mongoDB: unique index on a repeated value -

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so i'm pretty new mongodb figure misunderstanding on general usage. bear me. have document schema i'm working such

  {      name: "bob",      email: "bob@gmail.com",      logins: [        { u: 'a', p: 'b', public_id: '123' },        { u: 'x', p: 'y', public_id: 'abc' }     ]    }

my problem need ensure public ids unique within document , collection, furthermore there existing records being migrated mysql db dont have records, , therefore replaced null values in mongo.

i figure either index 

db.users.ensureindex({logins.public_id: 1}, {unique: true});

which isn't working because of missing keys , throwing e11000 duplicate key error index:

or more fundamental schema problem in shouldn't nesting objects in array structure that. in case, what? seperate collection user_logins??? seems go against idea of embedded document.

if expect u , p have same values on each insert (as in example snippet), might want use $addtoset operator on inserts ensure uniqueness of public_id field. otherwise think it's quite difficult make them unique across whole collection not working external maintenance or js functions.

if not, possibly store them in own collection , use public_id _id field ensure cross-document uniqueness inside collection. maybe contradict idea of embedded docs in doc database, according different requirements think that's negligible.


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