I've got a mongodb aggregation pipeline with a $project stage and I'd like to include certain fields only if conditions are met. Specifically, I'd like to exclude the _id in one condition and include a second field 'second_id' in the other condition.
I know that it's not possible (yet) to exclude fields from a mongodb $project, but is it possible to conditionally include them?
Is there a way to conditionally exclude the _id field? It accepts a 0 or 1, but what if I want to determine that 0 or 1 based on an if statement. How would this be done?
Pseudocode for _id:
$project: { _id: { $ifNull: [ "$user_id", 0 ] } }
The main use of this would be to use the doc.user_id as the result _id, or allow mongodb to create a new autoincrement _id if that user_id is null.
There isn't a way currently to do this within the $project stage, but you can use the $redact stage to remove the field conditionally (i.e. you set the value to 0 like you are doing in your example.
db.collection.aggregate(
... matching and stuff ...
{$project: { _id: { $ifNull: [ "$user_id", 0 ] } }},
{$redact: {
{$cond: {
if: { $eq: [ "$user_id", 0 ] },
then: '$$PRUNE',
else: '$$DESCEND'
}}
}
I have a trick for this one:
"_id": {"$cond": [{"$eq": ["$user_id", null]}, "$nonExistinField", "$user_id"]}
Maybe this was not doable when the question was asked, but now it is.
There is a way to conditionally include/exclude a particular field in $project stage.
This can be achieved using $cond to evaluate your condition, and then return either '$true' or '$false'
{
myConditionalField:{$cond:{
if:{
$eq:['$some_field','some_value']
},
then:'$false',
else:'$true'
}}
}
Here, the field myConditionalField will be projected only if the value of some_field from previous stage matched some_value.
NOTE: Its assumed here that myConditionalField already exists from a previous stage. If it doesn't, then the else should be $some_field or whatever field you want to be projected.
Related
I've been scratching my head with this problem. I've attempted to search for a solution but I didn't find anything relating to my specific use case.
Would anyone be able to help me out?
Say I have a collection of "discount" documents, and importantly they have an "amount" field to say how much the discount is worth. Whenever a discount is redeemed I currently want to track what the worth was at the time of the redemption.
To do this I've been attempting to use the following code:
await datastore.collection('discounts').updateOne(
{
$expr: { $gt: [ '$maxUses', '$uses' ] },
...criteria
},
{
$set: {
uses: 1
},
$push: {
redemptions: {
name: concatNames(user),
email: user.email,
amount: '$amount', // <-- use amount from root document
when: new Date()
}
}
}
)
Unfortunately $amount does not pull the value from the root document, instead it just becomes "$amount" as a string. I've also attempted to convert this update to use a pipeline but $push is not a valid pipeline stage.
Here's a quick Mongo playground link.
Thanks in advance.
In order to refer to another fields value, you'll need to use the aggregation pipeline form of update. However, '$push' is an update operator, not an aggregation operator.
$concatArrays gets most of the way there like
{$set: {redepmtions: {$concatArrays: [ "$redemptions", [{amount: "$amount"}]}}
That will throw an error if $redemptions doesn't already exist, so use $cond to subsitute an empty array in that case:
.updateOne(
{ ...criteria },
[{$set: {
redemptions: {$concatArrays: [
{$cond: [{$eq: ["array", {$type: "$redemptions"}]}, "$redemptions", []]},
[{amount: "$amount"}]
]}
}}]
)
Playground
Hello guys I'm writing a Message Application with Node.js and Mongoose. I keep datas in mongodb like that:
I want to list users who messaged before so I need to filter my 'Messages' collection but I can't do what exactly I want. If he sent a message to a person I need to take persons name but, if he take a message from a person I need to take persons name however in first situation person name in reciever, in second situation person name in sender. I made a table for explain more easily. I have left table and I need 3 name like second table.(Need to eliminate one John's name)
Sorry, if this problem asked before but I don't know how can I search this problem.
I tried this but it take user name who logged in and duplicate some names.
Message.find({$or: [{sender: req.user.username}, {reciever: req.user.username}]})
One option is to use an aggregation pipeline to create two sets and simply union them:
db.collection.aggregate([
{$match: {$or: [{sender: req.user.username}, {reciever: req.user.username}]}},
{$group: {
_id: 0,
recievers: {$addToSet: "$reciever"},
senders: {$addToSet: "$sender"}
}},
{$project: {
_id: req.user.username,
previousChats: {"$setDifference":
[
{$setUnion: ["$recievers", "$senders"]},
[req.user.username]
]
}
}}
])
See how it works on the playground example
This is a tricky one, but can be solved with a fairly simple aggregation pipeline.
Explanation
On our first stage of the pipeline, we will want to get all the messages sent or received by the user (in our case David), for that we will use a $match stage:
{
$match: {
$or: [
{sender: 'David'},
{receiver: 'David'}
]
}
}
After we found all the messages from or to David, we can start collecting the people he talks to, for that we will use a $group stage and use 2 operations that will help us to achieve this:
$addToSet - This will add all the names to a set. Sets only contain one instance of the same value and ignore any other instance trying to be added to the set of the same value.
$cond - This will be used to add either the receiver or the sender, depending which one of them is David.
