Mongodb aggregate collection - mongodb

Question

I'm learning aggregate in mongodb. I'm working with the collection:
{
"body" : ""
,
"email" : "oJJFLCfA#qqlBNdpY.com",
"author" : "Linnie Weigel"
},
{
"body" : ""
,
"email" : "ptHfegMX#WgxhlEeV.com",
"author" : "Dinah Sauve"
},
{
"body" : ""
,
"email" : "kfPmikkG#SBxfJifD.com",
"author" : "Zachary Langlais"
}
{
"body" : ""
,
"email" : "gqEMQEYg#iiBqZCez.com",
"author" : "Jesusa Rickenbacker"
}
]
I try to obtain the number of body of each author. But when I execute the command sum of aggregate mongodb, the result is 1(because the structure has only one element) . How can I do this operation?. I tried with $addToSet. But I don't know how to obtain each element of collection and to do the operation.

Answer 1

In order to count the comments by each author you want to $group by that author and $sum the occurrences. Basically just a "$sum: 1" operation. But it seems like you have "comments" as an array here based on your own comments and the closing bracket on your partial data listing. For that you need to process with $unwind first:
db.collection.aggregate([
{ "$unwind": "$comments" },
{ "$group": {
"_id": "$comments.author",
"count": { "$sum": 1 }
}}
])
That will obtain the total of all author comments by author for the entire collection. If you were just after getting the total comments by author per document ( or what looks like a blog post model ) then you use the document _id as part of the group statement:
db.collection.aggregate([
{ "$unwind": "$comments" },
{ "$group": {
"_id": {
"_id": "$_id"
"author": "$comments.author"
},
"count": { "$sum": 1 }
}}
])
And if you then want the summary of author counts per document with just a single document returned with all the authors in an array, then use $addToSet from here, with another $group pipeline stage:
db.collection.aggregate([
{ "$unwind": "$comments" },
{ "$group": {
"_id": {
"_id": "$_id"
"author": "$comments.author"
},
"count": { "$sum": 1 }
}},
{ "$group": {
"_id": "$_id._id",
"comments": {
"$addToSet": {
"author": "$_id.author",
"count": "$count"
}
}
}}
])
But really, the author values are already unique and "sets" are not ordered in any way, so you might change this using $push after first introducing a $sort to have the list ordered by the number of comments made:
db.collection.aggregate([
{ "$unwind": "$comments" },
{ "$group": {
"_id": {
"_id": "$_id"
"author": "$comments.author"
},
"count": { "$sum": 1 }
}},
{ "$sort": { "_id._id": 1, "count": -1 } },
{ "$group": {
"_id": "$_id._id",
"comments": {
"$push": {
"author": "$_id.author",
"count": "$count"
}
}
}}
])