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