I'm stuck in something very stupid but I can't get out from my own.
MongoDB v4.2 and I have a collection with documents like this:
{"_id":{"A":"***","B":0}}, "some other fields"...
I'm working on top of mongo-c driver and I want to query only the "_id.B" field but I don't know how I can do it. I have tried:
"projection":{"_id.B":1}: It returns me the whole _id object. _id.A & _id.B.
"projection":{"_id.A":0,"All other fields except _id.B":0}: Returns same as above.
"projection":{"_id.A":0,"_id.B":1}: Returns nothing.
How I can do it to get only some object elements when this object is inside the _id field? The first option works for me with objects that aren't inside the _id field but not here.
Best regards, thanks for your time.
Héctor
You can use MongoDB's $project in aggregation to do this. You can also use $addFields to get _id.B into new field + all other fields in document & finally project _id :0.
Code:
var coll = localDb.GetCollection("yourCollectionName");
var project = new BsonDocument
{
{
"$project",
new BsonDocument
{
{ "_id.B": 1 }
}
}
}
var pipeline = new[] { project };
var result = coll.Aggregate(pipeline);
Test : MongoDB-Playground
Related
I wish to return just the document id's from mongo that match a find() query.
I know I can pass an object to exclude or include in the result set, however I cannot find a way to just return the _id field.
My thought process is returning just this bit of information is going to be way more efficient (my use case requires no other document data just the ObjectId).
An example query that I expected to work was:
collection.find({}, { _id: 1 }).toArray(function(err, docs) {
...
}
However this returns the entire document and not just the _id field.
You just need to use a projection to find what ya want.
collection.find({filter criteria here}, {foo: 0, bar: 0, _id: 1});
Since I don't know what your document collection looks like this is all I can do for you. foo: 0 for example is exclude this property.
I found that using the cursor object directly I can specify the required projection. The mongodb package on npm when calling toArray() is returning the entire document regardless of the projection specified in the initial find(). Fixed working example below that satisfies my requirements of just getting the _id field.
Example document:
{
_id: new ObjectId(...),
test1: "hello",
test2: "world!"
}
Working Projection
var cursor = collection.find({});
cursor.project({
test1: 0,
test2: 0
});
cursor.toArray(function(err, docs) {
// Importantly the docs objects here only
// have the field _id
});
Because _id is by definition unique, you can use distinct to get an array of the _id values of all documents as:
collection.distinct('_id', function(err, ids) {
...
}
you can do like this
collection.find({},'_id').toArray(function(err, docs) {
...
}
I got a mongo db collection with structure
randomstring - means the string is actually random its diffrent in each document of the collection.
{
"notrandom":{
"randomstring":{
"randomstring":{
"randomstring":{
"notrandom2":"data"
}
}
}
}
}
how can i project this data out?
something like
db.mydb.aggregate( "notrandom[0][0].notrandom2":1}} , ] )
what i'm trying to achieve is a collection of all the notrandom2 values.
if you you to move notrandom2 to higer level in document structure,
you could use $project stage like this:
{
$project:{
_id:1,
notrandom2:"notrandom.randomstring.randomstring.randomstring.notrandom2",
// list all others fields with field:1 if you want them to appear down in pieline
}}
if this field is a part of an array then you need to $unwind first and then $project
You can use the following query
db.mydb.find({<findquery (in you have any)>},{"notrandom.randomstring.randomstring.randomstring.notrandom2" : 1}).
toArray(function(err, result)
{
console.log(result); //Array of objects with `notrandom2` values
})
I have a collection where the _id is of the form [message_code]-[language_code] and another where the _id is just [message_code]. What I'd like to do is find all documents from the first collection where the message_code portion of the _id does not appear in the second collection.
Example:
> db.colA.find({})
{ "_id" : "TRM1-EN" }
{ "_id" : "TRM1-ES" }
{ "_id" : "TRM2-EN" }
{ "_id" : "TRM2-ES" }
> db.colB.find({})
{ "_id" : "TRM1" }
I want a query that will return TRM2-EN and TRM-ES from colA. Of course in my live data, there are thousands of records in each collection.
