I have two collections, one that has _id and UserId, and another that has UserId (same unique identifier) and "other data".
I want to filter the latter collection based on a list of _ids from the former collection.
Can someone provide an example query for this scenario?
The only way to 'join' collections in MongoDB is a $lookup aggregation stage (available in version 3.2).
firstCollection.aggregate([
{ $match: { _id: {$in: [1,2,3] }}}, // filter by _ids
{
$lookup:
{
from: "secondCollection",
localField: "UserId",
foreignField: "UserId",
as: "data"
}
}
])
That will add 'data' field to the documents from the first collection which will contain all related documents from second collection. If relation is not 1:1, you can add $unwind stage to flatten results:
{$unwind: "$data"}
Related
Say I have a collection of elements with several fields, including userId and questionId. If I'm in an aggregation pipeline and I have a list of documents that all have userId and questionId as fields, but their values might already be in the collection (ie. a document of
{userId:1, questionId:1, score: 1}
but a similar document already exists in the collection
{userId:1, questionId:1, score:0}
How do I do a $merge into the collection while checking both fields? The $merge function does have an 'on: [field]' field to check overlap, but I don't think it can check two.
You can specify multiple fields in the on clause using an array.
db.toBe.aggregate([
{
"$merge": {
"into": "asIs",
"on": [
"userId",
"questionId"
],
"whenMatched": "merge"
}
}
])
Mongo Playground
I am having 2 collections -
Collection name is content
Collection name is surveys
I actually want to add the "type" information from the content collection to every content in every survey in the "surveys" collection.
I know in SQL we have to join these 2 tables first on _id and content_id commun columns and then add type column to suryeys table but not sure how should i do it here. I want the type value from content collection to add in every content field inside surveys collection. Please help me.
One option is:
Using $lookup to get the data from content collection
Merge the arrays using $mergeObjects and $indexOfArray. After the lookup, the content data is on someField array. This step finds the releant content from someField and put it inside the matching survey's content item, under the key type. This step also removes someField.
Format the result using $map. This step uses $map to iterate over the content array and format the data under type to contain only the wanted part.
Save it back using $merge. This step saves the result back into the collection.
db.surey.aggregate([
{$lookup: {
from: "content",
localField: "content.content_id",
foreignField: "_id",
as: "someField"
}},
{$set: {
someField: "$$REMOVE",
content: {$map: {
input: "$content",
in: {$mergeObjects: [
"$$this",
{type: {
$arrayElemAt: [
"$someField",
{$indexOfArray: ["$someField._id", "$$this.content_id"]}
]
}}
]}
}}
}},
{$set: {content: {$map: {
input: "$content",
in: {$mergeObjects: ["$$this", {type: "$$this.type.type"}]}
}}}},
{$merge: {into: "surey"}}
])
See how it works on the playground example
In mongoose (I assume you could be using mongoose, sind you added the tag) you can specify the relation in Schema definition and just just populate to add these fields to the output.
If you are not using mongoose you can use $lookup (https://www.mongodb.com/docs/manual/reference/operator/aggregation/lookup/) in your aggregation pipeline.
{
from: "content",
localField: "content._id",
foreignField: "_id",
as: "someField"
}
I am trying to delete orphaned documents in mongodb, which cross collections.
In the collection 'values', I have documents like this:
value:
{
resultId: <ObjectId>
...other data..
}
which reference documents in the collection 'results':
result:
{
_id: <ObjectId> //the resultId
}
A number of 'result' documents have been deleted, resulting in orphaned 'value' documents. How can I find all orphans and delete them?
What you will want to do is build up an aggregate pipeline and use the $lookup operator to fetch the corresponding result document. Then, add a $match operator to your aggregate pipeline to filter those that don't have corresponding result object.
db.values.aggregate([
{
$lookup: {
from: "results",
localField: "resultId",
foreignField: "id",
as: "resultDocument"
}
},
{ $match: { resultDocument: {$size:0} }}
])
This way, you have identified your orphaned documents and can delete them afterwards.
I have a collection (collectionA) that stores an event ID in an event array. The event array information comes from (collectionB).
Lately when an event is deleted from CollectionB via the web app, it sometimes does not get removed from Collection A as it should.
Is there a query i can do in mongo 3.0 to check to see what event_id's exist in CollectionA that are not in collectionB. Those will be the ones that need to be removed while the development team resolves the issue?
Here is a sample query that will give you list of such objects, assuming, collectionA has events array with IDs from collectionB
db.collectionA.aggregate([
{$unwind: '$events'},
{$lookup: {
from: 'collectionB',
localField: 'events',
foreignField: '_id',
as: 'event'
}},
{$unwind: {path: '$event', preserveNullAndEmptyArrays:true}},
{$match:{ 'event': {$exists:false}}},
])
I have a scenario in spring-mongo query. Mongo version is 3.2
Application have two collections (Collection A and Collection B).
**Sample contents**
Collection A :: {"_id":1, "name":"content 1"}...{"_id":100, "name":"content 100"}
Collection B :: {"_id":1, "name":"parent 1", "a":[1,2,58,67]}
{"_id":2, "name":"parent 2", "a":[2,85,96,99]}
Collection B holds reference ids of Collection A as a array.
Scenario:
I will past list of ids of Collection A to the query: I need to get list of ids of Collection A which are not associated anywhere in Collection B.
how to achieve this?
I am planning to proceed with Aggregation with following query.. Looking up with
preserveNullAndEmptyArrays saving my day.
db.a.aggregate([
{
$match: { "_id":{$in:["1","5","10"]} }
},
{
$lookup:
{
from: "b",
localField: "_id",
foreignField: "a",
as: "moneyid"
}
},
{
$unwind:
{"path":"$moneyid", "preserveNullAndEmptyArrays":true}
},
{
$match:{
"moneyid":{$eq:null}
}
}
])
It can be done using the following approach
1) Find Unique Id's in Collection B which are in array "a":[]
2) Execute a find query in Collection A - Use $nin in the find query and pass all the ids which you obtained from array "a":[] from Collection B
Note:If you need to find matching documents between two collections then you can use $lookup which works like a left outer join.