MongoDB: Combine data from multiple collections into one..how?

Question

How can I (in MongoDB) combine data from multiple collections into one collection?

Can I use map-reduce and if so then how?

I would greatly appreciate some

Answer 1

Very basic example with $lookup.

db.getCollection('users').aggregate([
    {
        $lookup: {
            from: "userinfo",
            localField: "userId",
            foreignField: "userId",
            as: "userInfoData"
        }
    },
    {
        $lookup: {
            from: "userrole",
            localField: "userId",
            foreignField: "userId",
            as: "userRoleData"
        }
    },
    { $unwind: { path: "$userInfoData", preserveNullAndEmptyArrays: true }},
    { $unwind: { path: "$userRoleData", preserveNullAndEmptyArrays: true }}
])

Here is used

 { $unwind: { path: "$userInfoData", preserveNullAndEmptyArrays: true }}, 
 { $unwind: { path: "$userRoleData", preserveNullAndEmptyArrays: true }}

Instead of

{ $unwind:"$userRoleData"} 
{ $unwind:"$userRoleData"}

Because { $unwind:"$userRoleData"} this will return empty or 0 result if no matching record found with $lookup.