When trying to do a $nin match with $elemMatch in .net I get no results. I've reconstructed the statement on Mongo Playground.
I guess I'm missing something around the $nin but can't find anything in the docs.
This post is talking about the issue but the solution they gave doesn't work if you set the data up with a valid exclude.
I've read through the docs and a few posts around this issue, I'm assuming there is a gap in my knowledge around how $nin and or $elemMatch works, I hope someone can point me in the correct direction and possible fix the example on mongo playground.
You are mistaking the behaviour of $elemMatch. $elemMatch will return the document if any of the array element match your criteria.
For your expected behaviour, you can simply do:
db.collection.aggregate([
{
$match: {
"ingredient.name": {
$nin: [
"X"
]
}
}
}
])
Mongo Playground
Related
Seems to be a simple question, but I can't find the answer. There is a collection of posts, documents of which have a field rubric. How to read ALL documents from this collection in mongoose using Post.find ({rubric: value}, function), but keeping rubric. What value has to be for this to work?
To find according if a field exists whatever value it has, you can use $exists in this way:
db.collection.find({
"rubric": {
"$exists": true
}
})
Using mongoose is the same query. Something like this:
yourModel.find({
"rubric": {
"$exists": true
}
})
Example here.
Also, to know about these world in mongo world you can check documentation every time you have an issue.
At the left of the page you see "Reference > Operators" and there are a lof of different operators you can use.
Does document db support NOT IN operator. I can see in this document https://docs.aws.amazon.com/documentdb/latest/developerguide/querying.html other examples but not for this. Need help with syntax to have something like
db.example.find( NOT IN { "Item": ["item1","item2"]} AND {"Code":"code1"} ).pretty()
DocumentDB supports $nin operator and you can use this operator to select documents where the field is not in a specified array. Please find below the updated query.
db.example.find({ $and: [{Item: {$nin:["item1","item2"]}} , {Code:"code1"}] })
I am trying a fairly complex aggregate command on two collections involving $lookup pipeline. This normally works just fine on simple aggregation as long as index is set on foreignField.
But my $lookup is more complex as the indexed field is not just a normal Int64 field but actually an array of Int64. When doing a simple find(), it is easy to verify using explain() that the index is being used. But explaining the aggregate pipeline does not explain whether index is being used in the $lookup pipeline. All my timing tests seem to indicate that the index is not being used. MongoDB version is 3.6.2. Db compatibility is set to 3.6.
As I said earlier, I am not using simple foreignField lookup but the 3.6-specific pipeline + $match + $expr...
Could using pipeline be showstopper for the index? Does anyone have any deep experience with the new $lookup pipeline syntax and/or the index on an array field?
Examples
Either of the following works fine and if explained, shows that index on followers is being used.
db.col1.find({followers: {$eq : 823778}})
db.col1.find({followers: {$in : [823778]}})
But the following one does not seem to make use of the index on followers [there are more steps in the pipeline, stripped for readability].
db.col2.aggregate([
{$match:{field: "123"}},
{$lookup:{
from: "col1",
let : {follower : "$follower"},
pipeline: [{
$match: {
$expr: {
$or: [
{ $eq : ["$follower", "$$follower"] },
{ $in : ["$$follower", "$followers"]}
]
}
}
}],
as: "followers_all"
}
}])
This is a missing feature which is going to part of 3.8 version.
Currently eq matches in lookup sub pipeline are optimised to use indexes.
Refer jira fixed in 3.7.1 ( dev version).
Also, this may be relevant as well for non-multi key indexes.
I am trying to search a list of values on a matching field in documents which is array of documents. Using $in makes it OR between the values I supply. Using $all seems to be more logical.
For eg:
Collection: Phrases
sample doc:
{
"locales": [
{
"name": "BPT",
"internal_desc": "Entre 2 e 3 horas"
},
{
"name": "JPN",
"internal_desc": "2 ～ 3 時間"
}
]
}
Query:
db.phrases.find({"locales.name":{"$all":["BPT", "JPN"]}})
But some posts suggesting $all is bad in terms of performance. Is there any other way to achieve this?
Using $and instead of $all will result in equivalent performance. The bottom line is that given what you are trying to accomplish using $all is your best bet (as far as my understanding goes). However, $all can be optimized by making the first element in the expression more selective. For example if you know that "BPT" shows up in 2% of documents and "JPN" shows up in 20% of documents then it makes sense to list "BPT" as the first element in the $all expression. This way mongo only needs to filter through fewer documents on each consecutive element in your $all expression. Im sure you've seen the documentation but here is a link nonetheless: $all - mongodb
You can use the $and syntax, as shown in the query below;
db.phrases.find({$and : [
{"locales.name" : "BPT"},
{"locales.name" : "JPN"}
]
});
You can get information about your query, to see what the db is doing when executing the query by using the explain command, as displayed below;
db.phrases.explain().find({$and : [
{"locales.name" : "BPT"},
{"locales.name" : "JPN"}
]
});
Although, the explain command is more relevant to dbs where indexes are used, since it sort of gives you information about, which index was utilised by the db on the search.
Have a quick look into MongoDB indexes and explain() for further information.
I hope this helps.
Regards,
Nick.
Imagine a collection of movies (stored in a MongoDB collection), with each one looking something like this:
{
_id: 123456,
name: 'Blade Runner',
buyers: [1123, 1237, 1093, 2910]
}
I want to get a list of movies, each one with an indication whether buyer 2910 (for example) bought it.
Any ideas?
I know I can change [1123, 1237, 1093, 2910] to [{id:1123}, {id:1237}, {id:1093}, {id:2910}] to allow the use of $elemMatch in the projection, but would prefer not to touch the structure.
I also know I can perhaps use the $unwind operator (within the aggregation framework), but that seems very wasteful in cases where buyer has thousands of values (basically exploding each document into thousands of copies in memory before matching).
Any other ideas? Am I missing something really simple here?
You can use the $setIsSubset aggregation operator to do this:
var buyer = 2910;
db.movies.aggregate(
{$project: {
name: 1,
buyers: 1,
boughtIt: {$setIsSubset: [[buyer], '$buyers']}
}}
)
That will give you all movie docs with a boughtIt field added that indicates whether buyer is contained in the the movie's buyers array.
This operator was added in MongoDB 2.6.
Not really sure of your intent here, but you don't need to change the structure just to use $elemMatch in projection. You can just issue like this:
db.movies.find({},{ "buyers": { "$elemMatch": { "$eq": 2910 } } })
That would filter the returned array elements to just the "buyer" that was indicated, or nothing where this was not present. It is true to point out that the $eq operator used here is not actually documented, but it does exist. So that may not be immediately clear that you can construct a condition in that way.
It seems a little wasteful to me though as you are returning "everything" regardless of whether the "buyer" is present or not. So a "query" seems more logical than a projection:
db.movies.find({ "buyers": 2910 })
And optionally either just keeping only that matched result:
db.movies.find({ "buyers": 2910 },{ "buyers.$": 1})
Set operators in the aggregation framework give you more options with $project which can do more to alter the document. But if you just want to know if someone "bought" the item, then a "query" seems the be logical and fastest way to do so.