适用对象:
MongoDB vCore
$elemMatch
运算符用于将包含数组字段的文档与至少一个与所有指定查询条件匹配的元素进行匹配。 需要查找具有指定元素的数组文档时,此运算符特别有用。
$elemMatch
运算符的基本语法如下所示:
db.collection.find({ <field>: { $elemMatch: { <query1>, <query2>, ... } } })
说明 | |
---|---|
field |
文档中包含要查询的数组的字段。 |
query |
数组中至少有一个元素必须满足的条件。 |
让我们了解 stores
数据集中的示例 json 的用法。
[
{
"_id": "91de5201-8194-44bf-848f-674e8df8bf5e",
"name": "Adatum Corporation",
"promotionEvents": [
{
"discounts": [
{ "categoryName": "DJ Cases", "discountPercentage": 6 },
{ "categoryName": "DJ Mixers", "discountPercentage": 14 }
]
},
{
"discounts": [
{ "categoryName": "DJ Headphones", "discountPercentage": 19 },
{ "categoryName": "DJ Speakers", "discountPercentage": 13 }
]
},
{
"discounts": [
{ "categoryName": "DJ Lighting", "discountPercentage": 12 },
{ "categoryName": "DJ Accessories", "discountPercentage": 6 }
]
}
]
}
]
本示例查找 stores
集合中至少有一个折扣的前两个文档,其 promotionEvents
数组中的类别名称为“DJ Lighting”。 查询仅返回这些文档的 _id
和 promotionEvents.discounts
字段。
db.stores.find( {"promotionEvents.discounts":{$elemMatch:{"categoryName":"DJ Lighting"}}}
,{ _id: 1, "promotionEvents.discounts": 1 }).limit(2)
我们收到了返回的以下文档。
{
"_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
"promotionEvents": [
{
"discounts": [
{
"categoryName": "DJ Turntables",
"discountPercentage": 18
},
{
"categoryName": "DJ Mixers",
"discountPercentage": 15
}
]
},
{
"discounts": [
{
"categoryName": "DJ Lighting",
"discountPercentage": 14
},
{
"categoryName": "DJ Cases",
"discountPercentage": 20
}
]
}
]
},
{
"_id": "91de5201-8194-44bf-848f-674e8df8bf5e",
"promotionEvents": [
{
"discounts": [
{
"categoryName": "DJ Cases",
"discountPercentage": 6
},
{
"categoryName": "DJ Mixers",
"discountPercentage": 14
}
]
},
{
"discounts": [
{
"categoryName": "DJ Headphones",
"discountPercentage": 19
},
{
"categoryName": "DJ Speakers",
"discountPercentage": 13
}
]
},
{
"discounts": [
{
"categoryName": "DJ Lighting",
"discountPercentage": 12
},
{
"categoryName": "DJ Accessories",
"discountPercentage": 6
}
]
}
]
}