Inline/combine other collection into one collection - mongodb

Question

I want to combine two mongodb collections.
Basically I have a collection containing documents that reference one document from another collection. Now I want to have this as a inline / nested field instead of a separate document.
So just to provide an example:
Collection A:
[{
"_id":"90A26C2A-4976-4EDD-850D-2ED8BEA46F9E",
"someValue": "foo"
},
{
"_id":"5F0BB248-E628-4B8F-A2F6-FECD79B78354",
"someValue": "bar"
}]
Collection B:
[{
"_id":"169099A4-5EB9-4D55-8118-53D30B8A2E1A",
"collectionAID":"90A26C2A-4976-4EDD-850D-2ED8BEA46F9E",
"some":"foo",
"andOther":"stuff"
},
{
"_id":"83B14A8B-86A8-49FF-8394-0A7F9E709C13",
"collectionAID":"90A26C2A-4976-4EDD-850D-2ED8BEA46F9E",
"some":"bar",
"andOther":"random"
}]
This should result in Collection A looking like this:
[{
"_id":"90A26C2A-4976-4EDD-850D-2ED8BEA46F9E",
"someValue": "foo",
"collectionB":[{
"some":"foo",
"andOther":"stuff"
},{
"some":"bar",
"andOther":"random"
}]
},
{
"_id":"5F0BB248-E628-4B8F-A2F6-FECD79B78354",
"someValue": "bar"
}]

Answer 1

I'd suggest something simple like this from the console:
db.collB.find().forEach(function(doc) {
var aid = doc.collectionAID;
if (typeof aid === 'undefined') { return; } // nothing
delete doc["_id"]; // remove property
delete doc["collectionAID"]; // remove property
db.collA.update({_id: aid}, /* match the ID from B */
{ $push : { collectionB : doc }});
});
It loops through each document in collectionB and if there is a field collectionAID defined, it removes the unnecessary properties (_id and collectionAID). Finally, it updates a matching document in collectionA by using the $push operator to add the document from B to the field collectionB. If the field doesn't exist, it is automatically created as an array with the newly inserted document. If it does exist as an array, it will be appended. (If it exists, but isn't an array, it will fail). Because the update call isn't using upsert, if the _id in the collectionB document doesn't exist, nothing will happen.
You can extend it to delete other fields as necessary or possibly add more robust error handling if for example a document from B doesn't match anything in A.
Running the code above on your data produces this:
{ "_id" : "5F0BB248-E628-4B8F-A2F6-FECD79B78354", "someValue" : "bar" }
{ "_id" : "90A26C2A-4976-4EDD-850D-2ED8BEA46F9E",
"collectionB" : [
{
"some" : "foo",
"andOther" : "stuff"
},
{
"some" : "bar",
"andOther" : "random"
}
],
"someValue" : "foo"
}

Answer 2

Sadly mapreduce can't produce full documents.
https://jira.mongodb.org/browse/SERVER-2517
No idea why despite all the attention, whining and upvotes they haven't changed it. So you'll have to do this manually in the language of your choice.
Hopefully you've indexed 'collectionAID' which should improve the speed of your queries. Just write something that goes through your A collection one document at a time, loading the _id and then adding the array from Collection B.

Answer 3

There is a much faster way than https://stackoverflow.com/a/22676205/1578508
You can do it the other way round and run through the collection you want to insert your documents in. (Far less executions!)
db.collA.find().forEach(function (x) {
var collBs = db.collB.find({"collectionAID":x._id},{"_id":0,"collectionA":0});
x.collectionB = collBs.toArray();
db.collA.save(x);
})