如何使用 Java 驱动程序对选定字段运行查找查询
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How to run find query with a selected field using Java driver
提问by Watt
I am trying to run following query using mongo java driver
我正在尝试使用 mongo java 驱动程序运行以下查询
db.myCollection.find({},{_id:1})
I need all the ids from the collection. The above query runs fine in mongo client.
我需要集合中的所有 ID。上述查询在 mongo 客户端中运行良好。
But, I need results through my java code.
I tried below non-workingcode.. because, as you can see below I am unable to create
{},{_id:1}for find()method from the java driver.
但是，我需要通过我的 Java 代码获得结果。我尝试以下非工作代码..因为，正如你可以看到下面我无法创建
{},{_id:1}用于find()从Java驱动程序的方法。
BasicDBObject query= new BasicDBObject("","").append("",new BasicDBObject("_id","1"));
DBCursor cursor = coll.find(query);
try {
while(cursor.hasNext()) {
System.out.println(cursor.next());
}
} finally {
cursor.close();
}
Please advise the appropriate way to initialize queryobject
请告知初始化query对象的适当方法
Edit:
编辑：
I can always get the ids by following:
我总是可以通过以下方式获取 ID：
DBCursor cursor = coll.find();
try {
while(cursor.hasNext()) {
System.out.println(cursor.next().get("_id"));
}
} finally {
cursor.close();
}
But, still it would be nicer to learn how to create the accurate query object, maybe that would be faster on big (+30gb) dataset as mine. So, I am keeping this question open.
但是，学习如何创建准确的查询对象仍然会更好，也许在我的大（+30gb）数据集上会更快。所以，我保持这个问题的开放性。
采纳答案by JohnnyHK
Keep your query conditions and selected fields objects as separate parameters to find:
将您的查询条件和选定的字段对象作为单独的参数保存到find：
BasicDBObject query = new BasicDBObject();
BasicDBObject fields = new BasicDBObject("_id", "1");
DBCursor cursor = coll.find(query, fields);
回答by Arun Kumar Giri
You can project fields from a collection as shown below.
您可以从集合中投影字段，如下所示。
// Create query and fields
BasicDBObject query = new BasicDBObject();
BasicDBObject fields = new BasicDBObject();
// Assign 1 or 0 for projecting a specific field.
fields.put("_id", 0);
fields.put("Student ID", 1);
fields.put("Student Age", 1);
fields.put("Gender", 1);
// Pass query and fields to find()
DBCursor cursor = dbCollection.find(query, fields);
回答by shiv
Below snippet gets all documents where “firstName” is “Gopi”.
<pre>
BasicDBObject query = new BasicDBObject();
query.put("firstName", "Gopi");
DBCursor cursor = collection.find(query);
import java.net.UnknownHostException;
import com.mongodb.BasicDBObject;
import com.mongodb.DBCursor;
import com.mongodb.MongoClient;
import com.mongodb.DB;
import com.mongodb.DBCollection;
public class FindDocument {
/* Step 1 : get mongoClient */
public static MongoClient getMongoClient(){
MongoClient mongoClient = null;
try {
mongoClient = new MongoClient( "localhost" , 27017 );
} catch (UnknownHostException e) {
e.printStackTrace();
}
return mongoClient;
}
public static void main(String args[]){
MongoClient mongoClient = getMongoClient();
/* Step 2: Connect to DB */
DB db = mongoClient.getDB("sample");
/*Step 3 : Get collection */
DBCollection collection = db.getCollection("employee");
/* Step 4 : Create Query object */
BasicDBObject query = new BasicDBObject();
query.put("firstName", "Gopi");
/* Step 5 : Get all documents */
DBCursor cursor = collection.find(query);
/* Step 6 : Print all documents */
while(cursor.hasNext()){
System.out.println(cursor.next());
}
}
}
</pre>
Output
{ "_id" : 3.0 , "firstName" : "Gopi" , "lastName" : "Battu"}
