Mongoose Compound Index Unique + Sparse - node.js

Question

I want to create an index which ensures, that I don't have a duplicate serialNr within the combination of a manufacturer + art. But some items don't have a serialNr. These I don't want to check/index.
Code
mySchema.index({ serialNr: 1, art: 1 , manufacturer: 1, deleted: 1}, { unique: true, sparse: true)
I tried it also with adding a partial Filter:
partialFilterExpression: { serialNr: {$ne:null} } to the index options.
Question
How can I index it that inserting:
[{art: 'a', manufacturer:'a', deleted:null}, {art: 'a', manufacturer:'a', deleted:null}] passes
and
[{serialNr: '123', art: 'a', manufacturer:'a', deleted:null}, {serialNr: '123', art: 'a', manufacturer:'a', deleted:null}]fails
I'm not trying to save:
[{serialNr: null, art: 'a', manufacturer:'a', deleted:null}, {serialNr: null, art: 'a', manufacturer:'a', deleted:null}]
The mongo docs state, that it should not be indexed if a field is missing in a compound index:
Sparse indexes selects documents to index solely based on the
existence of the indexed field, or for compound indexes, the existence
of the indexed fields.
https://docs.mongodb.org/manual/core/index-partial/
EDIT:
New Index
implantSchema.index({ serialNr: 1, art: 1 , manufacturer: 1, deleted: 1}, { unique: true, partialFilterExpression: { serialNr: {$exists:true} }})
Before I make changes on the index I drop my collection in my dev-database, to ensure that there is no old index existing. This is the new index which is returned on db.items.getIndexes()
/* 0 */
{
"0": {
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "LOC_db.items"
},
"1": {
"v" : 1,
"unique" : true,
"key" : {
"serialNr" : 1,
"art" : 1,
"manufacturer" : 1,
"deleted" : 1
},
"name" : "serialNr_1_art_1_manufacturer_1_deleted_1",
"ns" : "LOC_db.items",
"partialFilterExpression" : {
"serialNr" : {
"$exists" : true
}
},
"background" : true
}
}
Data
I have a pre save hook where I check the value of serialNr, which does not exist before, the dup error is returned.
When querying the database there is no doc existing which has null, "" as value for the serialNr.
The query db.items.find({ serialNr: {$exists:true} }) does not match any items.
Error
insertDocument :: caused by :: 11000 E11000 duplicate key error index: LOC_db.items.$serialNr_1_art_1_manufacturer_1_deleted_1 dup key: { : null, : "Robo", : "Olymp", : null }
Additional Information
Maybe the error is related to:
https://github.com/Automattic/mongoose/issues/2457

Answer 1

You were actually quite close to the solution, all you need to use this when creating index :
partialFilterExpression: { serialNr: {$exists:true} }
Be sure to have at least 3.2 version of mongo
Example :
db.col.createIndex({a:1, b:1},{unique:true, partialFilterExpression: { a: {$exists:true} })
db.col.save({a:50, b:27}) //pass
db.col.save({a:50, b:27}) //fails
db.col.save({b:27}) //pass
db.col.save({b:27}) //pass
db.col.save({b:27}) //pass
db.col.save({a:50, b:27}) //fails
db.col.save({a:50}) //pass
db.col.save({a:50}) //fails
PS : Maybe you can do little "hack" and use this : partialFilterExpression: { serialNr: {$exists:true}, serialNr: {$gte:0} }
I tried using $ne, but it was throwing me an error.