According to this question which is trying to do something similar, we have to save the results from a query against colB and use it in an $in condition in a query against colA. In my case, I need to strip the -[language_code] portion before doing this comparison, but I can't find a way to do so.
If all else fails, I'll just create a new field in colA that contains only the message code, but is there a better way do it?
Edit:
Based on Michael's answer, I was able to come up with this solution:
var arr = db.colB.distinct("_id")
var regexs = arr.map(function(elm){
return new RegExp(elm);
})
var result = db.colA.find({_id : {$nin : regexs}}, {_id : true})
Edit:
Upon closer inspection, the above method doesn't work after all. In the end, I just had to add the new field.
Disclaimer: This is a little hack it may not end well.
Get distinct _id using collection.distinct method.
Build a regular expression array using Array.prototype.map()
var arr = db.colB.distinct('_id');
arr.map(function(elm, inx, tab) {
tab[inx] = new RegExp(elm);
});
db.colA.find({ '_id': { '$nin': arr }})
I'd add a new field to colA since you can index it and if you have hundreds of thousands of documents in each collection splitting the strings will be painfully slow.
But if you don't want to do that you could make use of the aggregation framework's $substr operator to extract the [message-code] then do a $match on the result.
I have recently changed one of my fields from object to array of objects.
In my production I have only 14 documents with this field, so I decided to change those fields.
Is there any best practices to do that?
As it is in my production I need to do it in a best way possible?
I got the document Id's of those collections.like ['xxx','yyy','zzz',...........]
my doc structure is like
_id:"xxx",option1:{"op1":"value1","op2":"value2"},option2:"some value"
and I want to change it like(converting object to array of objects)
_id:"xxx",option1:[{"op1":"value1","op2":"value2"},
{"op1":"value1","op2":"value2"}
],option2:"some value"
Can I use upsert? If so How to do it?
Since you need to create the new value of the field based on the old value, you should retrieve each document with a query like
db.collection.find({ "_id" : { "in" : [<array of _id's>] } })
then iterate over the results and $set the value of the field to its new value:
db.collection.find({ "_id" : { "in" : [<array of _id's>] } }).forEach(function(doc) {
oldVal = doc.option1
newVal = compute_newVal_from_oldVal(oldVal)
db.collection.update({ "_id" : doc._id }, { "$set" : { "option" : newVal } })
})
The document structure is rather schematic, so I omitted putting in actual code to create newVal from oldVal.
Since it is an embedded document type you could use push query
db.collectionname.update({_id:"xxx"},{$push:{option1:{"op1":"value1","op2":"value2"}}})
This will create document inside embedded document.Hope it helps
So I have a document that has references to foreign ObjectIDs that may point to other documents or collections.
For example this is the pseudo-structure of the document
{
_id: ObjectID(xxxxxxxx),
....
reference: ObjectID(yyyyyyyy)
}
I can't find anything that does not involve providing the collection and given that I don't know for sure on which collection to search, I am wondering if there is a way for me to find the document in the entire database and find the collection ObjectID(yyyyyyyy) belongs to.
The only possible way to do this is by listing every collection in the database and performing a db.collection.find() on each one.
E.g. in the Mongo shell I would do something like
var result = new Array();
var collections = db.getCollectionNames();
for (var i = 0; i < collections.length; i++) {
var found = db.getCollection(collections[i]).findOne({ "_id" : ObjectId("yyyyyyyy") });
if (found) {
result.push(found);
}
}
print(result);
You need to run your query on all collections in your database.
db.getCollectionNames().forEach(function(collection){
db[collection].find({ $or : [
{ _id : ObjectId("535372b537e6210c53005ee5") },
{ reference : ObjectId("535372b537e6210c53005ee5")}]
}).forEach(printjson);
});