Mongo:You cannot currently mix including and excluding fields. Contact us if this is an issue.

在Mongo的Driver中用db.collection.find(<query>,<projection>)方法利用<projection>对象显示指定字段时，报错如上！

起始Mongo的官方文档里解释的是直接设置字段为1，则显示，0就不显示，如下面：

DBObject query = new DBObject();
	query.put("name",1);
	query.put("age",1);
	coll.find(new DBObject("name","lisa"),query);

如果按照上面的方式调用，就会报错，如下：

Exception in thread "main" com.mongodb.MongoException: You cannot currently mix including and excluding fields. Contact us if this is an issue.
	at com.mongodb.MongoException.parse(MongoException.java:82)
	at com.mongodb.DBApiLayer$MyCollection.__find(DBApiLayer.java:314)
	at com.mongodb.DBApiLayer$MyCollection.__find(DBApiLayer.java:295)
	at com.mongodb.DBCursor._check(DBCursor.java:368)
	at com.mongodb.DBCursor._hasNext(DBCursor.java:459)
	at com.mongodb.DBCursor._fill(DBCursor.java:518)
	at com.mongodb.DBCursor.toArray(DBCursor.java:553)
	at com.mongodb.DBCursor.toArray(DBCursor.java:542)
	at com.mongo.test.MongoTest.main(MongoTest.java:26)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)

经过尝试，我发现，其实只要在query对象中put一个字段，该字段就会显示，对应的，不显式的put一个字段，这个字段就不会显示，如下：

DBObject query = new DBObject();
	query.put("name","");
	query.put("age","");
	coll.find(new DBObject("name","lisa"),query);

这样结果就会自动显示name字段和age字段，如下所示

[{ "_id" : { "$oid" : "50ff5b28f5e86c4c3281c61c"} , "name" : "abc" , "age" : 13.0}]

在这里要注意一下，无论你要显示的字段是String类型还是int类型，都要填充”“形式，即：query.put("name","");query.put("age","");就可以了，

检索结果会自动出来的！