我是NoSQL数据库的新手，我选择MongoDB作为我的第一个NoSQL数据库。我创建了一个聚合管道来显示所需的数据，这是我的文档示例：
Users Collection中的文档样本
{
"_id": 9,
"name": "Sample Name",
"email": "email@example.com",
"password": "password hash"
}
Pages Collection中的文档样本（这一点并不重要）
{
"_id": 42,
"name": "Product Name",
"description": "Product Description",
"user_id": 8,
"rating_categories": [{
"_id": 114,
"name": "Build Quality"
}, {
"_id": 115,
"name": "Price"
}, {
"_id": 116,
"name": "Feature"
}, {
"_id": 117,
"name": "Comfort"
}, {
"_id": 118,
"name": "Switch"
}]
}
评论集中的文档样本
{
"_id": 10,
"page_id": 42, #ID reference from pages collection
"user_id": 8, #ID reference from users collection
"review": "The review of the product",
"ratings": [{
"_id": 114, #ID Reference from pages collection of what rating category it is
"rating": 5
}, {
"_id": 115,
"rating":4
}, {
"_id": 116,
"rating": 5
}, {
"_id": 117,
"rating": 3
}, {
"_id": 118,
"rating": 4
}],
"created": "1582825968963", #Date Object
"votes": {
"downvotes": [],
"upvotes": [9] #IDs of users who upvote this review
}
}
我想通过page_id获得评论，可以通过我制作的API进行访问，这是聚合的预期结果：
[
{
"_id": 10, #Review of the ID
"created": "Thu, 27 Feb 2020 17:52:48 GMT",
"downvote_count": 0, #Length of votes.downvotes from reviews collection
"page_id": 42, #Page ID
"ratings": [ #Stores what rate at what rating category id
{
"_id": 114,
"rating": 5
},
{
"_id": 115,
"rating": 4
},
{
"_id": 116,
"rating": 5
},
{
"_id": 117,
"rating": 3
},
{
"_id": 118,
"rating": 4
}
],
"review": "The Review",
"upvote_count": 0, #Length of votes.upvotes from reviews collection
"user": { #User who reviewed
"_id": 8, #User ID
"downvote_count": 0, #How many downvotes this user receive from all of the user's reviews
"name": "Sample Name", #Username
"review_count": 1, #How many reviews the user made
"upvote_count": 1 #How many upvotes this user receive from all of the user's reviews
},
"vote_state": 0 #Determining vote state from the user (who requested to the API) for this review, 0 for no vote, -1 for downvote, 1 for upvote
},
...
]
以下是我针对上述结果进行的评论收集汇总的管道：
user_id = 9
page_id = 42
pipeline = [
{"$group": {
"_id": {"user_id":"$user_id", "page_id": "$page_id"},
"review_id": {"$last": "$_id"},
"page_id": {"$last": "$page_id"},
"user_id" : {"$last": "$user_id"},
"ratings": {"$last": "$ratings"},
"review": {"$last": "$review"},
"created": {"$last": "$created"},
"votes": {"$last": "$votes"},
"upvote_count": {"$sum":
{"$cond": [
{"$ifNull": ["$votes.upvotes", False]},
{"$size": "$votes.upvotes"},
0
]}
},
"downvote_count": {"$sum":
{"$cond": [
{"$ifNull": ["$votes.downvotes", False]},
{"$size": "$votes.downvotes"},
0
]}
}}},
{"$lookup": {
"from": "users",
"localField": "user_id",
"foreignField": "_id",
"as": "user"
}},
{"$unwind": "$user"},
{"$lookup": {
"from": "reviews",
"localField": "user._id",
"foreignField": "user_id",
"as": "user.reviews"
}},
{"$addFields":{
"_id": "$review_id",
"user.review_count": {"$size": "$user.reviews"},
"user.upvote_count": {"$sum":{
"$map":{
"input":"$user.reviews",
"in":{"$cond": [
{"$ifNull": ["$$this.votes.upvotes", False]},
{"$size": "$$this.votes.upvotes"},
0
]}
}
}},
"user.downvote_count": {"$sum":{
