As comes from official mongo documentation you cannot create unique index on fields from embedded documents. Here is a workaround for those using $push to append values to array of embedded documents.

mongo_unique_index_on_embedded_document

Mongo shell command

> db.users.update(
    { community:'Coomunity1',
      'users.login': {'$ne': 'Login1'}
    },
    { $push:
      { users: {login:'Login1', pwd:'*****'} }
    },
    {upsert: true})

won't add new user into array of 'users' of a collection item as of condition "'users.login': {'$ne': 'Login1'}" is False