Return range of documents around ID in MongoDB

Question

I have an ID of a document and need to return the document plus the 10 documents that come before and the 10 documents after it. 21 docs total.

I do not have a start or end value from any key. Only the limit in either direction.

Best way to do this? Thank you in advance.

Answer 1

Did you know that ObjectID's contain a timestamp? And that therefore they always represent the natural insertion order. So if you are looking for documents before an after a known document _id you can do this:

Our documents:

{ "_id" : ObjectId("5307f2d80f936e03d1a1d1c8"), "a" : 1 }
{ "_id" : ObjectId("5307f2db0f936e03d1a1d1c9"), "b" : 1 }
{ "_id" : ObjectId("5307f2de0f936e03d1a1d1ca"), "c" : 1 }
{ "_id" : ObjectId("5307f2e20f936e03d1a1d1cb"), "d" : 1 }
{ "_id" : ObjectId("5307f2e50f936e03d1a1d1cc"), "e" : 1 }
{ "_id" : ObjectId("5307f2e90f936e03d1a1d1cd"), "f" : 1 }
{ "_id" : ObjectId("5307f2ec0f936e03d1a1d1ce"), "g" : 1 }
{ "_id" : ObjectId("5307f2ee0f936e03d1a1d1cf"), "h" : 1 }
{ "_id" : ObjectId("5307f2f10f936e03d1a1d1d0"), "i" : 1 }
{ "_id" : ObjectId("5307f2f50f936e03d1a1d1d1"), "j" : 1 }
{ "_id" : ObjectId("5307f3020f936e03d1a1d1d2"), "j" : 1 }

So we know the _id of "f", get it and the next 2 documents:

> db.items.find({ _id: {$gte: ObjectId("5307f2e90f936e03d1a1d1cd") } }).limit(3)

{ "_id" : ObjectId("5307f2e90f936e03d1a1d1cd"), "f" : 1 }
{ "_id" : ObjectId("5307f2ec0f936e03d1a1d1ce"), "g" : 1 }
{ "_id" : ObjectId("5307f2ee0f936e03d1a1d1cf"), "h" : 1 }

And do the same in reverse:

> db.items.find({ _id: {$lte: ObjectId("5307f2e90f936e03d1a1d1cd") } })
    .sort({ _id: -1 }).limit(3)
{ "_id" : ObjectId("5307f2e90f936e03d1a1d1cd"), "f" : 1 }
{ "_id" : ObjectId("5307f2e50f936e03d1a1d1cc"), "e" : 1 }
{ "_id" : ObjectId("5307f2e20f936e03d1a1d1cb"), "d" : 1 }

And that's a much better approach than scanning a collection.

Answer 2

Neil's answer is a good answer to the question as stated (assuming that you are using automatically generated ObjectIds), but keep in mind that there's some subtlety around the concept of the 10 documents before and after a given document.

The complete format for an ObjectId is documented here. Note that it consists of the following fields:

• timestamp to 1-second resolution,
• machine identifier
• process id
• counter

Generally if you don't specify your own _ids they are automatically generated by the driver on the client machine. So as long as the ObjectIds are generated on a single process on a client single machine, their order does indeed reflect the order in which they were generated, which in a typical application will also be the insertion order (but need not be). However if you have multiple processes or multiple client machines, the order of the ObjectIds for objects generated within a given second by those multiple sources has an unpredictable relationship to the insertion order.