I have items like these in my collection
{
user: data,
somestuff: [somedata ...],
terminals: [ {
label: data,
details: [{more content}, {}, ...]
}]
}
I would use 'find' to extract "details" field for a specific terminal 'label'
I know that there is an easy way to get the "terminals" array by:
collection.find({_id: "..."}, {_id:0, terminals: 1})
Wich return
{ terminals: [ {
label: data,
details: [{more content}, {}, ...]
}]
}
I tried
collection.find({ "terminals.label": data }, { _id: 0, "terminals.$.details": 1 })
As edirican has suggested
And it almost work, but it return the same structure than previously except that the terminals list contain only the labeled document
The result I expect is the details list, extracted from terminals
{ details: [{more content}, {}, ...] }
Thanks for your help !
Use positional ($) projection.
http://docs.mongodb.org/manual/reference/operator/projection/positional/
collection.find({ "terminals.label": 2 }, { _id: 0, "terminals.$.details": 1 })
Related
It's been 2 days (or nights should I say) since I am trying to figure out following so would appreciate your help guys.
in mongodb I have number of orders (I will simplify documents for the case).
I want to group all documents by $campRoundId and where there are installments, push all object to installments variable.
The problem I am facing is when document.installments array is empty it pushes it to the array too.
Initial documents (same campRoundId to group by - first with no instalmments, second with 2)
[
{
_id: ObjectId("62792d8a519af6ae8cdff779"),
campRoundId: ObjectId("620a790b2cbc52006c83115a"),
installments: [],
},
{
_id: ObjectId("62792d8a519af6ae8cdff77a"),
campRoundId: ObjectId("620a790b2cbc52006c83115a"),
installments: [
{
payment: 100,
paymentStatus: false,
},
{
payment: 20,
paymentStatus: false,
},
],
},
];
my aggregation
/**
* _id: The id of the group.
* fieldN: The first field name.
*/
{
_id : "$campRoundId",
installments: {
$push: {
$cond:[
{ $gte: ["$installments.length", 1] },
"$installments",
null
]
}
}
}
I want to get rid of empty object, so if there are no installments nothing will be pushed. (dotted lines)
This has been extensively covered here, but none of the solutions seems to be working for me. I'm attempting to remove an object from an array using that object's id. Currently, my Schema is:
const scheduleSchema = new Schema({
//unrelated
_id: ObjectId
shifts: [
{
_id: Types.ObjectId,
name: String,
shift_start: Date,
shift_end: Date,
},
],
});
I've tried almost every variation of something like this:
.findOneAndUpdate(
{ _id: req.params.id },
{
$pull: {
shifts: { _id: new Types.ObjectId(req.params.id) },
},
}
);
Database:
Database Format
Within these variations, the usual response I've gotten has been either an empty array or null.
I was able slightly find a way around this and accomplish the deletion by utilizing the main _id of the Schema (instead of the nested one:
.findOneAndUpdate(
{ _id: <main _id> },
{ $pull: { shifts: { _id: new Types.ObjectId(<nested _id>) } } },
{ new: true }
);
But I was hoping to figure out a way to do this by just using the nested _id. Any suggestions?
The problem you are having currently is you are using the same _id.
Using mongo, update method allows three objects: query, update and options.
query object is the object into collection which will be updated.
update is the action to do into the object (add, change value...).
options different options to add.
Then, assuming you have this collection:
[
{
"_id": 1,
"shifts": [
{
"_id": 2
},
{
"_id": 3
}
]
}
]
If you try to look for a document which _id is 2, obviously response will be empty (example).
Then, if none document has been found, none document will be updated.
What happens if we look for a document using shifts._id:2?
This tells mongo "search a document where shifts field has an object with _id equals to 2". This query works ok (example) but be careful, this returns the WHOLE document, not only the array which match the _id.
This not return:
[
{
"_id": 1,
"shifts": [
{
"_id": 2
}
]
}
]
Using this query mongo returns the ENTIRE document where exists a field called shifts that contains an object with an _id with value 2. This also include the whole array.
So, with tat, you know why find object works. Now adding this to an update query you can create the query:
This one to remove all shifts._id which are equal to 2.
db.collection.update({
"shifts._id": 2
},
{
$pull: {
shifts: {
_id: 2
}
}
})
Example
Or this one to remove shifts._id if parent _id is equal to 1
db.collection.update({
"_id": 1
},
{
$pull: {
shifts: {
_id: 2
}
}
})
Example
I have the following collection:
{
_id: 12345,
quizzes: [
{
_id: 111111,
questions: []
}
]
},
{
_id: 78910,
quizzes: [
{
_id: 22222
}
]
}
I want to select the documents of a certain quiz from the quizzes that do not have the questions array and want to make sure that it uses the appropriate questions index. So I use the following query:
Answer.find({ 'quizzes.0.questions': { $exists: false } }).explain('queryPlanner');
Which returns:
{
queryPlanner: {
plannerVersion: 1,
namespace: 'iquiz.answers',
indexFilterSet: false,
parsedQuery: { 'quizzes.0.questions': [Object] },
winningPlan: { stage: 'COLLSCAN', filter: [Object], direction: 'forward' },
rejectedPlans: []
}
}
The query is not using any index as seen from the output. I have tried the following indexes and none get used:
{ quizzes.$**: 1 }
{ quizzes.questions: 1 }
{ quizzes.[$**].questions: 1 }
{ quizzes: 1 }
The only 1 that actually gets used:
{ quizzes.0.questions: 1 }
However this is not really practical as I may target any quiz from the quizzes array not just the first one. Is there a certain syntax for the index in my case or this is a current limitation of mongodb? Thanks!
