Ember Data relationships not resolved

I'm still learning ember.js and have run into a roadblock with ember data not resolving lookup relationships in models. I have one model 'site' that will be basically a lookup table for every other model to differentiate data based on location.

At this point, I'm doing something wrong or missing a key concept - probably both... (or maybe it's the wee hour!)

Site Model (i.e. the lookup table)

import DS from 'ember-data';

export default DS.Model.extend({
    code: DS.attr(),
    name: DS.attr(),
});

The site model would have a hasMany relationship to all my other models (will be about 12 when complete)

Associate Model

import DS from 'ember-data';
import { belongsTo } from 'ember-data/relationships';

export default DS.Model.extend({
    site: belongsTo('site'),
    last: DS.attr(),
    first: DS.attr(),
    active: DS.attr('boolean'),

fullName: Ember.computed('first', 'last', function() {
  return `${this.get('first')} ${this.get('last')}`;
  }),
});

The 'associate model' will also be a lookup along with 'site' in some other models.

I'm providing data via the JSON API spec but I'm not including the relationship data because as I understand it, ember data it should be pulling down the site data using the site id attribute.

{
    "links": {
        "self": "/maint/associates"
    },
    "data": [
        {
            "type": "associate",
            "id": "1",
            "attributes": {
                "site": "6",
                "last": "Yoder",
                "first": "Steven",
                "active": "1"
            },
            "links": {
                "self": "/associates/1"
            }
        }
    ]
}

In my template file I'm referencing associate.site which gives me an error.

<(unknown mixin):ember431>

If I use associate.code or .name to match the site model, nothing will show in the template. The code from the 'site' table is the data I really want to displayed in the template.

So the obvious questions:

1. Am I wrong that Ember Data should be resolving this or do I need to include the relationship in my API response?

2. I realize that my belongsTo in the 'associate' model only references site while I want site.code, so how do I make that relationship known or access the field in my 'associate' model?

3. I didn't include hasMany relationship in the 'site' model because there would be many. Do I need to do an inverse relationship in other models? Examples I've seen don't all show the hasMany relationships setup.

4. When I look at the models in ember inspector the site field is not included in the model. Even if I wasn't getting the correct data should it still show up?

I like ember so far, just need to understand and get over this roadblock