Querying Array of Embedded Documents in MongoDB based on Range - mongodb

Question

How can I query array of embedded document in mongodb.
{
_id: 1,
zipcode: "63109",
students: [
{ name: "john", school: 102, age: 14 },
{ name: "jess", school: 102, age: 11 },
{ name: "jeff", school: 108, age: 15 }
]
}
{
_id: 2,
zipcode: "63110",
students: [
{ name: "ajax", school: 100, age: 7 },
{ name: "achilles", school: 100, age: 8 },
]
}
{
_id: 3,
zipcode: "63109"
}
Let's say for the above data how will retrieve only those lines where age>=7 and age<=10. For id_1 only the row with age 10 should be returned. Both the rows in id_2 and id_3. Also, id_4 has no field called students. So, I DO NOT, want to see it in the output.
Edit:-
The output I get looks something like this when I do a filter. But I DO NOT want the row that has "None".
The last document _id: 3 does not have the field students but filtering is done on students, so the output has a "None" corresponding to it.
I wish to handle 2 cases:
No filtering should be applied where the array "students" don't exist under an id.
Array students exists but is empty []
Both these cases end up in the output if simply filtered.
{'_id': ObjectId('5cdaefd393436906b016ddb4'),
'students': [{'name': ajax,
'school': '100',
'age': '7'},
{'name': achilles,
'school': '100',
'age': '8'}],
{'_id': ObjectId('5cdaefd393436906b016ddb3'), 'students': None}
Note: I am using a different but identical data set, so the output is not exact, but I hope you get the idea. The 'student' is an embedded document which may or may not be present in all documents.
Edit : Finally I used unwind to achieve what I wanted.

Answer 1

You can use below aggregation
db.collection.aggregate([
{ "$match": { "$expr": { "$gte": [{ "$size": { "$ifNull": ["$students", []] } }, 1] }}},
{ "$addFields": {
"students": {
"$filter": {
"input": { "$ifNull": ["$students", []] },
"cond": {
"$and": [
{ "$gte": ["$$this.age", 7] },
{ "$lte": ["$$this.age", 10] }
]
}
}
}
}}
])