英文:
MongoDB : How to multiply a field that appears only in $project?
问题
我正在使用以下方式连接两个集合（$lookup）：
const LEAD_PRICE = ....; // 不重要
Client.aggregate(
[
{
$lookup: {
from: "clientboughtleads",
localField: "_id",
foreignField: "Client",
as: "ClientLeads"
}
},
{
$project: {
ClientId: "$_id",
RegistrationDate: "$date",
TotalPurchasedLeads: {
$size: "$ClientLeads"
},
IncomeFromClient: { // 这个总是未定义
$multiply: [LEAD_PRICE / 2, "$TotalPurchasedLeads"]
}
}
}
]
)
我想通过常量和一个已计算的字段TotalPurchasedLeads的乘积来计算IncomeFromClient，但它返回了NULL：
[
{
_id: 5e0e43ae82a71e0017dccb20,
ClientId: 5e0e43ae82a71e0017dccb20,
RegistrationDate: 2020-01-02T21:25:34.000Z,
TotalPurchasedLeads: 4,
IncomeFromClient: null // 这个未计算
}
]
在计算一个被$project的字段时，如何获取$lookup集合的大小？
英文:
I'm joining ($lookup) two collection the following way :
const LEAD_PRICE = ....; // doesn't matter
Client.aggregate(
[
{
$lookup: {
from: "clientboughtleads",
localField: "_id",
foreignField: "Client",
as: "ClientLeads"
}
},
{
$project: {
ClientId: "$_id",
RegistrationDate: "$date",
TotalPurchasedLeads: {
$size: "$ClientLeads"
},
IncomeFromClient: { // This one is always undefined
$multiply: [LEAD_PRICE / 2, "$TotalPurchasedLeads"]
}
}
}
]
I want to calculate IncomeFromClient by the mul of a const and a field that is been also calculated - TotalPurchasedLeads , however it returns NULL :
[
[0] {
[0] _id: 5e0e43ae82a71e0017dccb20,
[0] ClientId: 5e0e43ae82a71e0017dccb20,
[0] RegistrationDate: 2020-01-02T21:25:34.000Z,
[0] TotalPurchasedLeads: 4,
[0] IncomeFromClient: null // This one is not calculated
[0] }
[0] ]
How can I get the size of the $lookup collection when calculating a field that is being $projected ?
答案1
得分: 1
请在我们可以在项目中使用关键字"addFields"之后使用"key"。
const LEAD_PRICE = ....; // doesn't matter
Client.aggregate(
[
{
$lookup: {
from: "clientboughtleads",
localField: "_id",
foreignField: "Client",
as: "ClientLeads"
}
},
{
$addFields: {
TotalPurchasedLeads: {
$size: "$ClientLeads"
}
}
},
{
$project: {
ClientId: "$_id",
RegistrationDate: "$date",
TotalPurchasedLeads: 1,
IncomeFromClient: { // This one is always undefined
$multiply: [LEAD_PRICE / 2, "$TotalPurchasedLeads"]
}
}
}
]
)
英文:
Please use addFields keyword after we can use key in project .
const LEAD_PRICE = ....; // doesn't matter
Client.aggregate(
[
{
$lookup: {
from: "clientboughtleads",
localField: "_id",
foreignField: "Client",
as: "ClientLeads"
}
},
{
$addFields: {
TotalPurchasedLeads: {
$size: "$ClientLeads"
}
}
},
{
$project: {
ClientId: "$_id",
RegistrationDate: "$date",
TotalPurchasedLeads:1,
IncomeFromClient: { // This one is always undefined
$multiply: [LEAD_PRICE / 2, "$TotalPurchasedLeads"]
}
}
}
]
答案2
得分: 1
你不能在相同的 $project 阶段中使用 $project 的值。所以，改为使用其值
{
"$project": {
"ClientId": "$_id",
"RegistrationDate": "$date",
"TotalPurchasedLeads": {
"$size": "$ClientLeads"
},
"IncomeFromClient": {
"$multiply": [LEAD_PRICE / 2, { "$size": "$ClientLeads" }]
}
}
}
英文:
You cannot use the $projected value in the same $projection stage. So instead use its value
{
"$project": {
"ClientId": "$_id",
"RegistrationDate": "$date",
"TotalPurchasedLeads": {
"$size": "$ClientLeads"
},
"IncomeFromClient": {
"$multiply": [LEAD_PRICE / 2, { "$size": "$ClientLeads" }]
}
}
}
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