Aggregate and reduce a nested array based upon an ObjectId - mongodb

Question

I have an Event document structured like so and I'm trying to query against the employeeResponses array to gather all responses (which may or may not exist) for a single employee:
[
{
...
eventDate: 2019-10-08T03:30:15.000+00:00,
employeeResponses: [
{
_id:"5d978d372f263f41cc624727",
response: "Available to work.",
notes: ""
},
...etc
];
}
];
My current mongoose aggregation is:
const eventResponses = await Event.aggregate([
{
// find all events for a selected month
$match: {
eventDate: {
$gte: startOfMonth,
$lte: endOfMonth,
},
},
},
{
// unwind the employeeResponses array
$unwind: {
path: "$employeeResponses",
preserveNullAndEmptyArrays: true,
},
},
{
$group: {
_id: null,
responses: {
$push: {
// if a response id matches the employee's id, then
// include their response; otherwise, it's a "No response."
$cond: [
{ $eq: ["$employeeResponses._id", existingMember._id] },
"$employeeResponses.response",
"No response.",
],
},
},
},
},
{ $project: { _id: 0, responses: 1 } },
]);
As you'll no doubt notice, the query above won't work after more than 1 employee records a response because it treats each individual response as a T/F condition, instead of all of the responses within the employeeResponses array as a single T/F condition.
As a result, I had remove all subsequent queries after the initial $match and do a manual reduce:
const responses = eventResponses.reduce((acc, { employeeResponses }) => {
const foundResponse = employeeResponses.find(response => response._id.equals(existingMember._id));
return [...acc, foundResponse ? foundResponse.response : "No response."];
}, []);
I was wondering if it's possible to achieve the same reduce result above, but perhaps using mongo's $reduce function? Or refactor the aggregation query above to treat all responses within the employeeResponses as a single T/F condition?
The ultimate goal of this aggregation is extract any previously recorded employee's responses and/or lack of a response from each found Event within a current month and place their responses into a single array:
["I want to work.", "Available to work.", "Not available to work.", "No response.", "No response." ...etc]

Answer 1

You can use $filter with $map to reshape your data and filter by _id. Then you can keep using $push with $ifNull to provide default value if an array is empty:
db.collection.aggregate([
{
$addFields: {
employeeResponses: {
$map: {
input: {
$filter: {
input: "$employeeResponses",
cond: {
$eq: [ "$$this._id", "5d978d372f263f41cc624727"]
}
}
},
in: "$$this.response"
}
}
}
},
{
$group: {
_id: null,
responses: { $push: { $ifNull: [ { $arrayElemAt: [ "$employeeResponses", 0 ] }, "No response" ] } }
}
}
])
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