Use mongo positional operator ($) and find in multiple arrays - mongodb

Question

Let me extend the example from the docs to:
{
_id: 4,
tags: ['good', 'great', 'nice'],
grades: [
{ grade: 80, mean: 75, std: 8 },
{ grade: 85, mean: 90, std: 5 },
{ grade: 90, mean: 85, std: 3 }
]
}
Now I want to update all grade=85 which have also an tag good. I would try to use:
db.students.update(
{ tags: "good", "grades.grade": 85 },
{ $set: { "grades.$.std" : 6 } }
)
However, mongo will set the $ to 0 because the tag good is the first in the tag list. This causes the data to change to:
{
_id: 4,
tags: ['good', 'great', 'nice'],
grades: [
{ grade: 80, mean: 75, std: 6 }, //←wrong
{ grade: 85, mean: 90, std: 5 },
{ grade: 90, mean: 85, std: 3 }
]
}
Question: Is there a way to tell mongo and/or mongoose to use the position in grades to reference the grade?

Answer 1

It happened because you use two arrays in a single query document. But unfortunately, it is not supported in MongoDB.
MongoDB documentation
The query document should only contain a single condition on the array
field being projected. Multiple conditions may override each other
internally and lead to undefined behavior.
Under these requirements, the following query is incorrect:
db.collection.find( { <array>: <value>, <someOtherArray>: <value2> },
{ "<array>.$": 1 } )
You can try to write something like that:
db.test.find({tags: "good", "grades.grade": 85 }).forEach(function(doc){ db.test.update(
{_id: doc._id, "grades.grade": 85 },
{ $set: { "grades.$.std" : 88 }},
{multi: true})})

Answer 2

With mongoose you should create two seperate models. One for Students and an other one for Grades. Then let Students contain references to Grades.
This will allow to update single Grades easily.
Something like:
var studentSchema = mongoose.Schema({
tags: [String],
grades: {
type: mongoose.Schema.Types.ObjectId,
ref: 'Grade'
}
});
var Student = mongoose.model('Student', studentSchema);
var gradeSchema = mongoose.Schema({
grade: Number,
mean: Number,
std: Number
});
var Grade = mongoose.model('Grade', studentSchema);
Your update code could look like this:
Student.find({tags: { $in: ['good'] }}).populate('grades', { grade: 85 }).exec(function(err, students) {
students.grades.forEach(function(grade) {
grade.update(null, { std: 6 }, function(err, numAffected) {
});
});
});