Question

例如，我有以下类型的集合：

[
    { batch: false, type: '' },
    { batch: false, type: '' },
    { batch: true, type: '123' },
    { batch: true, type: '123' },
    { batch: true, type: '123' },
    { batch: true, type: '234' },
    { batch: true, type: '234' },
    { batch: true, type: '234' },
    { batch: true, type: '234' },
    { batch: true, type: '567' },
    { batch: true, type: '567' }
]

所以问题是，如何返回具有{batch: false}且如果{batch: true}仅返回具有相同{type}字段的第一个对象的对象数组，基本上我想获得以下响应：

[
    { batch: false, type: '' },
    { batch: false, type: '' },
    { batch: true, type: '123' },
    { batch: true, type: '234' },
    { batch: true, type: '567' }
]

Answer 1

运行以下聚合管道，您需要在#ifndef RIDER_H #define RIDER_H #include <iostream> using std::cout; using std::cin; using std::endl; using std::boolalpha; #include <cstring> using std::strcpy; using std::strlen; #include <fstream> using std::ostream; using std::istream; using std::ofstream; using std::ifstream; class Rider { public: Rider( ) = default; Rider( const char* name, int age, const char* rank, const char* weyr ); void setName( const char* name ); void setRank( const char * rank ); void setWeyr( const char * weyr ); void setAge( int age ); // required when we enter a new Rider // from the keyboard const char* getName( ) const { return name; } const char* getRank( ) const { return rank; } const char* getWeyr( ) const { return weyr; } int getAge( ) const { return age; } friend ostream& operator << ( ostream& os, const Rider& r); friend istream& operator >> ( istream& is, Rider& r ); friend ofstream& operator << ( ofstream& os, const Rider& r ); friend ifstream& operator >> ( ifstream& is, Rider & r ); private: char * name = nullptr; char * rank = nullptr; char * weyr = nullptr; int age = 0; }; #endif键中具有满足给定条件的条件：

$group

Answer 2

尝试以下聚合管道

db.getCollection('yourCollection').aggregate([
    {
        $group: {
            _id: {
                k1: {
                    $cond: {
                        if: "$batch",
                        then: null,
                        else: "$_id"
                    }
                },
                k2: "$type"
            },
            batch: { $first: "$batch" },
            type: { $first: "$type" }
        }
    },
    {
        $project: {
            _id: 0,
            batch: 1,
            type: 1
        }
    }
])

导致

/* 1 */
{
    "batch" : false,
    "type" : ""
}

/* 2 */
{
    "batch" : false,
    "type" : ""
}

/* 3 */
{
    "batch" : true,
    "type" : "123"
}

/* 4 */
{
    "batch" : true,
    "type" : "567"
}

/* 5 */
{
    "batch" : true,
    "type" : "234"
}

按字段值返回第一个匹配的数组

2 个答案: