Mongodb - How to find documents sharing "partially" a set of fields predefined? - database

Question

I am trying to write a query in the MongoDB CLI to search for documents that are sharing an arbitrary number of fields, out of a specific, predetermined, set of fields.
To give you an example: let's consider a database composed of documents with variable fields. These fields can be shared among documents, but not necessarily.
{
_id: ObjectId("AAA"),
field1: "value_a",
field2: "value_b",
field3: "value_l",
field6: "value_n"
}
{
_id: ObjectId("BBB"),
field1: "value_c",
field3: "value_e"
}
{
_id: ObjectId("CCC"),
field2: "value_f",
field4: "value_g"
}
{
_id: ObjectId("DDD"),
field1: "value_m",
field5: "value_h",
field2: "value_i",
field6: "value_j",
field7: "value_k"
}
{
_id: ObjectId("EEE"),
field8: "value_o"
}
The challenge is to have a query in which one can not only specify the fields of interest, but also to specify/force the amount of fields to be shared among documents (instead of having an exact match, as mentioned below with the use of $exists). For the fields a document may have that are not in the list, it does not matter whether they are present or not.
For clarification, let's say we are interested in the list of fields ["field1", "field3", "field6", "field8"] in the documents shown above, and we want to know which documents share X fields (regardless which one, as long as they are from the list we defined).
Let's call QUERY1 the query returning the documents sharing exactly X fields, as described below:
Documents with exactly one field: returns the document with ObjectId("EEE")
Documents with exactly two fields: returns the documents with ObjectId("BBB") and ("DDD")
Documents with exactly three fields: returns the document with ObjectId("AAA")
Additionally, let's call QUERY2 the query returning documents that share at least X fields?
Documents with at least one field: returns the documents with ObjectId("AAA"), ("BBB"), ("DDD"), ("EEE")
Documents with at least two fields: returns the documents with ObjectId("AAA"), ("BBB"), ("DDD")
Documents with at least three fields: returns the document with ObjectId("AAA")
I have been trying with $exists, however the problem is that the query returns the documents sharing exactly those four fields, without the flexibility explained above: db.documents.find({'field1' : {$exists: true}, 'field3' : {$exists: true}, 'field6' : {$exists: true}, 'field8' : {$exists: true})
Does anyone know how to write QUERY1 and QUERY2?
Moreover, as I would like to create new collections out of the results from those queries, ideally the queries would work with the following function at the end : .forEach(function(x){db.newCollection.insert(x)});
Thank you very much for your help, very appreciated.

Answer 1

db.documents.aggregate([
{
"$project": {
"matchedFieldCount": {
$size: {
"$filter": {
"input": {
"$objectToArray": "$$ROOT"
},
"cond": {
"$in": [
"$$this.k",
[
"field1",
"field3",
"field6",
"field8"
]
]
}
}
}
}
}
},
{ $match: { matchedFieldCount: 1 } }
])
You can adjust the $match in the pipeline to do QUERY1 or QUERY2. And if you want you can project just the _id by adding one more pipeline step.