在上一篇 mongodb Aggregation聚合操作之$project 中详细介绍了mongodb聚合操作中的$project使用以及参数细节。本篇将开始介绍Aggregation聚合操作中的unwind操作。
说明：
解析输入文档中的数组字段，为每个元素输出一个文档。每个输出文档都是输入文档，数组字段的值被元素替换。如果字段值为null、缺失或数组为空，则$unwind不会输出文档。
语法：
{ $unwind: <field path> }
可以将文档传递给$unwind以指定各种行为选项。
{
$unwind:
{
path: <field path>,
includeArrayIndex: <string>,
preserveNullAndEmptyArrays: <boolean>
}
}
参数解析：
path：string类型，数组字段的字段路径。若要指定字段路径，请在字段名称前加上美元符号$并将其括在引号中。
includeArrayIndex：string类型，可选的。用于保存元素的数组索引的新字段的名称。新字段名称不能以美元符号$开头。
preserveNullAndEmptyArrays：boolean类型，可选的。如果为真，如果路径为空、丢失或数组为空，则$unwind输出文档。如果为false，如果路径为空、丢失或数组为空，则$unwind不输出文档。默认值为false。
1. 示例
1.1. 简单示例
初始化数据：
db.inventory.insertOne({ "_id" : 1, "item" : "ABC1", sizes: [ "S", "M", "L"] })
示例：按照sizes字段拆分数据
db.inventory.aggregate( [ { $unwind : "$sizes" } ] )
结果：
{ "_id" : 1, "item" : "ABC1", "sizes" : "S" }
{ "_id" : 1, "item" : "ABC1", "sizes" : "M" }
{ "_id" : 1, "item" : "ABC1", "sizes" : "L" }
1.2. 拆分数组字段是null或者[]数据
初始化数据：
db.inventory2.insertMany([
{ "_id" : 1, "item" : "ABC", price: NumberDecimal("80"), "sizes": [ "S", "M", "L"] },
{ "_id" : 2, "item" : "EFG", price: NumberDecimal("120"), "sizes" : [ ] },
{ "_id" : 3, "item" : "IJK", price: NumberDecimal("160"), "sizes": "M" },
{ "_id" : 4, "item" : "LMN" , price: NumberDecimal("10") },
{ "_id" : 5, "item" : "XYZ", price: NumberDecimal("5.75"), "sizes" : null }
])
示例：
下面的$unwind操作是等效的，并为size字段中的每个元素返回一个文档。如果size字段没有解析为数组，但没有丢失、null或空数组，则$unwind将非数组操作数视为单个元素数组。
db.inventory2.aggregate( [ { $unwind: "$sizes" } ] )
db.inventory2.aggregate( [ { $unwind: { path: "$sizes" } } ] )
结果：
{ "_id" : 1, "item" : "ABC", "price" : NumberDecimal("80"), "sizes" : "S" }
{ "_id" : 1, "item" : "ABC", "price" : NumberDecimal("80"), "sizes" : "M" }
{ "_id" : 1, "item" : "ABC", "price" : NumberDecimal("80"), "sizes" : "L" }
{ "_id" : 3, "item" : "IJK", "price" : NumberDecimal("160"), "sizes" : "M" }
示例：下面的$unwind操作使用includeArrayIndex选项在输出中包含数组索引。
db.inventory2.aggregate( [
{
$unwind:
{
path: "$sizes",
includeArrayIndex: "arrayIndex"
}
}])
结果：
{ "_id" : 1, "item" : "ABC", "price" : NumberDecimal("80"), "sizes" : "S", "arrayIndex" : NumberLong(0) }
{ "_id" : 1, "item" : "ABC", "price" : NumberDecimal("80"), "sizes" : "M", "arrayIndex" : NumberLong(1) }
{ "_id" : 1, "item" : "ABC", "price" : NumberDecimal("80"), "sizes" : "L", "arrayIndex" : NumberLong(2) }
{ "_id" : 3, "item" : "IJK", "price" : NumberDecimal("160"), "sizes" : "M", "arrayIndex" : null }
示例：下面的$unwind操作使用preserveNullAndEmptyArrays选项来包含size字段为null、缺失或空数组的文档。
db.inventory2.aggregate( [
{ $unwind: { path: "$sizes", preserveNullAndEmptyArrays: true } }
] )
{ "_id" : 1, "item" : "ABC", "price" : NumberDecimal("80"), "sizes" : "S" }
