[mongodb-user] Distinct() on indexed field returns undefined

[e***@hrs-pd.de]

Hi,

In one collection we have documents which look like this:

{
"A" : [],
}

OR

{
"A" : [
{
"_id" : 1,
"Name" : "ASDF"
},
{
"_id" : 2,
"Name" : "GHJKL"
}
],
}



So we need all the distinct values of this array for our UI.
Running
db.getCollection('X').distinct('A')

actually works fine and returns the values we expect. If we then add an
index on this Field ( db.X.createIndex({"A":1,{Name:"IX_A"}) and run this
query again, there is suddenly an item undefined:
[
undefined,
{
"_id" : 1,
"Name" : "ASDF"
}, ...
]



So how can we get rid of this undefined item in the index?

Thanks!
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['Wan Bachtiar' via mongodb-user]

actually works fine and returns the values we expect. If we then add an
index on this Field ( db.X.createIndex({“A”:1,{Name:”IX_A”}) and run this
query again, there is suddenly an item undefined:

Hi,

Depending on your use case, Partial Indexes
<https://docs.mongodb.com/manual/core/index-partial/> is likely what you’re
after.
For example you can create:

db.collection.createIndex({"A":1}, {partialFilterExpression:{"A":{"$exists":true}}})

Regards,
Wan.
​
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