I've seen similar questions but far to complex for a newbie like myself. Plus none of the answers offered a syntax break down.
My database:
name :"Kim"
points: [
{ category:"Purchase",
points: -50},
{ category: "Wage",
points : 275},
{ category: "Purchase",
points: -40}
]
name :"Meghan"
points: [
{ category:"Contest",
points: 130},
{ category: "Purchase",
points : -25},
{ category: "Games",
points : 50}
]
]
So in the mongo shell I'm trying to get it find every points.category that equals "Purchase" but only return to me the last one. I attempted to achieve this by using aggregate, $project and $slice. My issue is that I don't understand the syntax well enough to know whether $slice goes inside $project or outside separated by a comma. I understand the syntax when I use $project by itself or $slice by itself but I have no clue how to use all these things together to something magical. If someone could help me solve my problem and explain how to combine all these things properly together, I'd be forever indebted. I've read over the docs but nothing showed me how to use everything all at once.
I want to search the student who's name is Kim points to find the last Purchase she made. The result I want is
[{points:-40}]
If you can use more projection with other aggregation operator, you may try this.
db.students.aggregate([
{ $match : { name : "Kim" } },
{ $project:
{
points: {
$filter : { input: "$points", as:"pts", cond: {$eq: ["$$pts.category", "Purchase"]} }
}
}
},
{ $project:
{
_id : 0,
points: { $arrayElemAt: [{$slice: [ "$points.points", -1]},0] }
}
}
])
Related
new to Mongo. Trying to group across different sub fields of a document based on a condition. The condition is a regex on a field value. Looks like -
db.collection.aggregate([{
{
"$group": {
"$cond": [{
"upper.leaf": {
$not: {
$regex: /flower/
}
}
},
{
"_id": {
"leaf": "$upper.leaf",
"stem": "$upper.stem"
}
},
{
"_id": {
"stem": "$upper.stem",
"petal": "$upper.petal"
}
}
]
}
}])
Using api v4.0: cond in the docs shows - { $cond: [ <boolean-expression>, <true-case>, <false-case> ] }
The error I get with the above code is - "Syntax error: dotted field name 'upper.leaf' can not used in a sub object."
Reading up on that I tried $let to re-assign the dotted field name. But started to hit various syntax errors with no obvious issue in the query.
Also tried using $project to rename the fields, but got - Field names may not start with '$'
Thoughts on the best approach here? I can always address this at the application level and split my query into two but it's attractive potentially to solve it natively in mongo.
$group syntax is wrong
{
$group:
{
_id: <expression>, // Group By Expression
<field1>: { <accumulator1> : <expression1> },
...
}
}
You tried to do
{
$group:
<expression>
}
And even if your expression resulted in the same code, its invalid syntax for $group (check from the documentation where you are allowed to use expressions)
One other problem is that you use the query operator for regex, and not the aggregate regex operators (you can't do that, if you aggregate you can use only aggregate operators, only $match is the exception that you can use both if you add $expr)
You need this i think
[{
"$group" : {
"_id" : {
"$cond" : [ {
"$not" : [ {
"$regexMatch" : {
"input" : "$upper.leaf",
"regex" : "/flower/"}}]},
{"leaf" : "$upper.leaf","stem" : "$upper.stem"},
{"stem" : "$upper.stem","petal" : "$upper.petal"}]
}
}}]
Its similar code, but expression gets as value of the "_id" and $regexMatch
is used that is aggregate operator.
I didnt tested the code.
I'm trying to achieve a join on three String fields between two collections on MongoDB v. 4.4.3: one containing the original documents, the other the translations.
Both document types look like this:
{
"_id" : ObjectId("60644367b521563be8044f07"),
"dsId" : "2051918",
"lcId" : "data_euscreenXL_EUS_15541BBE705033639D4E06691D7A5D2E",
"pgId" : "1",
(...)
This MongoDB query does what I need, embedding the Translations in the result:
db.Original.aggregate([
{ $match: { query parameters } },
{ $lookup:
{
from: "Translation",
let: { "origDsId": "$dsId", origLcId: "$lcId", "origPgId": "$pgId" },
pipeline: [
{ $match:
{ $expr:
{ $and:
[
{ $eq: [ "$dsId", "$$origDsId" ] },
{ $eq: [ "$lcId", "$$origLcId" ] },
{ $eq: [ "$pgId", "$$origPgId" ] }
]
}
}
},
{ $project: { dsId: 0, _id: 0 } }
],
as: "translations"
}
}])
However, I can't figure out how to write the equivalent Morphia query. I updated to Morphia v.2.2, which adds the required features, but it's all very new and hasn't yet been documented on morphia.dev; I couldn't find much more on Javadoc either. This Morphia unit test on Github looked interesting and I tried copying that approach:
Aggregation<Original> query = datastore.aggregate(Original.class)
.match(eq("dsId", datasetId), eq("lcId", localId))
.lookup(Lookup.lookup(Translation.class)
.let("origDsId", value("$dsId"))
.let("origLcId", value("$lcId"))
.let("origPgId", value("$pgId"))
.pipeline(Match.match(expr(Expressions.of()
.field("$and",
array(Expressions
.of().field("$eq",
array(field("dsId"),
field("$origDsId"))),
Expressions
.of().field("$eq",
array(field("lcId"),
field("$origLcId"))),
Expressions
.of().field("$eq",
array(field("pgId"),
field("$origPgId"))))))))
.as("translations"));
...
This returns the Original documents, but fails to join the Translations.
The problem is that the syntax of the pipeline stage is rather puzzling. I wonder if anyone can shed some light on this?
the unit test example does not use (or need?) the double-$ form seen in "$$origDsId"? From the MongoDB documentation I understand that this form is used to refer to externally defined variables (eg in the "let" assignment before the "pipeline") but they don't work in the quoted example either;
what is the role of the static ArrayExpression "array"? It looks as if it's a kind of assignment container, where Expressions.of().field("$eq", array(field("dsId"), field("$origDsId"))) might mean something like "dsId" = "$origDsId" - which would be what I need (if it would work ;) )
I tried all sorts of combinations, using field("$origDsId"), value("$origDsId"), field("$$origDsId"), value("$$origDsId"), etcetera, but having no luck so far.
Thanks in advance!
I am fairly new to MongoDB and cant seem to find a solution to this problem.
I have a database of documents that has this structure:
{
id: 1
elements: [ {elementId: 1, nr1: 1, nr2: 3}, {elementId:2, nr1:5, nr2: 10} ]
}
I am looking for a query that can add a value nr3 which is for example nr2/nr1 to all the objects in the elements array, so that the resulting document would look like this:
{
id: 1
elements: [ {elementId: 1, nr1: 1, nr2: 3, nr3:3}, {elementId:2, nr1:5, nr2: 10, nr3: 2} ]
}
So I imagine a query along the lines of this:
db.collection.updateOne({id:1}, {$set:{"elements.$[].nr3": nr2/nr1}})
But I cant find how to get the value of nr2 and nr1 of the same object in the array.
I found some similar questions on stackoverflow stating this is not possible, but they were 5+ years old, so I thought maybe they have added support for something like this.
I realize I can achieve this with first querying the document and iterate over the elements-array doing updates along the way, but for the purpose of learning I would love to see if its possible to do this in one query.
You can use update with aggregation pipeline starting from MongoDB v4.2,
$map to iterate loop of elements
divide nr2 with nr1 using $divide
merge current object and new field nr3 using $mergeObjects
db.collection.updateOne(
{ id: 1 },
[{
$set: {
elements: {
$map: {
input: "$elements",
in: {
$mergeObjects: [
"$$this",
{ nr3: { $divide: ["$$this.nr2", "$$this.nr1"] } }
]
}
}
}
}
}]
)
Playground
db.collection.update(
{ id:1},
{ "$set": { "elements.$[elem].nr3":elements.$[elem].nr2/elements.$[elem].nr1} },
{ "multi": true }
);
I guess this should work
I am using aggregation with mongoDB now i am facing a problem here, i am trying to match my documents which are present in my input array by using $in operator. Now i want to know the index of the lement from the input array now can anyone please tell me how can i do that.
My code
var coupon_ids = ["58455a5c1f65d363bd5d2600", "58455a5c1f65d363bd5d2601","58455a5c1f65d363bd5d2602"]
couponmodel.aggregate(
{ $match : { '_id': { $in : coupons_ids }} },
/* Here i want to know index of coupon_ids element that is matched because i want to perform some operation in below code */
function(err, docs) {
if (err) {
} else {
}
});
Couponmodel Schema
var CouponSchema = new Schema({
category: {type: String},
coupon_name: {type: String}, // this is a string
});
UPDATE-
As suggested by user3124885 that aggregation is not better in performance, can anyone please tell me the performance difference between aggregation and normal query in mongodb. And which one is better ??
Update-
I read this question on SO mongodb-aggregation-match-vs-find-speed. Here the user himself commented that both take same time, also by seeing vlad-z answer i think aggregation is better. Please if anyone of you have worked on mongodb Then please tell me what are your opinion about this.
UPDATE-
I used sample json data containing 30,000 rows and tried match with aggregation v/s find query aggregation got executed in 180 ms where find query took 220ms. ALso i ran $lookup it is also taking not much than 500ms so think aggregation is bit faster than normal query. Please correct me guys if any one of you have tried using aggregation and if not then why ??
UPDATE-
I read this post where user uses below code as a replacement of $zip SERVER-20163 but i am not getting how can i solve my problem using below code. So can anybody please tell me how can i use below code to solve my issue.
{$map: {
input: {
elt1: "$array1",
elt2: "$array2"
},
in: ["$elt1", "$elt2"]
}
Now can anyone please help me, it would be really be a great favor for me.
So say we have the following in the database collection:
> db.couponmodel.find()
{ "_id" : "a" }
{ "_id" : "b" }
{ "_id" : "c" }
{ "_id" : "d" }
and we wish to search for the following ids in the collections
var coupons_ids = ["c", "a" ,"z"];
We'll then have to build up a dynamic projection state so that we can project the correct indexes, so we'll have to map each id to its corresponding index
var conditions = coupons_ids.map(function(value, index){
return { $cond: { if: { $eq: ['$_id', value] }, then: index, else: -1 } };
});
Then we can then inject this in to our aggregation pipeline
db.couponmodel.aggregate([
{ $match : { '_id' : { $in : coupons_ids } } },
{ $project: { indexes : conditions } },
{ $project: {
index : {
$filter: {
input: "$indexes", as: "indexes", cond: { $ne: [ "$$indexes", -1 ] }
}
}
}
},
{ $unwind: '$index' }
]);
Running the above will now output each _id and it's corresponding index within the coupons_ids array
{ "_id" : "a", "index" : 1 }
{ "_id" : "c", "index" : 0 }
However we can also add more items in to the pipeline at the end and reference $index to get the current matched index.
I think you could do it in a faster way simply retrieving the array and search manually. Remember that aggregation don't give you performance.
//$match,$in,$and
$match:{
$and:[
{"uniqueID":{$in:["CONV0001"]}},
{"parentID":{$in:["null"]}},
]
}
}])
Mongo DB: I'm looking to make one query to return both the first and last element of an array. I realize that I can do this multiple queries, but I would really like to do it with one.
Assume a collection "test" where each objects has an array "arr" of numbers:
db.test.find({},{arr:{$slice: -1},arr:{$slice: 1}});
This will result in the following:
{ "_id" : ObjectId("xxx"), "arr" : [ 1 ] } <-- 1 is the first element
Is there a way to maybe alias the results? Similar to what the mysql AS keyword would allow in a query?
This is not possible at the moment but will be with the Aggregation Framework that's in development now if I understand your functional requirement correctly.
You have to wonder about your schema if you have this requirement in the first place though. Are you sure there isn't a more elegant way to get this to work by changing your schema accordingly?
This can be done with the aggregation framework using the operators $first and $last as follows:
db.test.aggregate([
{ '$addFields': {
'firstElem': { '$first': '$arr' },
'lastElem': { '$last': '$arr' }
} }
])
or using $slice as
db.test.aggregate([
{ '$addFields': {
'firstElem': { '$slice': [ '$arr', 1 ] },
'lastElem': { '$slice': [ '$arr', -1 ] }
} }
])