Indexes generally do not help to answer queries in the form of "X does not exist". See also mongodb indexes covering missing values.
To verify whether the index is used, look up some data using a positive condition.
I have a document of this kind:
{
_id: "123",
arrayName: [
{ text: 'first', field: false},
{ text: 'second', field: true},
{ text: 'third', field: false}
]
}
Now I would like to my query to return the length of filtered arrayName to have only the objects where field === false.
I have tried many queries with db.collection.find({}) but I always receive the entire document and in the end and it count() => to 1.
Any suggestion?
thanks
You can use projection operator to achieve what you want.
Try this:
db.collection.find({
_id : givenId ,
"arrayName.field" : false
},{
"arrayName.$":1
},function(err,result){
if(!err){
len = result.arrayName.length;
//use len however you want.
}
});
"arrayName.$":1 will select only the matched elements of the array. Then you can get the length of array with field:false
Hope this helps!
Another user already give you the right answer for your question using the plain Mongo query. But I think it won't have much because you are using Mongo with Meteor, so here I present you another way to solve your problem by using transform function in find command:
Collection.find({
// ...
}, {
transform(doc) {
doc.filterArrayLength = doc.arrayName.filter(obj => obj.field === false).length;
return doc;
}
}).fetch();
With this command the result will be like:
[
// ...
{
_id: "123",
arrayName: [
{
text: 'first',
field: false
}, {
text: 'second',
field: true
}, {
text: 'third',
field: false
}
],
filterArrayLength: 2
}
]
Consider the following posts collection:
{
_id: 1,
title: "Title1",
category: "Category1",
comments: [
{
title: "CommentTitle1",
likes: 3
},
{
title: "CommentTitle2",
likes: 4
}
]
}
{
_id: 2,
title: "Title2",
category: "Category2",
comments: [
{
title: "CommentTitle3",
likes: 1
},
{
title: "CommentTitle4",
likes: 4
}
]
}
{
_id: 3,
title: "Title3",
category: "Category2",
comments: [
{
title: "CommentTitle5",
likes: 1
},
{
title: "CommentTitle6",
likes: 3
}
]
}
I want to retrieve all the posts, and if one post has a comment with 4 likes I want to retrieve this comment only under the "comments" array. If I do this:
db.posts.find({}, {comments: { $elemMatch: {likes: 4}}})
...I get this (which is exactly what I want):
{
_id: 1,
comments: [
{
title: "CommentTitle2",
likes: 4
}
]
}
{
_id: 2,
comments: [
{
title: "CommentTitle4",
likes: 4
}
]
}
{
_id: 3
}
But how can I retrieve the remaining fields of the documents without having to declare each of them like below? This way if added more fields to the post document, I wouldn't have to change the find query
db.posts.find({}, {title: 1, category: 1, comments: { $elemMatch: {likes: 4}}})
Thanks
--EDIT--
Sorry for the misread of your question. I think you'll find my response to this question here to be what you are looking for. As people have commented, you cannot project this way in a find, but you can use aggregation to do so:
https://stackoverflow.com/a/21687032/2313887
The rest of the answer stands as useful. So I think I'll leave it here
You must specify all of the fields you want or nothing at all when using projection.
You are asking here essentially that once you choose to alter the output of the document and limit how one field is displayed then can I avoid specifying the behavior. The bottom line is thinking of the projection part of a query argument to find just like SQL SELECT.It behaves in that * or all is the default and after that is a list of fields and maybe some manipulation of the fields format. The only difference is for _id which is always there by default unless specified otherwise by excluding it, i.e { _id: 0 }
Alternately if you want to filter the collection you nee to place your $elemMatch in thequery itself. The usage here in projection is to explicitly limit the returned document to only contain the matching elements in the array.
Alter your query:
db.posts.find(
{ comments: { $elemMatch: {likes: 4}}},
{ title: 1, category: 1, "comments.likes.$": 1 }
)
And to get what you want we use the positional $ operator in the projection portion of the find.
See the documentation for the difference between the two usages:
http://docs.mongodb.org/manual/reference/operator/query/elemMatch/
http://docs.mongodb.org/manual/reference/operator/projection/elemMatch/
This question is pretty old, but I just faced the same issue and I didn't want to use the aggregation pipeline as it was simple query and I only needed to get all fields applying an $elemMatch to one field.
I'm using Mongoose (which was not the original question but it's very frequent these days), and to get exactly what the question said (How can I retrieve all the fields when using $elemMatch?) I made this:
const projection = {};
Object.keys(Model.schema.paths).forEach(key => {
projection[key] = 1;
});
projection.subfield = { $elemMatch: { _id: subfieldId } };
Model.find({}, projection).then((result) => console.log({ result });