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!
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 $regexMatch
is used that is aggregate operator.
I didnt tested the code.
I have some documents with the following structure...
{
user: "Joe",
lists: [
{ listId: "1234", listName: "dogs" },
{ listId: "5678", listName: "cats" }
]
}
I am trying to prepend a string to each listId field but I am stuck. Amongst other things I have tried...
db.users.updateMany(
{"lists.listId": /^[0-9a-f]{20,}$/},
[{$set:
{"lists.listId.$[]": {"$concat": ["0000", "$lists.listId"]}}
}]
)
But I got the error message: "FieldPath field names may not start with '$'"
Variations on this write results into the appropriate field, but not the results I'm after.
I've bashed my head against the docs for a few hours now but all the references I can find to using the positional operator to reference the value of the field that is being updated use the field name directly, not referenced as a property like I am doing. I've not really messed with pipelines a lot before and I'm finding it all a bit confusing! Someone kindly helped me with a closely related problem yesterday, using $map, and that worked great for a plain array of strings but I haven't had any luck adapting that to an array of objects with string properties. Sorry if this is Mongo 101, the docs are good, but there's a lot of them and I'm not sure which bits are relevant to this.
You can do it like this:
db.collection.users({},
[
{
"$set": {
lists: {
$map: {
input: "$lists",
in: {
$mergeObjects: [
{
"listName": "$$this.listName",
"listId": {
$concat: [
"0000",
"$$this.listId"
]
}
}
]
}
}
}
}
}
],
{
"multi": true
})
Here is the working example: https://mongoplayground.net/p/Q8kUTB6X5JY
The aim that I am trying to achieve:
Update a value in MongoDB based on its current value.
After googling I have found that $cond operator potentially allows me to solve the problem, so I've written the following query:
db.getCollection('product').update({_id: ObjectId("77b2a57556a5e634d57d9977")},
{"$set": {"availability" :
{$cond: [ { "$availability": { $eq: true } }, "Yes", "No" ] }}});
but for some reason, it doesn't work and MongoDB throws an exception.
So could you please advise: Is it technically the right approach to use $cond within the update query?
If yes, why the query doesn't work, semantically it looks good to me,
or possibly some other options on how to solve the problem available, please suggest.
So it seems like you are trying to use Update with Aggregation pipeline which is supported in MongoDB >=4.2. And also if your DB version is >=4.2 the correct syntax would be
db.getCollection("product").update(
{ _id: ObjectId("77b2a57556a5e634d57d9977") },
[
{
$set: {
availability: {
$cond: [
{
$eq: ["$availability", true],
},
"Yes",
"No",
],
},
},
}
]
);
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: [ "$points.points", -1]},0] }
}
}
])
Mongo DB: I'm looking to make one query to return both the first and last element of an array. I realize that I can do this multiple queries, but I would really like to do it with one.
Assume a collection "test" where each objects has an array "arr" of numbers:
db.test.find({},{arr:{$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 ] }
} }
])