Convert Mysql query to Mongodb find() - mysql

Question

I have a mongodb collection structured like this:
{ "client" : "CLIENTIDHERE", "amount" : 90, "invoice": "SOMEIDHERE", "date" : ISODate("2014-07-09T11:13:49.273Z") }
And need to select somehow all the payments made from a client.
In mysql I would do something like this, but with mongodb I have really no clue.
SELECT SUM(amount) payments,client FROM invoices GROUP BY client;
What I tried:
db.invoices.find({
$group: {
amount: {$sum: "$amount"}
}
}, {
$project:{
amount: "$amount",
client: "$client"
}
})
But it didn't work. What did I do wrong?
EDIT: I get the following error:
error: { "$err" : "Unsupported projection option: amount", "code" : 13097 }

Answer 1

Your $group step doesn't define what to GROUP BY. MongoDB's equivalent for the SQL GROUP BY is the _id field of the $group statement which is missing from your query. Also, when you want to use aggregation, you need to use .aggregate( instead of .find(.
db.invoices.aggregate({
$group: {
_id: "$client",
amount: { $sum: "$amount"}
}
});
This will write the client into the _id field. When you would rather want to have the field named client, you can fix that in the $project step, but otherwise the projection is not required in this case.