Consider my database collection is having 5 documents and I need to delete the last 3 documents. So what is the query in Mongodb
Maybe something like this:
db.collection.aggregate([
{
$sort: {
_id: 1
}
},
{
$limit: 2
},
{
$out: "collection"
}
])
Explained:
Sort collection by inserted order
FIlter only the first 2 documents.
Store back the content to the original collection replacing the original.
Playground
Related
i have this collection on MongoDB
Collection structure
The main document has like main Field wineName. Now in this image the array of nested document (reviews) is only one but could be several reviews for each wines.
My question is how can I count the number of reviews that I have for each Wine?
Thank you in advance for the answer
You can use the $size aggregation operator for this:
db.collection.aggregate([
{
"$addFields": {
"reviewCount": {
$size: "$reviews"
}
}
}
])
Mongo Playground
I have a short but important question. I am new to MongoDB and querying.
My database looks like the following: I only have one document stored in my database (sorry for blurring).
The document consists of different fields:
two are blurred and not important
datum -> date
instance -> Array with an Embedded Document Object; Our instance has an id, two not important fields and a code.
Now I want to query how many times an object in my instance array has the group "a" and a text "sample"?
Is this even possible?
I only found methods to count how many documents have something...
I am using Mongo Compass, but i can also use Pymongo, Mongoengine or every other different tool for querying the mongodb.
Thank you in advance and if you have more questions please leave a comment!
You can try this
db.collection.aggregate([
{
$unwind: "$instance"
},
{
$unwind: "$instance.label"
},
{
$match: {
"instance.label.group": "a",
"instance.label.text": "sample",
}
},
{
$group: {
_id: {
group: "$instance.label.group",
text: "$instance.label.text"
},
count: {
$sum: 1
}
}
}
])
I have a below aggregate query and I wanted to insert/update this result into another collection.
db.coll1.aggregate([
{
$group:
{
_id: "$collId",
name: {$first:"$name"} ,
type: {$first:"$Type"} ,
startDate: {$first:"$startDate"} ,
endDate: {$first:"$endDate"}
}
}
])
I have another collection coll2, which has fields in the above query and some additional fields too.
I may already have the document created in Coll2 matching the _id:collId in the above query. If this Id matches, I want to update the document with the field values of the result and keep the values of other fields in the document.
If the Id does not exists, it should just create a new document in Coll2.
Is there a way to do it in the MongoDB query. I want to implement this in my Spring application.
We can use $merge to do that. It's supported from Mongo v4.2
db.collection.aggregate([
{
$group:{
"_id":"$collId",
"name": {
$first:"$name"
},
"type":{
$first:"$Type"
},
"startDate":{
$first:"$startDate"
},
"endDate":{
$first:"$endDate"
}
}
},
{
$merge:{
"into":"collection2",
"on":"_id",
"whenMatched":"replace",
"whenNotMatched":"insert"
}
}
])
Is there any possible way to combine two documents in mongodb?
Imagine I have a collection with this documents:
{ "_id":ObjectId("142342432"), "name":"Chris", "surname":"Patrick", "pets":["dog", "cat"] }
{ "_id":ObjectId("142342752"), "name":"Chris", "surname":"Patrick", "pets":["lizard"] }
more than x2000 documents
And some other documentes. My idea would be to group each of the entries by name and surname and in case they match, join the pets array $addToSet.
The idea is to delete those two documents and add a new one with a new generated id and the combination. Deleting one document and appending the array to the other one, would also be ok.
My main problem is that I should be updating the collection, so it wouldn't be just a dump. And it can't be inefficient to dump it into a file and import it later on.
Thanks!
Update: Using mongodb 3.4
you can achieve this in a single query :
first, unwind pets array
then, group by name and surname, and combine arrays with $addToSet
finally, write the output to the same collection using $out
Be carefull, this will overwrite the original collection!
before running this, you should create a dump of your current collection to avoid any data loss
here is the query:
db.collectionName.aggregate([
{ $unwind: {
path: "$pets",
preserveNullAndEmptyArrays: true
}
},
{
$group:{
_id:{
name:"$name",
surname:"$surname"
},
pets:{
$addToSet:"$pets"
},
otherField: {
$first: "$otherField"
}
}
},
{
$out:"collectionName"
}
])
I have a document that I want to split up into multiple documents.
The document looks like this.
{
a:1,
b:2,
c:3,
d:4,
}
{
a:5,
b:6
c:7
d:8
}
I want to pull out "d" into separate documents. I know that $unwind would unwind an array, but how would I unwind a situation like this?
Document 1
{
d:4
}
Document 2
{
d:8
}