{ "_id" : 1, "item" : "ABC", "price" : NumberDecimal("80"), "sizes" : "M" }
{ "_id" : 1, "item" : "ABC", "price" : NumberDecimal("80"), "sizes" : "L" }
{ "_id" : 2, "item" : "EFG", "price" : NumberDecimal("120") }
{ "_id" : 3, "item" : "IJK", "price" : NumberDecimal("160"), "sizes" : "M" }
{ "_id" : 4, "item" : "LMN", "price" : NumberDecimal("10") }
{ "_id" : 5, "item" : "XYZ", "price" : NumberDecimal("5.75"), "sizes" : null }
1.3. 拆分数组字段并分组
初始化数据：
db.inventory2.insertMany([
{ "_id" : 1, "item" : "ABC", price: NumberDecimal("80"), "sizes": [ "S", "M", "L"] },
{ "_id" : 2, "item" : "EFG", price: NumberDecimal("120"), "sizes" : [ ] },
{ "_id" : 3, "item" : "IJK", price: NumberDecimal("160"), "sizes": "M" },
{ "_id" : 4, "item" : "LMN" , price: NumberDecimal("10") },
{ "_id" : 5, "item" : "XYZ", price: NumberDecimal("5.75"), "sizes" : null }
])
示例：下面的管道将展开大小数组，并将产生的文档按展开后的大小值进行分组:
db.inventory2.aggregate( [
// First Stage
{
$unwind: { path: "$sizes", preserveNullAndEmptyArrays: true }
},
// Second Stage
{
$group:
{
_id: "$sizes",
averagePrice: { $avg: "$price" }
}
},
// Third Stage
{
$sort: { "averagePrice": -1 }
}
] )
结果：
{ "_id" : "M", "averagePrice" : NumberDecimal("120") }
{ "_id" : "L", "averagePrice" : NumberDecimal("80") }
{ "_id" : "S", "averagePrice" : NumberDecimal("80") }
{ "_id" : null, "averagePrice" : NumberDecimal("45.25") }
1.4. 拆分嵌入式数组文档
初始化数据：
db.sales.insertMany([
{
_id: "1",
"items" : [
{
"name" : "pens",
"tags" : [ "writing", "office", "school", "stationary" ],
"price" : NumberDecimal("12.00"),
"quantity" : NumberInt("5")
},
{
"name" : "envelopes",
"tags" : [ "stationary", "office" ],
"price" : NumberDecimal("1.95"),
"quantity" : NumberInt("8")
}
]
},
{
_id: "2",
"items" : [
{
"name" : "laptop",
"tags" : [ "office", "electronics" ],
"price" : NumberDecimal("800.00"),
"quantity" : NumberInt("1")
},
{
"name" : "notepad",
"tags" : [ "stationary", "school" ],
"price" : NumberDecimal("14.95"),
"quantity" : NumberInt("3")
}
]
}
])
示例：下面的操作将按标签销售的商品分组，并计算每个标签的总销售额。
db.sales.aggregate([
// First Stage
{ $unwind: "$items" },
// Second Stage
{ $unwind: "$items.tags" },
// Third Stage
{
$group:
{
_id: "$items.tags",
totalSalesAmount:
{
$sum: { $multiply: [ "$items.price", "$items.quantity" ] }
}
}
}
])
结果：
{ "_id" : "writing", "totalSalesAmount" : NumberDecimal("60.00") }
{ "_id" : "stationary", "totalSalesAmount" : NumberDecimal("264.45") }
{ "_id" : "electronics", "totalSalesAmount" : NumberDecimal("800.00") }
{ "_id" : "school", "totalSalesAmount" : NumberDecimal("104.85") }
{ "_id" : "office", "totalSalesAmount" : NumberDecimal("1019.60") }
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