Aggregate by array field with object ids - mongodb

Question

Is there a better way of retrieving values from a collection2 based on an array field with object ids in collection1? I've tried to use $project but failed to get all required fields
Collections to aggregate:
collection1:
{
"_id" : ObjectId("5a58910de202796cfef41c6a"),
"sortOrder" : 5,
"title" : "Question 1 ?",
"freeTextIncluded" : false,
"freeText" : false,
"resultChart" : "pieChart",
"answer" : [
ObjectId("5a579fefd5554706b446cc71"),
ObjectId("5a587f17e4b2de0d683f96a4"),
ObjectId("5a587f20e4b2de0d683f96a5"),
ObjectId("5a587f29e4b2de0d683f96a6")
],
"state" : "active",
"__v" : 1,
"description" : ""
}
collection2:
{
"_id" : ObjectId("5a579fefd5554706b446cc71"),
"slug" : "answer-1",
"title" : "Answer 1",
"state" : "active",
"__v" : 0,
"author" : ObjectId("5a2e6b56e593c8525ced34b8"),
"body" : "<p>Lipsum...</p>"
}
{
"_id" : ObjectId("5a587f17e4b2de0d683f96a4"),
"slug" : "answer-2",
"title" : "Answer 2",
"state" : "active",
"__v" : 0,
"body" : ""
}
{
"_id" : ObjectId("5a587f20e4b2de0d683f96a5"),
"slug" : "answer-3",
"title" : "Answer 3",
"state" : "active",
"__v" : 0,
"body" : "",
"isCorrect" : true,
"sortOrder" : 3
}
{
"_id" : ObjectId("5a587f29e4b2de0d683f96a6"),
"slug" : "answer-4",
"title" : "Answer 4",
"state" : "active",
"__v" : 0,
"body" : ""
}
This aggregation works ok but I'm just wondering if there's a better/shorter way of aggregating 2 collections...
db.getCollection('questions').aggregate([
{
$match: {'_id': ObjectId('5a58910de202796cfef41c6a') }
},
{
$unwind: "$answer"
},
{
$lookup:
{
from: "answers",
localField: "answer",
foreignField: "_id",
as: "answers"
}
},
{
$match: { "answers": { $ne: [] }}
},
{
$unwind: "$answers"
},
{
$group: {
_id : ObjectId('5a58910de202796cfef41c6a'),
answerList: {$push: "$answers"},
title: {$first: "$title"},
sortOrder: {$first: "$sortOrder"},
description: {$first: "$description"},
resultChart: {$first: "$resultChart"},
freeTextIncluded: {$first: "$freeTextIncluded"},
}
}
]);

Answer 1

You need to improve your query like this:
db.getCollection('test').aggregate([{
$match: {
'_id': ObjectId('5a58910de202796cfef41c6a')
}
},
{
$lookup: {
from: "answers",
localField: "answer",
foreignField: "_id",
as: "answers"
}
},
{
$unwind: {
path: "$answers",
preserveNullAndEmptyArrays: true
}
},
{
$group: {
_id: ObjectId('5a58910de202796cfef41c6a'),
answerList: {
$push: "$answers"
},
title: {
$first: "$title"
},
sortOrder: {
$first: "$sortOrder"
},
description: {
$first: "$description"
},
resultChart: {
$first: "$resultChart"
},
freeTextIncluded: {
$first: "$freeTextIncluded"
},
}
}])