const UserSchema = new Schema({
  email: String
  isVerified: { default: false, type; Boolean }
  accounts: {
    ref: 'Account',
    type: Schema.Types.ObjectId
  }
})

const AccountSchema = new Schema({
  facebook: {
    ref: 'FacebookAccount',
    type: Schema.Types.?
  },
  local: {
    ref: 'LocalAccount',
    type: Schema.Types.?
  },
  twitter: {
    ref: 'TwitterAccount',
    type: Schema.Types.?
  },
})

const LocalAccount = new Schema({
  email: String,
  name: String,
  phone: String,
  password: String,
  _user: {
    ref: 'User',
    type: Schema.Types.ObjectId
  }
})

{
  _id: '12345789',
  email: 'turdFerguson@gmail.com',
  accounts: {
    facebook: { ... }
    local: { ... }
  }
}