查询MongoDB以获取有序的不同值 [英] Query MongoDB for ordered distinct values

问题描述

我使用Morphia Java驱动程序查询包含以下格式集合的MongoDB：

I am using Morphia Java driver for querying a MongoDB that contains a collection of the following form:

MyCollection {
   TypeA
   TypeB
}

我想检索所有不同的值我使用以下代码执行TypeB：

I want to retrieve all distinct values of TypeB which I do using the following code:

DBCollection myCol = getDatastore().getCollection(MyCollection.class);
List typeBs = myCol.distinct("TypeB");

上面的代码按预期工作，但不同值的列表当然没有排序。

Above code works as expected, but list of distinct values is of course not sorted.

我已经尝试了以下代码：

I have experimented with the following code:

DBCollection myCol = getDatastore().getCollection(MyCollection.class);
DBObject orderBy = new BasicDBObject("$orderby", new BasicDBObject("TypeB", 1);
List typeBs = myCol.distinct("TypeB", orderBy);

但在这种情况下，清单是空的，我的假设在哪里错了？

But in this case the list is empty, where are my assumptions wrong?

UPDATE

通过使用CLI我发现以下查询返回了预期结果：

By using the CLI I found out that the following query returned the expected result:

> db.mycollection.find({$query : {}, $orderby : { TypeB : 1 }})

所以我相应调整了我的代码：

So I adjusted my code accordingly:

DBCollection myCol = getDatastore().getCollection(MyCollection.class);

BasicDBObject ascending = new BasicDBObject("TypeB", 1);
BasicDBObject filter = new BasicDBObject();
filter.put("$query", new BasicDBObject());
filter.put("$orderby", ascending);

List typeBs = myCol.distinct("TypeB", filter);

结果仍包含0个条目！

真正的结果我同意融合是相同的查询工作，如果我使用.find而不是.distinct：

What really makes me confused is that the same query works if I use .find instead of .distinct:

DBCollection myCol = getDatastore().getCollection(MyCollection.class);

BasicDBObject ascending = new BasicDBObject("TypeB", 1);
BasicDBObject filter = new BasicDBObject();
filter.put("$query", new BasicDBObject());
filter.put("$orderby", ascending);

myCol.find(filter).hasNext();  <-- Evaluates to TRUE

为什么过滤器在使用<时不起作用？ em> distinct 方法调用？

Why is it that the filter doesn't work when using the distinct method-call?

推荐答案

DBCollection.distinct（）的第二个DBObject参数是查询对象，用于定义匹配条件。要对列表进行排序，您可以使用：

the second DBObject parameter of DBCollection.distinct() is a query object, used to define the matching criteria. To sort the list distincts you can use:

List typeBs = myCol.distinct("TypeB");
java.util.Collections.sort(typeBs);

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