"$map":{
"input":"$user.reviews",
"in":{"$cond": [
{"$ifNull": ["$$this.votes.downvotes", False]},
{"$size": "$$this.votes.downvotes"},
0
]}
}
}},
"vote_state": {"$switch": {
"branches": [
{"case": { "$and" : [
{"$ifNull": ["$votes.upvotes", False]},
{"$in": [user_id, "$votes.upvotes"]}
]}, "then": 1
},
{"case": { "$and" : [
{"$ifNull": ["$votes.downvotes", False]},
{"$in": [user_id, "$votes.downvotes"]}
]}, "then": -1
},
],
"default": 0
}},
}},
{"$project":{
"user.password": 0,
"user.email": 0,
"user_id": 0,
"review_id" : 0,
"votes": 0,
"user.reviews": 0
}},
{"$sort": {"created": -1}},
{"$match": {"page_id": page_id}},
]
注意：用户可以对同一page_id进行多条评论，但只会显示最新的评论
我正在使用pymongo btw，这就是为什么运算符带有引号
我的问题是：
是否有空间优化我的聚合管道？
是否执行多个小聚合执行以获取上述数据是一种好习惯，还是最好进行一次大聚合（或尽可能少地获取我想要的数据）？< / p>
如您所见，每次我想访问votes.upvotes集合上的文档中的votes.downvotes或review时，我都会检查该字段是否为空，即因为在用户进行评论时不会创建votes.upvotes和votes.downvotes字段，而是在用户对该评论进行投票时创建。用户进行审核并删除votes.upvotes时，我应该在votes.downvotes和$ifNull上留空字段吗？这样会提高聚合的性能吗？
谢谢
答案 0 :(得分：1)
检查此聚合是否具有更好的性能。
如果还没有创建这些索引：
db.reviews.create_index([("page_id", 1)])
注意：我们可以进一步提高性能，而无需再次进行$lookup 评论。
db.reviews.aggregate([
{
$match: {
page_id: page_id
}
},
{
$addFields: {
request_user_id: user_id
}
},
{
$group: {
_id: {
page_id: "$page_id",
user_id: "$user_id",
request_user_id: "$request_user_id"
},
data: {
$push: "$$ROOT"
}
}
},
{
$lookup: {
"from": "users",
"let": {
root_user_id: "$_id.user_id"
},
"pipeline": [
{
$match: {
$expr: {
$eq: [
"$$root_user_id",
"$_id"
]
}
}
},
{
$lookup: {
"from": "reviews",
"let": {
root_user_id: "$$root_user_id"
},
"pipeline": [
{
$match: {
$expr: {
$eq: [
"$$root_user_id",
"$user_id"
]
}
}
},
{
$project: {
user_id: 1,
downvote_count: {
$size: "$votes.downvotes"
},
upvote_count: {
$size: "$votes.upvotes"
}
}
},
{
$group: {
_id: null,
review_count: {
$sum: {
$cond: [
{
$eq: [
"$$root_user_id",
"$user_id"
]
},
1,
0
]
}
},
upvote_count: {
$sum: "$upvote_count"
},
downvote_count: {
$sum: "$downvote_count"
}
}
},
{
$unset: "_id"
}
],
"as": "stats"
}
},
{
$project: {
tmp: {
$mergeObjects: [
{
_id: "$_id",
name: "$name"
},
{
$arrayElemAt: [
"$stats",
0
]
}
]
}
}
},
{
$replaceWith: "$tmp"
}
],
"as": "user"
}
},
{
$addFields: {
first: {
$mergeObjects: [
"$$ROOT",
{
$arrayElemAt: [
"$data",
0
]
},
{
user: {
$arrayElemAt: [
"$user",
0
]
},
created: {
$toDate: {
$toLong: {
$arrayElemAt: [
"$data.created",
0
]
}
}
},
downvote_count: {
$reduce: {
input: "$data.votes.downvotes",
initialValue: 0,
in: {
$add: [
"$$value",
{
$size: "$$this"
}
]
}
}
},
upvote_count: {
$reduce: {
input: "$data.votes.upvotes",
initialValue: 0,
in: {
$add: [
"$$value",
{
$size: "$$this"
}
]
}
}
},
vote_state: {
$cond: [
{
$gt: [
{
$size: {
$filter: {
input: "$data.votes.upvotes",
cond: {
$in: [
"$_id.request_user_id",
"$$this"
]
}
}
}
},
0
]
},
1,
{
$cond: [
{
$gt: [
{
$size: {
$filter: {
input: "$data.votes.downvotes",
cond: {
$in: [
"$_id.request_user_id",
"$$this"
]
}
}
}
},
0
]
},
-1,
0
]
}
]
}
}
]
}
}
},
{
$unset: [
"first.data",
"first.votes",
"first.user_id",
"first.request_user_id"
]
},
{
$replaceWith: "$first"
},
{
"$sort": {
"created": -1
}
}
])