Create dependencies between documents in MongoDB - mongodb

Question

I want to create a dependency between some documents in my collection.
For example I have document A, and I want to use some fields of it in document B as a subsection. So if I change document A, the fields that are also in document B will also change.
Here's a better explanation with actual JSONs:
Here's document A
{
"_id": ObjectId("1"),
"value": 10,
"name": "A_document_name",
"owner": "some_owner"
}
Here's document B
{
"_id": ObjectId("2"),
"description": "some_description",
"valid": true,
"name": "some_name",
"items": [
{
"id": "1",
"name": "A_object_name"
}
]
}
Now, what I want to happen is, if I change the name of document A, I want to see that it also changed in the items section of document B. Currently, when I change A's name, it changes only in A and B remains the same since it was created. Is it achievable in Mongo? How can I create a reference between items section in B with A? Do I need to change something in my document structure?

Answer 1

You need to use mongoDB's $lookup
You you'd a unique identifier from that you can use match the documents,
e.g:
The A document would be:
{
"_id": ObjectId("1"),
"b_id": ObjectId("2"),
"value": 10,
"name": "A_document_name",
"owner": "some_owner"
}
While the B document would just be:
{
"_id": ObjectId("2"),
"description": "some_description",
"valid": true,
"name": "some_name"
}
You'll then do an aggregate query like so:
db.B.aggregate([
{
$lookup:
{
from: "A", //The collection you're getting the items from
localField: "_id", //The local field you're using to lookup
foreignField: "b_id", //The field the `A` document you're using to match
as: "items" //The name of the field that will be populated with the results
}
}
])
There's a lot more you can do with aggregate take a minute to look through the docs. Let me know if this helps.