Question

似乎几乎没有关于如何为MongoDB设计数据库的文档。所以我想我会先在这里发布我的问题。

假设此集合（fruits_inventory）为例：

{
    "name"      : "Orange",
    "type"      : "citric",
    "available" : 3
}
{
    "name"      : "Apple",
    "type"      : "pome",
    "available" : 0
    "note"      : "Not shipping this month"
}
{
    "name"      : "Pear",
    "type"      : "pome",
    "available" : 2
}

(No indexes set)

1）字段选择

db.fruits_inventory.findOne({name:"Orange"},{"note":1});

此查询是否会搜索仅包含值为name的字段Orange的文档，并在第一次匹配时返回，即使它没有设置note字段？或者它会继续搜索包含note字段的文档吗？

2）具有唯一索引

如果我在name上设置了唯一索引，那么上一个问题的答案是否会改变？

现在只有这两个问题。答案将不胜感激。

Answer 1

我写了以下脚本：

// sofruit.js
db = db.getSiblingDB('test');

db.fruits_inventory.drop();
db.fruits_inventory.save({
    "name"      : "Orange",
    "type"      : "citric",
    "available" : 3
});
db.fruits_inventory.save({
    "name"      : "Apple",
    "type"      : "pome",
    "available" : 0,
    "note"      : "Not shipping this month"
});
db.fruits_inventory.save({
    "name"      : "Pear",
    "type"      : "pome",
    "available" : 2 
});

var a1 = db.fruits_inventory.findOne({name:"Orange"},{"note":1});

db.fruits_inventory.ensureIndex({name:1}, {unique:true});

var a2 = db.fruits_inventory.findOne({name:"Orange"},{"note":1});

然后我从mongo shell运行它并得到：

> load('../mongojs/sofruit.js');
> a1
{ "_id" : ObjectId("4e7d119e9b3e59bf2e0c5199") }
> a2
{ "_id" : ObjectId("4e7d119e9b3e59bf2e0c5199") }  
>

所以，答案是“是的”，它将返回第一个命中，即使它没有设置“注释”字段。添加索引不会改变它。

Answer 2

您可以直接连接到mongodb并进行检查：

MongoDB shell version: 2.0.0
connecting to: test
> use test
switched to db test
> db.fruits_inventory.save({
...     "name"      : "Orange",
...     "type"      : "citric",
...     "available" : 3
... });
> db.fruits_inventory.save({
...     "name"      : "Pear",
...     "type"      : "pome",
...     "available" : 2
... })
> db.fruits_inventory.save({
...     "name"      : "Apple",
...     "type"      : "pome",
...     "available" : 0,
...     "note"      : "Not shipping this month"
... })
> db.fruits_inventory.find()
{ "_id" : ObjectId("4e7d0fa5626e0ab7b5074bb0"), "name" : "Orange", "type" : "citric", "available" : 3 }
{ "_id" : ObjectId("4e7d101b626e0ab7b5074bb1"), "name" : "Pear", "type" : "pome", "available" : 2 }
{ "_id" : ObjectId("4e7d1059626e0ab7b5074bb2"), "name" : "Apple", "type" : "pome", "available" : 0, "note" : "Not shipping this month" }
> db.fruits_inventory.find({name: "Orange"},{"note":1})
{ "_id" : ObjectId("4e7d0fa5626e0ab7b5074bb0") }
> db.fruits_inventory.ensureIndex({name:1}, {unique:true})
> db.fruits_inventory.find({name: "Orange"},{"note":1})
{ "_id" : ObjectId("4e7d0fa5626e0ab7b5074bb0") }

因此，在回答您的问题时，当您查询注释字段时，它将只返回id并且具有唯一索引没有区别。

MongoDB有多聪明？

1）字段选择

2）具有唯一索引

2 个答案: