mongoose-indexing.js

var mongoose = require('mongoose')
  , Schema = mongoose.Schema

mongoose.connect('mongodb://localhost/identities');

var IdentitySchema = new Schema({ 
    type : { type: String, enum: ['twitter', 'facebook', 'google'] }
  , uid  : { type: String }
})

var UserSchema = new Schema({ 
    identities : [ IdentitySchema ]
})

UserSchema.index({ "identities.type": 1, "identities.uid"   : 1 }, { unique: true })
    
var User = mongoose.model('User', UserSchema)

// clean up db before tests
User.remove({}, function(){
    
    // creates an user from identities array
    function testUserInsert(){
        var user = new User()
          , identities = [].slice.call(arguments)
          
        identities.forEach(function(id){
            user.identities.push(id)
        })
        
        user.save(function(err){
            if (err){
                console.log("error creating user\n", identities, "\n", err.message)
            } else {
                console.log("user created\n", identities)
            }
        })
    }
    
    // multiple ids
    testUserInsert(
        { uid: 'Jim', type: 'twitter' }
      , { uid: 'Jim', type: 'facebook' }
    )
    
    // different service, ok
    testUserInsert(
        { uid: 'Jim', type: 'google' }
    )
    
    // same service, should raise an error
    testUserInsert(
        { uid: 'Jim', type: 'twitter' }
    )
})