java mongo驱动程序3.0+ [英] java mongo driver 3.0+

问题描述

使用新的 3.0+ java驱动程序来自mongo检查文档是否存在于集合中的最佳方法是什么。

Using the new 3.0+ java driver from mongo what is the best way to check if a document exists in a collection.

我看过这里并尝试做类似的事情。我只得到这个：

I have looked at here and tried to do something similar. I have only gotten as far as this:

FindIterable<Document> iterable = collection.find(eq("code", "abcdefg")).projection(Projections.include("_id")).limit(1);

这会返回一个FindIterable但你怎么检查它发现了什么？如果你能提供代码示例。

This returns a FindIterable but how do you check it has found anything at all ? If you can please provide a code example.

我试过：

if (!iterable.first().isEmpty()){System.out.println(" RESILT IS FOUND ");}else{System.out.println(" RESULT IS NOT FOUND ");}

但当查询返回任何内容时它会因以下错误而死亡：

but when the query returns nothing it dies with the following error:

Exception in thread "main" java.lang.NullPointerException
    at com.oss.niagaramqtt.MongoLib.exists(MongoLib.java:58)
    at com.oss.niagaramqtt.MongoLib.<init>(MongoLib.java:47)
    at com.oss.niagaramqtt.startup.main(startup.java:24)

确实这是检查文件是否存在的正确方法吗？

Indeed is this the correct approach overall for checking the existence of a document?

编辑：
这可能是答案，请确认：

This could be the answer please confirm:

MongoCursor<Document> iterable = collection.find(eq("code", "abcdefg")).projection(Projections.include("_id")).limit(1).iterator();                
if (iterable.hasNext()){System.out.println(" RESILT IS FOUND ");}else{System.out.println(" RESULT IS NOT FOUND ");}

推荐答案

如果你需要加载这个文件，你的方式很好。如果您不需要加载它，那么您可以使用MongoCollection.count方法，如：

Your way is good if you need to load this document in case it exists. If you don't need to load it then you can use MongoCollection.count method like:

    long count = collection.count(new BsonDocument("code", new BsonString("abcdefg")));
    if (count > 0){System.out.println(" RESILT IS FOUND ");}else{System.out.println(" RESULT IS NOT FOUND ");}

[更新]如果数据存储在分片群集中，db.collection.count（）如果孤立则可能导致计数不准确存在文件或正在进行块迁移。因此，使用聚合函数更安全：

[Update] In case data is stored on a sharded cluster, db.collection.count() can result in an inaccurate count if orphaned documents exist or if a chunk migration is in progress. So it's safer to use aggregate function instead:

    Iterator<Document> it = collection.aggregate(Arrays.asList(
            new Document("$match", new Document("code", "abcdefg")),
            new Document("$group", new Document("_id", null).append("count", 
                    new Document("$sum", 1))))).iterator();
    int count = it.hasNext() ? (Integer)it.next().get("count") : 0;

参见 http://docs.mongodb.org/manual/reference/sql-aggregation-comparison/ 了解更多详情。

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