How to incorporate $lt and $divide in mongodb - mongodb

Question

I am a newbie in MongoDB but I am trying to query to identify if any of my field meets the requirements.
Consider the following:
I have a collection where EACH document is formatted as:
{
"nutrition" : [
{
"name" : "Energy",
"unit" : "kcal",
"value" : 150.25,
"_id" : ObjectId("fdsfdslkfjsdf")
}
{---then there's more in the array---}
]
"serving" : 4
"id": "Food 1"
}
My current code looks something like this:
db.recipe.find(
{"nutrition": {$elemMatch: {"name": "Energy", "unit": "kcal", "value": {$lt: 300}}}},
{"id":1, _id:0}
)
Under the array nutrition, there's a field with its name called Energy with it's value being a number. It checks if that value is less than 300 and outputs all the documents that meets this requirement (I made it only output the field called id).
Now my question is the following:
1) For each document, I have another field called "serving" and I am supposed to find out if "value"/"serving" is still less than 300. (As in divide value by serving and see if it's still less than 300)
2) Since I am using .find, I am guessing I can't use $divide operator from aggregation?
3) I been trying to play around with aggregation operators like $divide + $cond, but no luck so far.
4) Normally in other languages, I would just create a variable a = value/serving then run it through an if statement to check if it's less than 300 but I am not sure if that's possible in MongoDB
Thank you.

Answer 1

In case anyone was struggling with similar problem, I figured out how to do this.
db.database.aggregate([
{$unwind: "$nutrition"}, //starts aggregation
{$match:{"nutrition.name": "Energy", "nutrition.unit": "kcal"}}, //breaks open the nutrition array
{$project: {"Calories per serving": {$divide: ["$nutrition.value", "$ingredients_servings"]}, //filters out anything not named Energy and unit is not kcal, so basically 'nutrition' array now has become a field with one data which is the calories in kcal data
"id": 1, _id:0}}, //show only the food id
{$match:{"Calories per serving": {$lt: 300}}} //filters out any documents that has their calories per serving equal to or greater than 300
]}
So basically, you open the array and filter out any sub-fields you don't want in the document, then display it using project along with any of your math evaluations that needs to be done. Then you filter out any condition you had, which for me was that I don't want to see any foods that have their calories per serving more than 300.