Unique Constraint with Two Fields in MongoDB

Question

I have a collection with fields "email" and "friends_email". I would like to setup a unique-ness constraint like the following, using MongoDB:

1. No record will have the same value for email and friends_email. So this would be invalid:

   {"email": "[email protected]", "friends_email": "[email protected]" } 

2. No 2 records will have the same values for all fields. So the following examples would ALL be invalid:

   {     { "email": "[email protected]", "friends_email": "[email protected]" },     { "email": "[email protected]", "friends_email": "[email protected]" } } {     { "email": "[email protected]", "friends_email": null },     { "email": "[email protected]", "friends_email": null } } {     { "email": null, "friends_email": "[email protected]" },     { "email": null, "friends_email": "[email protected]" } } 

   In plain english, it would be something like, the concatenation of email and friends_email will be unique, with null and undefined being coalesced into empty-string.

What's the best way to enforce this rule in MongoDB?

Answer 1

It sounds like you need a compound unique index:

db.users.createIndex( { "email": 1, "friends_email": 1 }, { unique: true } ) 

... and you can verify at the ORM layer that email =/= friends_email.