MongoDB $lookup with <collection to join> coming from the input document - mongodb

Question

New to mongodb, so maybe this is a dumb question. I am using MongoDB lookup aggregation, but the 'from' collection is a field in the input document. How do I indicate that field in the 'from' rather than a string literal?
A simplified version of the collection I am starting with ("Groups") has documents that look like this:
{
_id: "<ObjectId>",
collectionName: "MyCollectionA",
list: ["<Foreign ObjectId>", "<Foreign ObjectId>", "<Foreign ObjectId>"]
}
I am joining to another collection. In this case, "MyCollectionA".
My lookup is working and looks like this:
{
$lookup: {
from: "MyCollectionA",
localField: "list",
foreignField: "_id",
as: "myJoinedItems"
}
}
However, I want to be able to use the field 'collectionName' rather than hardcoding 'MyCollectionA' in the lookup. How can I do that? I've tried '$collectionName' and { $literal: '$collectionName }, but no luck.

Answer 1

The following query can do the trick. We are iterating over each record of Groups collection and performing the find operation on the collection specified in each document for the list of object IDs. Since there is a default index on _id, the search operation would be fast.
db.Groups.find().forEach(doc=>{
var myJoinedItems = [];
doc["list"].forEach(id=>{
myJoinedItems.push(
db.getCollection(doc["collectionName"]).find({"_id":id})[0]
);
});
doc["myJoinedItems"]=myJoinedItems;
print(tojson(doc));
});
Output:
{
"_id" : ObjectId("5d6cac366bc2ad3b23f7de74"),
"collectionName" : "MyCollectionA",
"list" : [
ObjectId("5d6cabd16bc2ad3b23f7de72")
],
"myJoinedItems" : [
{
"_id" : ObjectId("5d6cabd16bc2ad3b23f7de72"),
"collectionDetails" : {
"name" : "MyCollectionA",
"info" : "Cool"
}
}
]
}
{
"_id" : ObjectId("5d6cb82a6bc2ad3b23f7de76"),
"collectionName" : "MyCollectionB",
"list" : [
ObjectId("5d6cb7fd6bc2ad3b23f7de75")
],
"myJoinedItems" : [
{
"_id" : ObjectId("5d6cb7fd6bc2ad3b23f7de75"),
"collectionDetails" : {
"name" : "MyCollectionB",
"info" : "Super-Cool"
}
}
]
}
Data set:
Collection: Groups
{
"_id" : ObjectId("5d6cac366bc2ad3b23f7de74"),
"collectionName" : "MyCollectionA",
"list" : [
ObjectId("5d6cabd16bc2ad3b23f7de72")
]
}
{
"_id" : ObjectId("5d6cb82a6bc2ad3b23f7de76"),
"collectionName" : "MyCollectionB",
"list" : [
ObjectId("5d6cb7fd6bc2ad3b23f7de75")
]
}
Collection: MyCollectionA
{
"_id" : ObjectId("5d6cabd16bc2ad3b23f7de72"),
"collectionDetails":{
"name":"MyCollectionA",
"info":"Cool"
}
}
Collection: MyCollectionB
{
"_id" : ObjectId("5d6cb7fd6bc2ad3b23f7de75"),
"collectionDetails":{
"name":"MyCollectionB",
"info":"Super-Cool"
}
}

Answer 2

Here's a variation that uses $lookup directly.
db.foo.drop();
db.foo1.drop();
db.foo2.drop();
db.foo.insert(
[
{_id:0, collectionName:"foo1", list: ['A','B','C'] },
{_id:1, collectionName:"foo2", list: ['D','E'] }
]);
db.foo1.insert([
{_id:0, key:"A", foo1data:"goodbye"},
{_id:1, key:"B", foo1data:"goodbye"},
{_id:2, key:"C", foo1data:"goodbye"}
]);
db.foo2.insert([
{_id:0, key:"C", foo2data:"goodbye"},
{_id:1, key:"D", foo2data:"goodbye"}
]);
// Pass 1: Collect unique collections.
c = db.foo.distinct("collectionName");
// Pass 2: Get 'em:
c.forEach(function(collname) {
c2 = db.foo.aggregate([
{$match: {"collectionName": collname}}
,{$lookup: {"from": collname,
// Clever twist: if localField is a list, then the lookup
// behaves like an in-list:
localField: "list",
foreignField: "key",
as: "X" }}
]);
});