How to match multiple subdocuments in MongoDB? - mongodb

Question

Assuming that I have the following data in my books collection:
[
{
name: "Animal Farm",
readers: [
{
name: "Johny"
},
{
name: "Lisa"
}
],
likes: [
{
name: "Johny"
}
]
},
{
name: "1984",
readers: [
{
name: "Fred"
},
{
name: "Johny"
},
{
name: "Johny",
type: "bot"
}
],
likes: [
{
name: "Fred"
}
]
}
]
How do I retrieve all readers and likes that match name "Johny", with end result something like this:
[
{
name: "Animal Farm",
readers: [
{
name: "Johny"
}
],
likes: [
{
name: "Johny"
}
]
},
{
name: "1984",
readers: [
{
name: "Johny"
},
{
name: "Johny",
type: "bot"
}
],
likes: []
}
]
A following query is not possible:
db.books.find(
{ $or: [{ "readers.name": "Johny" }, { "likes.name": "Johny" }] },
{ name: 1, "readers.$": 1, "likes.$": 1 })
MongoDB complains with the following error: Cannot specify more than one positional array element per query (currently unsupported).
I have tried to use aggregation framework but did not succeed. So is this possible with MongoDB or do I have to run two queries to retrieve needed results?

Answer 1

As Sammaye has pointed already, specifying more than one positional array element is currently not supported.
However, you can use $elemMatch projection operator to get the results you want. $elemMatch projection operator limits the contents of the array to contain elements that matche the $elemMatch condition:
db.books.find(
{ $or: [{ "readers.name": "Johny" }, { "likes.name": "Johny" }] },
{
readers : { $elemMatch : { name : "Johny" }},
likes : { $elemMatch : { name : "Johny" }}
}
);
Edit
Altough MongoDB doesn't have a built in operator to do what you want, using existing operators, you can achieve what you want. But, embrace yourself, this is going to be a long one:
db.books.aggregate([
// find only documents that have correct "name"
{ $match: { $or: [{ "readers.name": "Johny" }, { "likes.name": "Johny" }]}},
// unwind the documents so we can push them to a array
{ $unwind: '$likes' },
// do a group to conditionally push the values into the array
{ $group : {
_id : '$_id',
likes : {
$push : {
$cond : [
{ $eq : ["$likes.name", "Johny"]},
"$likes",
null
]
}
},
readers : { $first : "$readers" },
name : { $first : "$name" }
}},
// the process is repeated for the readers array
{ $unwind: '$readers' },
{ $group : {
_id : '$_id',
readers : {
$push : {
$cond : [
{ $eq : ["$readers.name", "Johny"]},
"$readers",
null
]
}
},
likes : { $first : "$likes" },
name : { $first : "$name" }
}},
// final step: remove the null values from the arrays
{ $project : {
name : 1,
readers : { $setDifference : [ "$readers", [null] ] },
likes : { $setDifference : [ "$likes", [null] ] },
}}
]);
As you can see, you can do a "conditional" $push by using $cond operator inside the $push. But after the group stage, your array will contain null values. You have to filter them out by using setDifference.
Also note that you need to do unwind/group stages for each array you're building, otherwise a double unwind will duplicate the documents and you will end up with duplicate values in your arrays.

Answer 2

Following on from #ChristianP's Answer:
db.books.aggregate(
// So we don't have to random do this to docs we don't need to
{$match: { $or: [{ "readers.name": "Johny" }, { "likes.name": "Johny" }] }},
{$unwind: '$readers'},
{$match: { "readers.name": "Johny" }},
{$unwind: '$likes'},
{$match: { "likes.name": "Johny" }},
{$group: {_id: '$_id', likes: {$push: '$likes'}, readers: {$push: '$readers'}}}
)
Something like that should be able to do what you want, the functionality to do this in query was shunned in favour of doing it this way.