I have a collection named Vote that looks like the following:
{
postId: "1",
comment:{
text_sentiment: "positive",
topic: "A"
}
}, // DOC-1
{
postId: "2",
comment:{
text_sentiment: "negative",
topic: "A"
}
}, // DOC-2
{
postId: "3",
comment:{
text_sentiment: "positive",
topic: "B"
}
},..//DOC-3 ..
and a collection named post which looks as follows?
{
_id: "1",
category: "a"
},
{
_id: "2",
category: "b",
}
I want to do an aggregation on this collection such that it returns the following structure. (which also takes into account the other post collection`
[
{
_id: "hash",
topic: "A",
topicOccurance: 2,
sentiment: {
positive: 1,
negative: 1,
neutral: 0
},
postIds: [1,2],
categories: [a,b]
},
..
]
I created the following aggregation:
db.Vote.aggregate([
{
$match: {
surveyId: "e6d38e1ecd",
"comment.topic": {
$exists: 1
},
}
},
{
$group: {
_id: {
topic: "$comment.topic",
text_sentiment: "$comment.text_sentiment"
},
total: {
$sum: 1
},
postIds: {
$push: "$postId"
}
}
},
{
$group: {
_id: "$_id.topic",
total: {
$sum: "$total"
},
text_sentiments: {
$push: {
k: "$_id.text_sentiment",
v: "$total"
}
},
postIds: {
"$push": "$postIds"
}
}
},
{
$project: {
topic: "$_id",
topicOccurance: "$total",
sentiment: {
"$arrayToObject": "$text_sentiments"
},
postIds: {
$reduce: {
input: "$postIds",
initialValue: [],
in: {
$concatArrays: [
"$$value",
"$$this"
]
}
}
}
}
},
{
$sort: {
"topicOccurance": -1
}
}
])
This works fine but I do not know, how to also get categories which exist in a separate collection named posts. How can I query a different collection while performing aggregation?
CodePudding user response：
A simple $lookup will suffice:
db.Vote.aggregate([
{
$match: {
"comment.topic": {
$exists: 1
},
}
},
{
$group: {
_id: {
topic: "$comment.topic",
text_sentiment: "$comment.text_sentiment"
},
total: {
$sum: 1
},
postIds: {
$push: "$postId"
}
}
},
{
$group: {
_id: "$_id.topic",
total: {
$sum: "$total"
},
text_sentiments: {
$push: {
k: "$_id.text_sentiment",
v: "$total"
}
},
postIds: {
"$push": "$postIds"
}
}
},
{
$project: {
topic: "$_id",
topicOccurance: "$total",
sentiment: {
"$arrayToObject": "$text_sentiments"
},
postIds: {
$reduce: {
input: "$postIds",
initialValue: [],
in: {
$concatArrays: [
"$$value",
"$$this"
]
}
}
}
}
},
{
$sort: {
"topicOccurance": -1
}
},
{
$lookup: {
from: "post",
localField: "postIds",
foreignField: "_id",
as: "categories"
}
},
{
$addFields: {
categories: {
"$setUnion": {
$map: {
input: "$categories",
in: "$$this.category"
}
}
}
}
}
])