Mongoose Aggregation match an array of objectIds - node.js

Question

I have a schema that Looks like this
var Post = new mongoose.Schema({
author: {
type: mongoose.Schema.Types.ObjectId,
ref: 'User'
},
created: {
type: Date,
Default: Date.now
})
I have a User Table as well. I Have a array of user ids and i am trying to search the post table based on an array of user ids
For Example
var userIds = ["575e96652473d2ab0ac51c1e","575e96652473d2ab0ac51c1d"] .... and so on
I want to return all posts created by these users. And posts should be sorted by their creation date. Is there a way to group this post based on the user ids provided, basically match the posts for an individual user?
The result I am trying to attain is something like this:
[{
userAId : "56656.....",
post : [postA, postB],
},{
userBId :"12345...",
post : [postA, postB]
}]
How do I write this query?
This is what I have so far
Post.aggregate([{
// {"$unwind" : ""},
// "$group": {
// _id: "$author",
// "created" : {"$sum" : 1 }
// }
"$match" : { author : id}
}]).exec(function(error, data) {
if(error){
return console.log(error);
}else{
return console.log(data)
}
})
{
"_id" : ObjectId("575e95bc2473d2ab0ac51c1b"),
"lastMod" : ISODate("2016-06-13T11:15:08.950Z"),
"author" : ObjectId("575dac62ec13010678fe41cd"),
"created" : ISODate("2016-06-13T11:15:08.947Z"),
"type" : "photo",
"end" : null,
"commentCount" : 0,
"viewCount" : 0,
"likes" : 0,
"tags" : [],
"title" : "Today is a good day",
"__v" : 0
}

Answer 1

To return all posts created by users depicted in a list of ids, use the $in operator in your query and then chain the sort() method to the query to order the results by the created date field:
Post.find({ "author": { "$in": userIds } })
.sort("-created") // or .sort({ field: 'asc', created: -1 });
.exec(function (err, data){
if(err){
return console.log(err);
} else {
return console.log(data);
}
});
To get a result where you have the post id's grouped per user, you need to run the following aggregation operation:
Post.aggregate([
{ "$match" : { "author": { "$in": userIds } } },
{ "$sort": { "created": -1 } },
{
"$group" : {
"_id" : "$author",
"posts" : { "$push": "$_id" }
}
},
{
"$project": {
"_id": 0,
"userId": "$_id",
"posts": 1
}
}
]).exec(function (err, result){
if(err){
return console.log(err);
} else {
return console.log(result);
}
});
Or with the fluent API:
Post.aggregate()
.match({ "author": { "$in": userIds } })
.sort("-created")
.group({
"_id" : "$author",
"posts" : { "$push": "$_id" }
})
.project({
"_id" : 0,
"userId" : "$_id",
"posts": 1
})
.exec(function (err, result){
if(err){
return console.log(err);
} else {
return console.log(result);
}
});

Answer 2

This should be possible without aggregation.
Post
.find({ author: { $in: userIds } })
.sort({ created: -1 })
If you get CastError: Cast to ObjectId failed, make sure to map your userIds array from an array of strings to an array of mongoose id's.
userIds = userIds.map(userId => new mongoose.Types.ObjectId(userId))