Fetching HasMany Ember Relationship data asynchronously form Mirage

I have model which has a hasMany async relationship,

export default Model.extend({
    name: attr(),
    description: attr(),
    metricNamespaces: hasMany('metric-namespace', {
        async: true
    })
});

Here is a sample jSONAPI doc of a model instance

{
  "data": {
    "type": "context",
    "id": "1",
    "attributes": {
      "name": "O'Reilly Inc",
      "description": "Aut mollitia mollitia esse inventore est consequatur mollitia ex dolores. Deserunt est dicta temporibus eius. Id dicta molestiae nostrum odio."
    },
    "relationships": {
      "metricNamespaces": {
        "data": [
          {
            "type": "metric-namespace",
            "id": "8"
          }
        ]
      }
    }
  }
}

I have an afterModel hook in a route which tries to fetch the relationships

model.get('metricNamespaces').then((success) => {
    console.log("Success", success);
}, (error) => {
    console.log("Error", error);
});

I expect this to trigger a get request to Mirage. But this never triggers a get request (Which should fail since I don't have a mirage route for the metric-namespace get)

Can someone help me figure out what I am missing?