MongoDB - simple sub query example - mongodb

Question

Given the data:
> db.parameters.find({})
{ "_id" : ObjectId("56cac0cd0b5a1ffab1bd6c12"), "name" : "Speed", "groups" : [ "
123", "234" ] }
> db.groups.find({})
{ "_id" : "123", "name" : "Group01" }
{ "_id" : "234", "name" : "Group02" }
{ "_id" : "567", "name" : "Group03" }
I would like to supply a parameter _id an make a query return all groups that are within the groups array of the given document in parameters table.
The straightforward solution seems to make several DB calls in PyMongo:
Get parameter from parameters table based on the supplied _id
For each element of groups array select a document from groups collection
But this will have so much unnecessary overhead. I feel there must be a better, faster way to do this within MongoDB (without running custom JS in the DB). Or should I re-structure my data by normalising it a little bit (like a table of relationships), neglecting the document-based approach?
Again, please help me find a solution that would work from PyMongo DB interface

Answer 1

You can do this within a single query using the aggregation framework. In particular you'd need to run an aggregation pipeline that uses the $lookup operator to do a left join from the parameters collection to the groups collection.
Consider running the following pipeline:
db.parameters.aggregate([
{ "$unwind": "$groups" },
{
"$lookup": {
"from": "groups",
"localField": "groups",
"foreignField": "_id",
"as": "grp"
}
},
{ "$unwind": "$grp" }
])
Sample Output
/* 1 */
{
"_id" : ObjectId("56cac0cd0b5a1ffab1bd6c12"),
"name" : "Speed",
"groups" : "123",
"grp" : {
"_id" : "123",
"name" : "Group01"
}
}
/* 2 */
{
"_id" : ObjectId("56cac0cd0b5a1ffab1bd6c12"),
"name" : "Speed",
"groups" : "234",
"grp" : {
"_id" : "234",
"name" : "Group02"
}
}
If your MongoDB server version does not support the $lookup pipeline operator, then you'd need execute two queries as follows:
# get the group ids
ids = db.parameters.find_one({ "_id": ObjectId("56cac0cd0b5a1ffab1bd6c12") })["groups"]
# query the groups collection with the ids from previous query
db.groups.find({ "_id": { "$in": ids } })
EDIT: matched the field name in the aggregation query to the field name in example dataset (within the question)