在上一篇mongodb Aggregation聚合操作之$collStats中详细介绍了mongodb聚合操作中的$collStats使用以及参数细节。本篇将开始介绍Aggregation聚合操作中的$facet操作。
说明：
在同一组输入文档的单一阶段中处理多个聚合管道。每个子管道在输出文档中都有自己的字段，其结果存储在文档数组中。$facet阶段允许您在单个聚合阶段内创建多面聚合，这些聚合描述了跨多个维度(或多个方面)的数据。多面聚合提供多个过滤器和分类来指导数据浏览和分析，$facet在同一组输入文档上支持各种聚合，而不需要多次检索输入文档。
语法：
{ $facet:
{
<outputField1>: [ <stage1>, <stage2>, ... ],
<outputField2>: [ <stage1>, <stage2>, ... ],
...
}
}
注意：
$facet阶段及其子管道不能使用索引，即使它的子管道使用$match，或者$facet是管道中的第一阶段。$facet阶段将始终在执行期间执行COLLSCAN。
1. 示例
初始化数据：
db.artwork.insertMany([{ "_id" : 1, "title" : "The Pillars of Society", "artist" : "Grosz", "year" : 1926,
"price" : NumberDecimal("199.99"),
"tags" : [ "painting", "satire", "Expressionism", "caricature" ] },
{ "_id" : 2, "title" : "Melancholy III", "artist" : "Munch", "year" : 1902,
"price" : NumberDecimal("280.00"),
"tags" : [ "woodcut", "Expressionism" ] },
{ "_id" : 3, "title" : "Dancer", "artist" : "Miro", "year" : 1925,
"price" : NumberDecimal("76.04"),
"tags" : [ "oil", "Surrealism", "painting" ] },
{ "_id" : 4, "title" : "The Great Wave off Kanagawa", "artist" : "Hokusai",
"price" : NumberDecimal("167.30"),
"tags" : [ "woodblock", "ukiyo-e" ] },
{ "_id" : 5, "title" : "The Persistence of Memory", "artist" : "Dali", "year" : 1931,
"price" : NumberDecimal("483.00"),
"tags" : [ "Surrealism", "painting", "oil" ] },
{ "_id" : 6, "title" : "Composition VII", "artist" : "Kandinsky", "year" : 1913,
"price" : NumberDecimal("385.00"),
"tags" : [ "oil", "painting", "abstract" ] },
{ "_id" : 7, "title" : "The Scream", "artist" : "Munch", "year" : 1893,
"tags" : [ "Expressionism", "painting", "oil" ] },
{ "_id" : 8, "title" : "Blue Flower", "artist" : "O'Keefe", "year" : 1918,
"price" : NumberDecimal("118.42"),
"tags" : [ "abstract", "painting" ] }])
示例：
db.artwork.aggregate( [
{
$facet: {
"categorizedByTags": [
{ $unwind: "$tags" },
{ $sortByCount: "$tags" }
],
"categorizedByPrice": [
// Filter out documents without a price e.g., _id: 7
{ $match: { price: { $exists: 1 } } },
{
$bucket: {
groupBy: "$price",
boundaries: [ 0, 150, 200, 300, 400 ],
default: "Other",
output: {
"count": { $sum: 1 },
"titles": { $push: "$title" }
}
}
}
],
"categorizedByYears(Auto)": [
{
$bucketAuto: {
groupBy: "$year",
buckets: 4
}
}
]
}
}
])
结果：
[ { categorizedByTags:
[ { _id: 'painting', count: 6 },
{ _id: 'oil', count: 4 },
{ _id: 'Expressionism', count: 3 },
{ _id: 'abstract', count: 2 },
{ _id: 'Surrealism', count: 2 },
{ _id: 'ukiyo-e', count: 1 },
{ _id: 'woodblock', count: 1 },
{ _id: 'woodcut', count: 1 },
{ _id: 'satire', count: 1 },
{ _id: 'caricature', count: 1 } ],
categorizedByPrice:
[ { _id: 0, count: 2, titles: [ 'Dancer', 'Blue Flower' ] },
{ _id: 150,
count: 2,
titles: [ 'The Pillars of Society', 'The Great Wave off Kanagawa' ] },
{ _id: 200, count: 1, titles: [ 'Melancholy III' ] },
{ _id: 300, count: 1, titles: [ 'Composition VII' ] },
{ _id: 'Other',
count: 1,
titles: [ 'The Persistence of Memory' ] } ],
'categorizedByYears(Auto)':
[ { _id: { min: null, max: 1902 }, count: 2 },
{ _id: { min: 1902, max: 1918 }, count: 2 },
{ _id: { min: 1918, max: 1926 }, count: 2 },
{ _id: { min: 1926, max: 1931 }, count: 2 } ] } ]
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