MongoDB 家庭作业(job)帮助.不确定如何在 mongodb 中访问文档中的变量

问题1:使用点符号进行筛选:"subject.book.Year Pub"

db.Subject.aggregate([
  {
    "$unwind": "$subject.book"
  },
  {
    "$match": {
      "subject.book.yearPub": 2022
    }
  },
  {
    "$project": {
      "subject.book.ISBN": 1,
      "subject.book.bookTitle": 1
    }
  }
])

Demo Q1 @ Mongo Playground个

Updated: To filter the document in the array个个

db.collection.aggregate([
  {
    "$match": {
      "subject.book.yearPub": 2022
    }
  },
  {
    $set: {
      "subject.book": {
        $filter: {
          input: "$subject.book",
          cond: {
            $eq: [
              "$$this.yearPub",
              2022
            ]
          }
        }
      }
    }
  },
  {
    "$project": {
      "subject.book.ISBN": 1,
      "subject.book.bookTitle": 1
    }
  }
])

Demo Q1.1 @ Mongo Playground个

问题2:我怀疑你对结果的观察不正确.当索引从0开始时，subject.book.author.3将返回subject.book.author数组中的第四个元素.

对于您当前的查询，它将返回具有subject.book.author个数组中的第三个和第四个元素的文档，这意味着只要该数组包含more than 2个元素，就会返回该文档.

相反，您需要使用$size运算符来获取数组的大小.虽然有些文档没有book属性，因此使用$ifNull返回空数组，以防止在使用$size时出错.

db.Subject.aggregate([
  {
    "$match": {
      $expr: {
        $in: [
          {
            $size: {
              $ifNull: [
                "$subject.book.author",
                []
              ]
            }
          },
          [
            2,
            3
          ]
        ]
      }
    }
  },
  {
    "$project": {
      "subject.book.ISBN": 1,
      "subject.book.bookTitle": 1,
      "subject.book.publisher": 1,
      "_id": 0
    }
  }
])

Demo Q2 @ Mongo Playground个

Updated: To filter the document in the array个个

db.collection.aggregate([
  {
    "$match": {
      $expr: {
        $in: [
          {
            $size: {
              $ifNull: [
                "$subject.book.author",
                []
              ]
            }
          },
          [
            2,
            3
          ]
        ]
      }
    }
  },
  {
    $set: {
      "subject.book": {
        $filter: {
          input: "$subject.book",
          cond: {
            $in: [
              {
                $size: {
                  $ifNull: [
                    "$subject.book.author",
                    []
                  ]
                }
              },
              [
                2,
                3
              ]
            ]
          }
        }
      }
    }
  },
  {
    "$project": {
      "subject.book.ISBN": 1,
      "subject.book.bookTitle": 1,
      "subject.book.publisher": 1,
      "_id": 0
    }
  }
])

Demo Q2.1 @ Mongo Playground个

Note: You attach too many questions into one. Please remember to split it into different questions (posts) when you ask a new question in the future.个