MongoDB $elemMatch操作符

document中可以内嵌document，

document中也可以有数组，

如果document的数组中是document又是什么情况，

例如有个school的collection有如下的document

{
 _id: 1,
 zipcode: "63109",
 students: [
              { name: "john", school: 102, age: 10 },
              { name: "jess", school: 102, age: 11 },
              { name: "jeff", school: 108, age: 15 }
           ]
}
{
 _id: 2,
 zipcode: "63110",
 students: [
              { name: "ajax", school: 100, age: 7 },
              { name: "achilles", school: 102, age: 8 },
           ]
}

{
 _id: 3,
 zipcode: "63109",
 students: [
              { name: "jimi", school: 100, age: 8 },
              { name: "messi", school: 102, age: 7 },
           ]
}

{
 _id: 4,
 zipcode: "63109",
 students: [
              { name: "barney", school: 102, age: 7 },
           ]
}

db.schools.find({'students.age':7})  
结果显示_id为2，3，4的三条记录

db.schools.find({'students.0.age':7}) 
结果显示_id为2，4的两条记录

db.schools.find({'students.age':7，'students.school':100})  
结果显示_id为2，3的两条记录
和想象中结果是_id=2一条记录不一样

这时 该$elemMatch出场了

db.schools.find(
	{
	   students:{
		    $elemMatch:{age:7,school:100}
		    } 
	}

)

$elemMatch还有一种用法，用在第二个参数中

db.schools.find( { zipcode: "63109" },
                 { students: { $elemMatch: { school: 102 } } } )

db.schools.find( { zipcode: "63109" },
                 { students: { $elemMatch: { school: 102, age: { $gt: 10} } } } )

db.schools.find(
                 { zipcode: 63109 },
                 { students: { $elemMatch: { school: 102 } } }
               ).sort( { "students.age": -1 } ) //age倒序