Node.js - Creating Relationships with Mongoose

I have 2 Schemas, Custphone and Subdomain. Custphone belongs_to a Subdomain and Subdomain has_many Custphones.

The problem is in creating the relationship using Mongoose. My goal is to do: custphone.subdomain and get the Subdomain that the Custphone belongs to.

I have this in my schemas:

SubdomainSchema = new Schema
    name : String

CustphoneSchema = new Schema
    phone : String
    subdomain  : [SubdomainSchema]

When I print the Custphone result I get this:

{ _id: 4e9bc59b01c642bf4a00002d,
  subdomain: [] }

When the Custphone result has {"$oid": "4e9b532b01c642bf4a000003"} in MongoDB.

I want to do custphone.subdomain and get the subdomain object of the custphone.