MongoDB: Unique index on array element’s property

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As far as I know, unique indexes only enforce uniqueness across different documents, so this would throw a duplicate key error:

db.cats.insert( { id: 123, kittens: [ { id: 456 } ] } )
db.cats.insert( { id: 123, kittens: [ { id: 456 } ] } )

But this is allowed:

db.cats.insert( { id: 123, kittens: [ { id: 456 }, { id: 456 } ] } )

I’m not sure if there’s any way enforce the constraint you need at the Mongo level, maybe it’s something you could check in the application logic when you insert of update?

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