Finding multiple docs using same id not working, using meteor + react and mongoDB - mongodb

Question

How do I get the email address of the students in the same class_id, take it as there are more then 2 students in different class in the DB as well?
I have this but it return empty array []
Meteor.users.find({"course_learn_list.$.class_id": {$in: [classId]}},
{field: {"emails.address": 1}}
).fetch()
Collections
{
"_id": "LMZiLKs2MRhZiiwoS",
"course_learn_list": [
{
"course_id": "M8EiKfxAAzy25WmFH",
"class_id": "jePhNgEuXLM3ZCt98"
},
{
"course_id": "5hbwrfbfxAAzy2nrg",
"class_id": "dfbfnEuXLM3fngndn"
}
],
"emails": [
{
"address": "student1#gmail.com",
"verified": false
}
]
},
{
"_id": "JgfdLKs2MRhZJgfNgk",
"course_learn_list": [
{
"course_id": "M8EiKfxAAzy25WmFH",
"class_id": "jePhNgEuXLM3ZCt98"
},
{
"course_id": "5hbwrfbfxAAzy2nrg",
"class_id": "dfbfnEuXLM3fngndn"
}
],
"emails": [
{
"address": "student2#gmail.com",
"verified": false
}
]
}

Answer 1

I think you want:
Meteor.users.find({ "course_learn_list.class_id": classId },
{ "course_learn_list.$": 1, "emails.address": 1 }).fetch()
This should find the first instance in each course_learn_list array where the classId is your classId.

Answer 2

In this case you probably don't need to use a projection to get the right answer. Here's an example of extracting the verified email addresses using only the . operator in the selector:
const ids = ['jePhNgEuXLM3ZCt98', 'some-other-id'];
const emails =
Meteor.users.find({ 'course_learn_list.class_id': { $in: ids } })
.fetch()
.map(user => _.findWhere(user.emails, { verified: true }).address);

Answer 3

This works for me!
Meteor.publish("getMyClassStudents", function(classId) {
console.log("Publish getMyClassStudents")
var self = this
if (self.userId) {
var data = Meteor.users.find({
"course_learn_list.class_id": classId
}, {
"fields": {
"emails.address": 1
}
})
return data
}
else {
return self.ready()
}
})