MongoDB findOne() 方法只返回一个符合输入的 criteria的文档. 如果输入的标准对应于多个文档,该方法只返回一个文档按照自然排序,这反映了文档存储在数据库中的顺序。
MongoDB 找到一
MongoDB findOne() syntax is: db.collection.findOne(<criteria>, <projection>) criteria - specifies the selection criteria entered. projection - specifies the list of fields to be displayed in the returned document. Few important points about MongoDB findOne:
- 投影参数接受 1 或 true, 0 或 false 的布尔值. 如果投影字段未指定,所有字段将被检索
- MongoDB findOne() 总是包含 _id 字段,即使在投影参数中没有明确指定,除非它被排除
- MongoDB findOne() 只返回文档,而不是路由器
MongoDB findOne - 空查询规格
此操作返回指定的集合中的单个文档. 例如, db.car.findOne() 输出:
1{
2"_id" : 2,
3"name" : "Polo", "color" : "White",
4"cno" : "H411", "speed" : 45, "mfdcountry" : "Japan"
5}
仅从汽车集合中获取了一个记录,请注意Polo首先被插入数据库。
MongoDB findOne - 查询规格
此 MongoDB findOne 操作返回指定的集合中的第一个匹配文档,以及输入的选择标准。
1>db.car.findOne(
2... {
3... $or:[
4... {name:"Zen"},
5... {speed: {$gt:60}} ... ]
6... }
7... )
8
9{
10"_id" : ObjectId("546cb92393f464ed49d620db"),
11"name" : "Zen",
12"color" : "JetRed",
13"cno" : "H671",
14"speed" : 67,
15"mfdcountry" : "Rome"
16}
此操作搜索名为Zen的汽车或超过60的速度,并从汽车收集中获取第一个符合输入的标准的文件。
在 MongoDB findOne() 中投影
投影参数也适用于 MongoDB findOne 方法. 让我们看看我们可以在 findOne 中使用投影的某些场景。
MongoDB findOne - 指定要返回的字段
此操作只显示查询中指定的字段,例如:
1>db.car.findOne(
2... { },
3... {name:1,color:1}
4... )
5
6{ "_id" : 2, "name" : "Polo", "color" : "White" }
显示了来自汽车收藏的第一个文件,其中包含ID、名称和颜色字段。
MongoDB findOne - 返回除排除外的所有字段
此操作会检索第一份文档,不包括选项标准中指定的字段;例如;
1>db.car.findOne(
2... { name:"Volkswagen" },
3... {_id:0, mfdcountry:0,cno:0 }
4... )
5
6{ "name" : "Volkswagen", "color" : "JetBlue", "speed" : 62 }
与 Volkswagen 的汽车名称被检索,不包括 id、 mfdcountry 和 cno 字段。
MongoDB findOne 结果文件
指标方法不会在此操作中工作,因为该方法只返回一个文档. 为了从文档中打印单个字段值,我们可以使用下面的代码。
1>var car = db.car.findOne();
2> if (car) {
3... var carName = car.name;
4... print (tojson(carName));
5... }
6
7"Polo"
这只恢复了汽车的名称Polo。
MongoDB find一个Java例子
下面是 Java 程序,展示了我们可以使用 MongoDB findOne()的不同选项。
1package com.journaldev.mongodb;
2
3import com.mongodb.BasicDBObject;
4import com.mongodb.DB;
5import com.mongodb.DBCollection;
6import com.mongodb.DBObject;
7import com.mongodb.MongoClient;
8
9import java.net.UnknownHostException;
10
11public class MongoDBFindOne {
12
13 // method retrieves the first document without any criteria
14 public static void emptyFindOne() throws UnknownHostException {
15
16 // Get a new connection to the db assuming that it is running
17 MongoClient mongoClient = new MongoClient("localhost");
18
19 // use test as a datbase,use your database here
20 DB db = mongoClient.getDB("test");
21
22 // fetch the collection object ,car is used here,use your own
23 DBCollection coll = db.getCollection("car");
24
25 // invoking findOne() method
26 DBObject doc = coll.findOne();
27
28 // prints the resultant document
29 System.out.println(doc);
30 }
31
32 // method that retrieves the document based on the selection criteria
33 public static void querySpecification() throws UnknownHostException {
34 // getting a connection everytime is not needed (could be done once
35 // globally).
36 MongoClient mongoClient = new MongoClient("localhost");
37 DB db = mongoClient.getDB("test");
38 DBCollection coll = db.getCollection("car");
39
40 // query to filter the document based on name and speed values by
41 // creating new object
42 DBObject query = new BasicDBObject("name", "Zen").append("speed",
43 new BasicDBObject("$gt", 30));
44
45 // resultant document fetched by satisfying the criteria
46 DBObject d1 = coll.findOne(query);
47
48 // prints the document on console
49 System.out.println(d1);
50 }
51
52 public static void projectionFields() throws UnknownHostException {
53
54 MongoClient mongoClient = new MongoClient("localhost");
55 DB db = mongoClient.getDB("test");
56 DBCollection coll = db.getCollection("car");
57
58 // creates new db object
59 BasicDBObject b1 = new BasicDBObject();
60
61 // criteria to display only name and color fields in the resultant
62 // document
63 BasicDBObject fields = new BasicDBObject("name", 1).append("color", 1);
64
65 // method that retrieves the documents by accepting fields and object
66 // criteria
67 DBObject d1 = coll.findOne(b1, fields);
68
69 System.out.println(d1);
70
71 }
72
73 public static void excludeByfields() throws UnknownHostException {
74 MongoClient m1 = new MongoClient("localhost");
75
76 DB db = m1.getDB("test");
77 DBCollection col = db.getCollection("car");
78
79 // filter criteria for car name volkswagen
80 DBObject query = new BasicDBObject("name", "Volkswagen");
81
82 // excluding the fields mfdcountry,cno and id fields
83 BasicDBObject fields = new BasicDBObject("mfdcountry", 0).append("cno",
84 0).append("_id", 0);
85
86 DBObject d1 = col.findOne(query, fields);
87 System.out.println(d1);
88 }
89
90 public static void printDoc() throws UnknownHostException {
91 MongoClient m1 = new MongoClient("localhost");
92 DB db = m1.getDB("test");
93 DBCollection col = db.getCollection("car");
94
95 DBObject d1 = col.findOne();
96
97 // prints only the name of the car
98 System.out.println((d1.get("name")));
99 }
100
101 public static void main(String[] args) throws UnknownHostException {
102 // invoking all the methods from main
103 emptyFindOne();
104 querySpecification();
105 projectionFields();
106 excludeByfields();
107 printDoc();
108 }
109
110}
上述方案的结果是:
1{ "_id" : 2.0 , "name" : "Polo" , "color" : "White" , "cno" : "H411" , "speed" : 45.0 , "mfdcountry" : "Japan"}
2{ "_id" : { "$oid" : "546cb92393f464ed49d620db"} , "name" : "Zen" , "color" : "JetRed" , "cno" : "H671" , "speed" : 67 , "mfdcountry" : "Rome"}
3{ "_id" : 2.0 , "name" : "Polo" , "color" : "White"}
4{ "name" : "Volkswagen" , "color" : "JetBlue" , "speed" : 62}
5Polo
这就是MongoDB findOne()方法的全部,我们将在即将到来的帖子中查看更多MongoDB选项。