$elemMatch vs. Dot Notation

[msforest]

$elemMatch 和点运算符都是匹配数组里面嵌套的字段

db.inventory.find( { "instock": { $elemMatch: { qty: { $gt: 10, $lte: 20 } } } } )
db.inventory.find( { "instock.qty": { $gt: 10,  $lte: 20 } } )

两个都是匹配10<qty<=20，查出来的结果却有点不一样；$elemMatch是对数组嵌套的同一条记录的字段值要同时满足条件，即 qty>10 且 qty<=20；点运算符是数组中包含一条记录 a 存在 qty>10 和一条记录 b 存在 qty<=20（a 不一定等于 b）

例如：

> db.inventory.insertMany( [
   { item: "journal", instock: [ { warehouse: "A", qty: 5 }, { warehouse: "C", qty: 15 } ] },
   { item: "notebook", instock: [ { warehouse: "C", qty: 5 } ] },
   { item: "paper", instock: [ { warehouse: "A", qty: 60 }, { warehouse: "B", qty: 15 } ] },
   { item: "planner", instock: [ { warehouse: "A", qty: 40 }, { warehouse: "B", qty: 5 } ] },
   { item: "postcard", instock: [ { warehouse: "B", qty: 15 }, { warehouse: "C", qty: 35 } ] }
]);

> db.inventory.find( { "instock": { $elemMatch: { qty: { $gt: 10, $lte: 20 } } } } , {_id: 0}) //匹配的结果得在同一个记录里
{ "item" : "journal", "instock" : [ { "warehouse" : "A", "qty" : 5 }, { "warehouse" : "C", "qty" : 15 } ] }
{ "item" : "paper", "instock" : [ { "warehouse" : "A", "qty" : 60 }, { "warehouse" : "B", "qty" : 15 } ] }
{ "item" : "postcard", "instock" : [ { "warehouse" : "B", "qty" : 15 }, { "warehouse" : "C", "qty" : 35 } ] }

> db.inventory.find( { "instock.qty": { $gt: 10,  $lte: 20 } } ,{_id: 0}) //匹配的结果可以不在同一个记录里
{ "item" : "journal", "instock" : [ { "warehouse" : "A", "qty" : 5 }, { "warehouse" : "C", "qty" : 15 } ] }
{ "item" : "paper", "instock" : [ { "warehouse" : "A", "qty" : 60 }, { "warehouse" : "B", "qty" : 15 } ] }
{ "item" : "planner", "instock" : [ { "warehouse" : "A", "qty" : 40 }, { "warehouse" : "B", "qty" : 5 } ] }
{ "item" : "postcard", "instock" : [ { "warehouse" : "B", "qty" : 15 }, { "warehouse" : "C", "qty" : 35 } ] }

> db.inventory.find( { "instock.qty": 5, "instock.warehouse": "A" } ,{_id: 0}) //匹配的结果可以不在同一个记录里
{ "item" : "journal", "instock" : [ { "warehouse" : "A", "qty" : 5 }, { "warehouse" : "C", "qty" : 15 } ] }
{ "item" : "planner", "instock" : [ { "warehouse" : "A", "qty" : 40 }, { "warehouse" : "B", "qty" : 5 } ] }

资料参考
query-array-of-documents

$elemMatch vs dot notation