Question

这个mongo命令（1）返回100

db.collection.aggregate([{$match: {"events._id":{$exists:true}}},{$unwind: "$events"},{$match: {"events._id":{$exists:true}}},{$group: {_id: null, count: {$sum: 1}}}])

这个mongo命令（2）返回80

db.collection.aggregate([{$match: {"events.field":{$exists:true}}},{$unwind: "$events"},{$match: {"events.field":{$exists:true}}},{$group: {_id: null, count: {$sum: 1}}}])

这个mongo命令（2）返回15

db.collection.aggregate([{$match: {"events.field":{$exists:false}}},{$unwind: "$events"},{$match: {"events.field":{$exists:false}}},{$group: {_id: null, count: {$sum: 1}}}])

换句话说，第一个命令返回我的集合中的事件总数，100。第二个命令返回存在'field'的事件，80。第三个命令返回那些'field'不存在的事件，15。不应该（1）+（2）= 100？如何获得5个丢失的事件？

Answer 1

在#include <stdio.h> #include "SDL2/SDL.h" #include "SDL2/SDL_image.h" int main() { SDL_Init(SDL_INIT_VIDEO); SDL_Window* win = SDL_CreateWindow("test", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 640, 480, 0); Uint32 render_flags = SDL_RENDERER_ACCELERATED; SDL_Renderer* rend = SDL_CreateRenderer(win, -1, render_flags); SDL_Surface* surf = IMG_Load("Workspace/learnSDL/red.png"); if (!surf) { printf("%s\n", IMG_GetError()); } SDL_Texture* tex = SDL_CreateTextureFromSurface(rend, surf); SDL_Delay(10000); SDL_DestroyTexture(tex); SDL_DestroyRenderer(rend); SDL_DestroyWindow(win); SDL_Quit(); }管道中，您需要使用字段$unwind保留空数组或空数组，否则将丢失缺失或空数组，并且不会将其传递给下一个管道。

在您的收藏中，某些文档没有preserveNullAndEmptyArrays或events字段

events.field

样本集

db.collection.aggregate(
    [
        { $unwind:  { path : "$events", preserveNullAndEmptyArrays : true } },
        { $match: {"events.field":{ $exists:false } } },
        { $group: { _id: null, count: { $sum: 1 } } }
    ]
)

总计数

> db.collection.find()
{ "_id" : ObjectId("5a5b0a3dc2a7348cbc592b9e"), "name" : "1", "events" : { "field" : "1" } }
{ "_id" : ObjectId("5a5b0a3dc2a7348cbc592b9f"), "name" : "2" }
{ "_id" : ObjectId("5a5b0a3dc2a7348cbc592ba0"), "name" : "3", "events" : { "field" : "2" } }
{ "_id" : ObjectId("5a5b0a3dc2a7348cbc592ba1"), "name" : "1", "events" : { "field" : "1" } }
>

计数> db.collection.count() 4 > >

events.field exists

计数> db.collection.aggregate( [ { $unwind: { path : "$events", preserveNullAndEmptyArrays : true } }, { $match: { "events.field": { $exists : true } } }, { $group: { _id: null, count: { $sum: 1 } } } ] ) { "_id" : null, "count" : 3 } >

events.field does not exist

MongoDB聚合计数奇怪的行为

1 个答案: