在MongoDb中查找下一个文档 [英] Finding The Next Document in MongoDb

问题描述

如果这是我的收藏结构:

If this is my collection structure:

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

我查询产品1，是否可以查询下一个文档，在这种情况下为产品2?

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

推荐答案

如果想要可预测的结果顺序，最好添加明确的sort()标准.

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

假设您遵循的顺序是插入顺序"，并且您使用的是MongoDB的默认生成的ObjectId，那么您可以基于ObjectId进行查询:

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)

请注意，此示例之所以有效，是因为:

Note that this example only works because:

• ObjectId的前四个字节是根据unix样式的时间戳计算的(请参阅: ObjectId规格)
• 仅对_id的查询将使用默认的_id索引(按ID排序)来查找匹配项

• 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

实际上，这种隐式排序与:

So really, this implicit sort is the same as:

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

如果您在查询中添加了更多条件以限定如何查找下一个"产品(例如，category)，则查询可能使用其他索引，并且顺序可能与您期望的不一样.

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.

您可以通过 explain()检查索引的使用情况.

You can check index usage with explain().

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