Finding The Next Document in MongoDb

Question

If this is my collection structure:

{   \"_id\": ObjectId(\"4fdbaf608b446b0477000142\"),
    \"name\": \"product 1\",
},
{   \"_id\": ObjectId(\"fghfghfgjhjg         


        

Answer 1

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().

Answer 2

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.