The stage will look like this:
{
$group: {
_id: null,
chats: {$addToSet: {$cond: {
if: {$eq: ['$sender', 'David']},
then: '$receiver',
else: '$sender'
}}}
}
}
Combining these 2 stages together will give us the expected result, one document looking like this:
{
"_id": null, // We don't care about this
"chats": [
"John",
"James",
"Daniel"
]
}
Final Solution
Message.aggregate([{
$match: {
$or: [
{
sender: req.user.username
},
{
receiver: req.user.username
}
]
}
}, {
$group: {
_id: null,
chats: {
$addToSet: {
$cond: {
'if': {
$eq: [
'$sender',
req.user.username
]
},
then: '$receiver',
'else': '$sender'
}
}
}
}
}])
Sources
Aggregation
$match aggregation stage
$group aggregation stage
$addToSet operation
$cond operation
I have the following aggregation pipeline consisting of a single $redact step:
(which is supposed to return all products for which the recorded number of sales exceeds the stock)
$redact:
{
$cond: [
{ $gt: ["$sales", "$productInfo.0.stock"] },
"$$KEEP",
"$$PRUNE"
]
}
(the syntax above is specific to Compass, so no issues here)
Where the entities look something like:
{
_id: 123,
sales: 60,
price: 80,
productInfo: [
{
stock: 100
}
]
}
However, the query above does not seem to work. My presumption is that the issue is caused by the comparison with $productInfo.0.stock, as replacing it with another direct attribute of the entity (e.g. price) runs without any issues.
How should such a query ($cond by $gt where one of the values is from an array/list) be specified?
The productionInfo.0.stock syntax is the Mongo Query Language (MQL), which can be used in a $match or a find.
That particular construct is not availabl when using aggregation syntax, such as "$fieldName". Instead, you need to use $arrayElemAt, but that unfortunately doesn't support accessing fields in the reference element.
To get what you want, you will need to add a field in a prior stage that retrieves the desired element from the array, reference that object in the $redact, and then project out the temporary field, such as:
{$addFields: {firstProduct: {$arrayElemAt: [ "$productInfo", 0 ]}}},
{$redact:{
$cond: [
{ $gt: ["$sales", "$productInfo.0.stock"] },
"$$KEEP",
"$$PRUNE"
]
}},
{$project: {firstProduct: 0}}
How can I get documents from mongo with an array containing some elements but IN THE SAME ORDER?
I know that $all do the job but ignoring the order of elements. The order in my case is important and I can't sort my arrays since it's describing a path that I want to keep the order.
111,222,333 is not the same as 222,111,333
Is there a way to do it using $all or maybe another operator in mongo aggregation framework?
You can avoid the first "intersect" field, is just to give you back as debug what MongoDB make with this command. You should create the $and operator dynamically.
db.Test6.aggregate([
{
$project: {
_id:1,
pages:1,
intersect: {$setIntersection: [[111,666], "$pages"]},
theCondition: {$let: {
vars: {
intersect: {$setIntersection: [[111,666], "$pages"]}
},
in: {
$cond:[ {$and:[
{$eq:[{$arrayElemAt:["$$intersect", 0]}, 111]},
{$eq:[{$arrayElemAt:["$$intersect", 1]}, 666]}
]} , true, false]
}
}
}
}
}
]);
I have a "class" document as:
{
className: "AAA",
students: [
{name:"An", age:"13"},
{name:"Hao", age:"13"},
{name:"John", age:"14"},
{name:"Hung", age:"12"}
]
}
And i want to get the student who has name is "An", get only matching element in array "students". I can do that with function find() as:
>db.class.find({"students.name":"An"}, {"students.$":true})
{
"_id" : ObjectId("548b01815a06570735b946c1"),
"students" : [
{
"name" : "An",
"age" : "13"
}
]}
It's fine, but when i do the same with Aggregation as following, it get error:
db.class.aggregate([
{$match:{"students.name":'An'}},
{$project:{"students.$":true}}
])
Error is:
uncaught exception: aggregate failed: {
"errmsg" : "exception: FieldPath field names may not start with '$'.",
"code" : 16410,
"ok" : 0
}
Why? I can't use "$" for array in $project operator of aggregate() while can use this one in project operator of find().
From the docs:
Use $ in the projection document of the find() method or the findOne()
method when you only need one particular array element in selected
documents.
The positional operator $ cannot be used in an aggregation pipeline projection stage. It is not recognized there.
This makes sense, because, when you execute a projection along with a find query, the input to the projection part of the query is a single document that has matched the query.The context of the match is known even during projection. So for each document that matches the query, the projection operator is applied then and there before the next match is found.
db.class.find({"students.name":"An"}, {"students.$":true})
In case of:
db.class.aggregate([
{$match:{"students.name":'An'}},
{$project:{"students.$":true}}
])
The aggregation pipeline is a set of stages. Each stage is completely unaware and independent of its previous or next stages. A set of documents pass a stage completely before being passed on to the next stage in the pipeline. The first stage in this case being the $match stage, all the documents are filtered based on the match condition. The input to the projection stage is now a set of documents that have been filtered as part of the match stage.
So a positional operator in the projection stage makes no sense, since in the current stage it doesn't know on what basis the fields had been filtered. Therefore, $ operators are not allowed as part of the field paths.
Why does the below work?
db.class.aggregate([
{ $match: { "students.name": "An" },
{ $unwind: "$students" },
{ $project: { "students": 1 } }
])
As you see, the projection stage gets a set of documents as input, and projects the required fields. It is independent of its previous and next stages.
Try using the unwind operator in the pipeline: http://docs.mongodb.org/manual/reference/operator/aggregation/unwind/#pipe._S_unwind
Your aggregation would look like
db.class.aggregate([
{ $match: { "students.name": "An" },
{ $unwind: "$students" },
{ $project: { "students": 1 } }
])
You can use $filter to selects a subset of an array to return based on the specified condition.
db.class.aggregate([
{
$match:{
"className": "AAA"
}
},
{
$project: {
$filter: {
input: "$students",
as: "stu",
cond: { $eq: [ "$$stu.name", "An" ] }
}
}
])
The following example filters the Students array to only include documents that have a name equal to "An".