Exclude fields in matching element retrieved by positional operator - mongodb

Question

I have a texts collection with documents looking like this:
{
title: 'A title',
author: 'Author Name',
published: 1944,
languages: [
{
code: 'en',
text: 'A long english text by the author...'
},
{
code: 'da',
text: 'En lang dansk tekst skrevet af forfatteren...'
}
// + many more languages
]
}
and would like to make a query that retrieves the title, author and published date, and the text in a given language, so I do this:
texts.findOne(
{ title: titleArg, language.code: languageArg },
{ 'title': 1, 'author': 1, 'published': 1, 'languages.$': 1 } ...
but I would like to return the matching language element WITHOUT mongodb's _id field.
If I do this in the projection:
{ '_id': 0, 'title': 1, 'author': 1, 'published': 1, 'languages.$': 1 }
I get the document back without it's main _id, but if I do this:
{ 'languages.$._id': 0, 'title': 1, 'author': 1, 'published': 1, 'languages.$': 1 }
or this:
{ 'languages._id': 0, 'title': 1, 'author': 1, 'published': 1, 'languages.$': 1 }
nothing at all is returned.
Does anyone know how to create a projection that returns an element in an array AND exludes some fields in that element?

Answer 1

You seem to be actually saying that your documents really look like this:
{
"_id": ObjectId("53b25ad420edfc7d0df16a0c"),
"title": "A title",
"author": "Author Name",
"published": 1944,
"languages": [
{
"_id": ObjectId("53b25af720edfc7d0df16a0d"),
"code": "en",
"text": "A long english text by the author..."
},
{
"_id": ObjectId("53b25b0720edfc7d0df16a0e"),
"code": "da",
"text": "En lang dansk tekst skrevet af forfatteren..."
}
]
}
Just to clear up the point, MongoDB does not insert the _id values in the array elements. This is something that certain Object Document Mappers or ODM's do, one such ODM is mongoose. But it is other software that does this as MongoDB and default drivers only place this field at the "top" level of the documents in a collection unless another value is specified in it's place.
Being specific or precise in the fields within in array that you wish to "project", is beyond the scope of what you can do with .find(). You actually need the .aggregate() method in order to "re-shape" the document and remove all the _id fields the way you want:
db.collection.aggregate([
// Match the document(s) that meet the conditions
{ "$match": {
"title": "A title",
"languages.code": "en"
}},
// Unwind the array to de-normalize for processing
{ "$unwind": "$languages" },
// Match to "filter" the actual array documents
{ "$match": { "languages.code": "en" } },
// Group back the array per document and keep only the wanted fields
{ "$group": {
"_id": "$_id",
"title": { "$first": "$title" },
"author": { "$first": "$author" },
"published": { "$first": "$published" },
"languages": {
"$push": {
"code": "$languages.code",
"text": "$languages.text"
}
}
}},
// Finally project to remove the "root" _id field
{ "$project": {
"_id": 0,
"title": 1,
"author": 1,
"published": 1,
"languages": 1
}}
])
MongoDB 2.6 introduces some new operators to make this process possible in a single $project stage:
db.collection.aggregate([
{ "$match": {
"title": "A title",
"languages.code": "en"
}},
{ "$project": {
"_id": 0,
"title": 1,
"author": 1,
"published": 1,
"languages": {
"$setDifference": [
{ "$map": {
"input": "$languages",
"as": "el",
"in": {
"$cond": [
{ "$eq": [ "$$el.code", "en" ] },
{ "code": "$$el.code", "text": "$$el.text" },
false
]
}
}},
[false]
]
}
}}
])
The general intent of such operations is usually for more involved document re-shaping than what you are doing, including the need to match multiple array elements which is something you cannot do with positional projection.
But this is also the only present way to "alter" the fields returned from within the array elements as you wish to happen.
Also look at the full list of aggregation operators for a more detailed explanation of each one.