MongoDB average aggregation doesn't work properly - mongodb

Question

I'm newbie to NoSQL databases. What I want is to show the title, url and the avg(ratings).
The sample data looks like the following:
{
"_id" : ObjectId("52b3833bd3e98582d2bfb628"),
"author" : {
"name" : "Graydon Hoare",
"email" : "graydon#gmail.com"
},
"title" : "Why Rust ditched pure functions",
"body" : "sth",
"url" : "http://thread.gmane.org/gmane.comp.lang.rust.devel/3674/focus=3855",
"date" : ISODate("2013-04-30T13:23:00.000Z"),
"starred" : 105,
"ratings" : [
3,
5,
3,
2,
4,
1,
3,
3,
3,
2,
3
],
"comments" : [
{
"user" : "tr0lltherapy",
"upVotes" : 18,
"downVotes" : 2,
"text" : "something",
"replies" : [
{
"user" : "thedeemon",
"upVotes" : 10,
"downVotes" : 0,
"text" : "something"
},
{
"user" : "mcandre",
"upVotes" : 0,
"downVotes" : 5,
"text" : "Performance? There are already a slew of performant languages. Assembler, C, C++, Go. What does Rust actually offer that's new and useful in this category, other than using my favorite abbreviation for the named function keyword, fn?"
},
{
"user" : "lacosaes0",
"upVotes" : 30,
"downVotes" : 6,
"text" : "Particular emphasis on memory safety."
}
]
},
{
"user" : "hypster",
"upVotes" : 30,
"downVotes" : 2,
"text" : "tl;dr everybody was type-fu fighting",
"replies" : [
{
"user" : "homoiconic",
"upVotes" : 15,
"downVotes" : 0,
"text" : "Here comes the Big Boss, Hu! Simon Peyton-Jones."
}
]
}
],
"tags" : [
"Rust",
"Computer",
"Programming"
],
"draft" : true,
"published" : true
}
I have tried the following query, but it doesn't work properly and puts null value in the average. I don't know how should I fix it.
db.getCollection('links').aggregate(
[
{
$match: {
"author.email": /#gmail.com$/
}
},
{
$project: {
_id: 0,
title: 1,
url: 1,
avgRatings: {
$avg: "$Ratings"
}
}
}
])
Expected output is:
title: "Why Rust ditched pure functions", url: "http://thread.gmane.org/gmane.comp.lang.rust.devel/3674/focus=3855",
avgRatings: 2.90

Answer 1

You have a typo, $Ratings; use $ratings as below. Aggregation syntax is case sensitive.
db.getCollection('links').aggregate(
[
{
$match: {
"author.email": /#gmail.com$/
}
},
{
$project: {
_id: 0,
title: 1,
url: 1,
avgRatings: {
$avg: "$ratings"
}
}
}
])