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})]}}
)
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 } });
})