mongo 更新数组 键值对

{ "_id" : 1, "grades" : [ 80, 85, 90 ] }
{ "_id" : 2, "grades" : [ 88, 90, 92 ] }
{ "_id" : 3, "grades" : [ 85, 100, 90 ] }

To update 80 to 82 in the grades array
in the first document, use the positional $ operator if you do not know the position of the element
in the array:

如果不知道值在数组的下标，可以用$操作把数组中的 80 更新成82

db.students.update(
{ _id: 1, grades: 80 },
{ $set: { "grades.$" : 82 } }
)

{
_id: 4,
grades: [
{ grade: 80, mean: 75, std: 8 },
{ grade: 85, mean: 90, std: 5 },
{ grade: 90, mean: 85, std: 3 }
]
}

用$操作符 来更新 std 字段的内嵌文档

db.students.update(
{ _id: 4, "grades.grade": 85 },
{ $set: { "grades.$.std" : 6 } }
)