Finding the next document in MongoDb

Question

If this is my collection structure:

{ _id: ObjectId("4fdbaf608b446b0477000142"), name: "product 1" }
{ _id: ObjectId("4fdbaf608b446b0477000143"), name: "product 2" }
{ _id: ObjectId("4fdbaf608b446b0477000144"), name: "product 3" }

and I query product 1, is there a way to query the next document, which in this case would be "product 2"?

So you want to query for the first document after "product 1"? How are you defining the documents' order (what's considered 'after')? — JohnnyHK, Aug 17 '12 at 20:08
I have not reordered them at all, they are in the original insertion order. The way I got product 1 is someone goes to testsite.com/4fdbaf608b446b0477000142 I take the number and query by id. — Claremont, Aug 17 '12 at 20:35

score 24 · Answer 1 · answered Aug 18 '12 at 07:11

It is best to add explicit sort() criteria if you want a predictable order of results.

Assuming the order you are after is "insertion order" and you are using MongoDB's default generated ObjectIds, then you can query based on the ObjectId:

// Find next product created
db.products.find({_id: {$gt: ObjectId("4fdbaf608b446b0477000142") }}).limit(1)

Note that this example only works because:

the first four bytes of the ObjectId are calculated from a unix-style timestamp (see: ObjectId Specification)
a query on _id alone will use the default _id index (sorted by id) to find a match

So really, this implicit sort is the same as:

db.products.find({_id: {$gt: ObjectId("4fdbaf608b446b0477000142" )}}).sort({_id:1}).limit(1);

If you added more criteria to the query to qualify how to find the "next" product (for example, a category), the query could use a different index and the order may not be as you expect.

You can check index usage with explain().

score 1 · Answer 2 · answered Nov 27 '21 at 17:13

Starting in Mongo 5, it's a perfect use case for the new $setWindowFields aggregation operator:

// { _id: 1, name: "product 1" }
// { _id: 2, name: "product 2" }
// { _id: 3, name: "product 3" }
db.collection.aggregate([
  { $setWindowFields: {
    sortBy: { _id: 1 },
    output: { next: { $push: "$$ROOT", window: { documents: [1, 1] } } }
  }}
])
// { _id: 1, name: "product 1", next: [{ _id: 2, name: "product 2" }] }
// { _id: 2, name: "product 2", next: [{ _id: 3, name: "product 3" }] }
// { _id: 3, name: "product 3", next: [ ] }

This:

$sorts documents by their order of insertion using their _ids (ObjectIds contain the timestamp of insertion): sortBy: { _id: 1 }
adds the next field in each document (output: { running: { ... }})
- by $pushing the whole document $$ROOT ($push: "$$ROOT")
- on a specified span of documents (the window) which is in this case is a range of only the next document document: window: { documents: [1, 1] }.

score 0 · Answer 3 · answered Aug 17 '12 at 20:24

You can get the items back in insertion order using ObjectId. See: http://www.mongodb.org/display/DOCS/Optimizing+Object+IDs#OptimizingObjectIDs

If you want to get them back in some other order then you will have to define what that order is and store more information in your documents.

Finding the next document in MongoDb

3 Answers3

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