Mongodb Aggregate calculate average and add it to the document - mongodb

Question

I have websites which contains 2 documents:
{
"_id" : ObjectId("58503934034b512b419a6eab"),
"website" : "https://www.stackoverflow.com",
"name" : "Stack Exchange",
"keywords" : [
"helping",
"C#",
"PYTHON"
]
}
{
"_id" : ObjectId("58503934034b512b419a6eab"),
"website" : "https://www.google.com.com",
"name" : "Stack Exchange",
"keywords" : [
"search",
"engine",
]
}
I also have another seo_tracking which contains:
{
"_id" : ObjectId("587373d6f6325811c8a0b3ad"),
"position" : "2",
"real_url" : "https://www.stackoverflow.com",
"created_at" : ISODate("2017-01-09T11:28:22.104Z"),
"keyword" : "helping"
},
{
"_id" : ObjectId("587373d6f6325811c8a0b3ad"),
"position" : "4",
"real_url" : "https://www.stackoverflow.com",
"created_at" : ISODate("2017-01-09T11:28:22.104Z"),
"keyword" : "C#"
}
etc.. This contains around 100+ documents
What I want to do is is aggregate the seo_tracking with website on the specific URL (www.stackexchange (in websites) would match www.stackoverflow.com in (seo_tracking)) which I can do fine. However, I would like to return for each of the websites the following:
{
"_id" : ObjectId("587373d6f6325811c8a0b3ad"),
"website":"https://www.stackoverflow.com",
"avg_position" : "2"
}
Then for Google etc.. Even if the avg_position is 0 .. I have tried the following:
db.seo_tracking.aggregate([
{
$lookup:
{
from: "websites",
localField: "real_url",
foreignField: "website",
as: "post_websites"
},
},
{
"$group": {
_id:null,
avg_position:{$avg:"$position"}
}
}
])
However, this just produces:
{
"_id" : null,
"avg_position" : 2.0
}
What I need to do is have website and ideally also need the ID
Any ideas to where I'm going wrong here?

Answer 1

You can try something like this. You'll need to $unwind to access the fields from joined collection and change your grouping key to use the _id from joined collection to get average for each website:
db.seo_tracking.aggregate([{
$lookup: {
from: "website",
localField: "real_url",
foreignField: "website",
as: "post_websites"
},
}, {
$unwind: "$post_websites"
}, {
"$group": {
_id: "$post_websites._id",
avg_position: {
$avg: "$position"
},
website: {
$first: "$real_url"
}
}
}])