How can I retrieve all the fields when using $elemMatch? - mongodb

Question

Consider the following posts collection:
{
_id: 1,
title: "Title1",
category: "Category1",
comments: [
{
title: "CommentTitle1",
likes: 3
},
{
title: "CommentTitle2",
likes: 4
}
]
}
{
_id: 2,
title: "Title2",
category: "Category2",
comments: [
{
title: "CommentTitle3",
likes: 1
},
{
title: "CommentTitle4",
likes: 4
}
]
}
{
_id: 3,
title: "Title3",
category: "Category2",
comments: [
{
title: "CommentTitle5",
likes: 1
},
{
title: "CommentTitle6",
likes: 3
}
]
}
I want to retrieve all the posts, and if one post has a comment with 4 likes I want to retrieve this comment only under the "comments" array. If I do this:
db.posts.find({}, {comments: { $elemMatch: {likes: 4}}})
...I get this (which is exactly what I want):
{
_id: 1,
comments: [
{
title: "CommentTitle2",
likes: 4
}
]
}
{
_id: 2,
comments: [
{
title: "CommentTitle4",
likes: 4
}
]
}
{
_id: 3
}
But how can I retrieve the remaining fields of the documents without having to declare each of them like below? This way if added more fields to the post document, I wouldn't have to change the find query
db.posts.find({}, {title: 1, category: 1, comments: { $elemMatch: {likes: 4}}})
Thanks

Answer 1

--EDIT--
Sorry for the misread of your question. I think you'll find my response to this question here to be what you are looking for. As people have commented, you cannot project this way in a find, but you can use aggregation to do so:
https://stackoverflow.com/a/21687032/2313887
The rest of the answer stands as useful. So I think I'll leave it here
You must specify all of the fields you want or nothing at all when using projection.
You are asking here essentially that once you choose to alter the output of the document and limit how one field is displayed then can I avoid specifying the behavior. The bottom line is thinking of the projection part of a query argument to find just like SQL SELECT.It behaves in that * or all is the default and after that is a list of fields and maybe some manipulation of the fields format. The only difference is for _id which is always there by default unless specified otherwise by excluding it, i.e { _id: 0 }
Alternately if you want to filter the collection you nee to place your $elemMatch in thequery itself. The usage here in projection is to explicitly limit the returned document to only contain the matching elements in the array.
Alter your query:
db.posts.find(
{ comments: { $elemMatch: {likes: 4}}},
{ title: 1, category: 1, "comments.likes.$": 1 }
)
And to get what you want we use the positional $ operator in the projection portion of the find.
See the documentation for the difference between the two usages:
http://docs.mongodb.org/manual/reference/operator/query/elemMatch/
http://docs.mongodb.org/manual/reference/operator/projection/elemMatch/

Answer 2

This question is pretty old, but I just faced the same issue and I didn't want to use the aggregation pipeline as it was simple query and I only needed to get all fields applying an $elemMatch to one field.
I'm using Mongoose (which was not the original question but it's very frequent these days), and to get exactly what the question said (How can I retrieve all the fields when using $elemMatch?) I made this:
const projection = {};
Object.keys(Model.schema.paths).forEach(key => {
projection[key] = 1;
});
projection.subfield = { $elemMatch: { _id: subfieldId } };
Model.find({}, projection).then((result) => console.log({ result });