: "$parent" } } }
            ],
            as: "category"
         }
   },
   { $set: { category: { $first: "$category.parent" } } }
])

If you like to update existing collection, then you have to create a loop for it:

db.books.aggregate([...]).forEach(function (doc) {
   db.books.updateOne({ _id: doc._id }, { $set: { category: doc.category } });
})