How to get documents with non unique array elements? - node.js

Question

I have the following MongoDB documents:
{
_id: ObjectId('09de14821345dda65c471c99'),
items: [
_id: ObjectId('34de64871345dfa655471c99'),
_id: ObjectId('34de64871345dfa655471c91'),
_id: ObjectId('34de64871345dfa655471c99'),
]
},
{
_id: ObjectId('09de14821345dda65c471c98'),
items: [
_id: ObjectId('24de64871345dfa61271c10'),
_id: ObjectId('24de64871345dfa61271c11'),
_id: ObjectId('24de64871345dfa61271c11'),
]
},
{
_id: ObjectId('09de14821345dda65c471c07'),
items: [
_id: ObjectId('24de64871345dfa61271c05'),
_id: ObjectId('24de64871345dfa61271c06'),
_id: ObjectId('24de64871345dfa61271c07'),
]
}
I need to find all documents with repeated items array elements. So from the documents above I want to get the following result:
db.collection.documents.find({/** need query*/}).toArray(function (err, documents) {
console.dir(documents); // documents with id's 09de14821345dda65c471c99 and 09de14821345dda65c471c98
});
How could I do that?

Answer 1

In order to group and match results you will need to use the Aggregation Framework or Map/Reduce rather than a simple find() query.
Example data
Your example document include some errors: a few of the ObjectIDs are too short and the array elements should either be embedded documents ({_id: ObjectId(...)}) or simple values.
For test data I've used:
db.mydocs.insert([
{
_id: ObjectId('09de14821345dda65c471c99'),
items: [
ObjectId('34de64871345dfa655471c99'),
ObjectId('34de64871345dfa655471c91'),
ObjectId('34de64871345dfa655471c99')
]
},
{
_id: ObjectId('09de14821345dda65c471c98'),
items: [
ObjectId('24de64871345ddfa61271c10'),
ObjectId('24de64871345ddfa61271c11'),
ObjectId('24de64871345ddfa61271c11')
]
},
{
_id: ObjectId('09de14821345dda65c471c07'),
items: [
ObjectId('24de64871345ddfa61271c05'),
ObjectId('24de64871345ddfa61271c06'),
ObjectId('24de64871345ddfa61271c07')
]
}])
Aggregation query
Here is an aggregation query using the mongo shell:
db.mydocs.aggregate(
// Unpack items array into stream of documents
{ $unwind: "$items" },
// Group by original document _id and item
{ $group: {
_id: { _id: "$_id", item: "$items" },
count: { $sum: 1 }
}},
// Limit to duplicated array items (1 or more count per document _id)
{ $match: {
count: { $gt: 1 }
}},
// (Optional) clean up the result formatting
{ $project: {
_id: "$_id._id",
item: "$_id.item",
count: "$count"
}}
)
Sample results
{
"_id" : ObjectId("09de14821345dda65c471c98"),
"count" : 2,
"item" : ObjectId("24de64871345ddfa61271c11")
}
{
"_id" : ObjectId("09de14821345dda65c471c99"),
"count" : 2,
"item" : ObjectId("34de64871345dfa655471c99")
}