localField只能支持string类型的值，表示字段名。
当您使用管道查找$lookup时，通过将那些course字段设置为courses数组，并在查找管道中使用$in运算符执行过滤。

db.students.aggregate([
  {
    $lookup: {
      "from": "courses",
      "let": {
        courses: [
          "$course1",
          "$course2",
          "$course3",
          "$course4"
        ]
      },
      "pipeline": [
        {
          $match: {
            $expr: {
              $in: [
                "$_id",
                "$$courses"
              ]
            }
          }
        }
      ],
      "as": "studentCourses"
    }
  }
])

Demo @ Mongo Playground

• 观点：*

Thinking that your Student schema is not a good design. You may think how about the students may take a different number of courses. Possible that the referredcoursefield may not exist. Or you need to modify your query again if thecourse` field is added/removed each time.

You shall consider a m1n7o1p array field to store those m1n8o1p ids.`
您可以参考下面的查询，而不是硬编码字段名，而是获得带有前缀的字段的值：“Course”。

db.students.aggregate([
  {
    $lookup: {
      "from": "courses",
      "let": {
        courses: {
          $reduce: {
            input: {
              $filter: {
                input: {
                  $objectToArray: "$$ROOT"
                },
                cond: {
                  "$regexMatch": {
                    "input": "$$this.k",
                    "regex": "^course[\\d]$"
                  }
                }
              }
            },
            initialValue: [],
            in: {
              $concatArrays: [
                "$$value",
                [
                  "$$this.v"
                ]
              ]
            }
          }
        }
      },
      "pipeline": [
        {
          $match: {
            $expr: {
              $in: [
                "$_id",
                "$$courses"
              ]
            }
          }
        }
      ],
      "as": "studentCourses"
    }
  }
])

Demo 2 @ Mongo Playground