如何使用Mongo Java驱动程序3.0+检查集合中是否存在文档 [英] How to check if document exists in collection using mongo Java driver 3.0+

问题描述

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

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

我在此处查看了并尝试做类似的事情.我只了解到了这一点:

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);

这将返回一个FindIterable，但是您如何检查它是否找到了任何东西?如果可以的话，请提供代码示例.

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

我确实尝试过:

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

但是当查询不返回任何内容时，它将死于以下错误:

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)

实际上，这是检查文档是否存在的正确方法吗?

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

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

如果需要加载此文档(如果存在的话)，则您的方法很好.如果您不需要加载它，则可以使用MongoCollection.count方法，例如:

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，则如果存在孤立文档或正在进行块迁移，则db.collection.count()可能导致计数不正确.因此，改为使用aggregate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;

请参见 http://docs.mongodb.org/manual/reference/sql-aggregation-comparison/了解更多详细信息.

See http://docs.mongodb.org/manual/reference/sql-aggregation-comparison/ for more details.

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