MongoDb $lookup query with multiple fields from objects array - mongodb

Question

This question has previously been marked as a duplicate of this question I can with certainty confirm that it is not.
This is not a duplicate of the linked question because the elements in question are not an array but embedded in individual objects of an array as fields. I am fully aware of how the query in the linked question should work, however that scenario is different from mine.
I have a question regarding the $lookup query of MongoDb. My data structure looks as follows:
My "Event" collection contains this single document:
{
"_id": ObjectId("mongodbobjectid..."),
"name": "Some Event",
"attendees": [
{
"type": 1,
"status": 2,
"contact": ObjectId("mongodbobjectidHEX1")
},
{
"type": 7,
"status": 4,
"contact": ObjectId("mongodbobjectidHEX2")
}
]
}
My "Contact" collection contains these documents:
{
"_id": ObjectId("mongodbobjectidHEX1"),
"name": "John Doe",
"age": 35
},
{
"_id": ObjectId("mongodbobjectidHEX2"),
"name": "Peter Pan",
"age": 60
}
What I want to do is perform an aggregate query with the $lookup operator on the "Event" collection and get the following result with full "contact" data:
{
"_id": ObjectId("mongodbobjectid..."),
"name": "Some Event",
"attendees": [
{
"type": 1,
"status": 2,
"contact": {
"_id": ObjectId("mongodbobjectidHEX1"),
"name": "John Doe",
"age": 35
}
},
{
"type": 7,
"status": 4,
"contact": {
"_id": ObjectId("mongodbobjectidHEX2"),
"name": "Peter Pan",
"age": 60
}
}
]
}
I have done the same with single elements of "Contact" referenced in another document but never when embedded in an array. I am unsure of which pipeline arguments to pass to get the above shown result?
I also want to add a $match query to the pipeline to filter the data, but that is not really part of my question.

Answer 1

Try this one
db.getCollection('Event').aggregate([{ "$unwind": "$attendees" },
{ "$lookup" : { "from" : "Contact", "localField" : "attendees.contact", "foreignField": "_id", "as" : "contactlist" } },
{ "$unwind": "$contactlist" },
{ "$project" :{
"attendees.type" : 1,
"attendees.status" : 1,
"attendees.contact" : "$contactlist",
"name": 1, "_id": 1
}
},
{
"$group" : {
_id : "$_id" ,
"name" : { $first : "$name" },
"attendees" : { $push : "$attendees" }
}
}
])