Mirage server GETs data but POST fails

I have the mirage models:

// mirage/models/country.js
import { Model, belongsTo, hasMany } from 'miragejs';

export default Model.extend({
    name: '',
    iso3166_1_alpha3: '',
    capitol_city: belongsTo('city', {inverse: null}),
    cities: hasMany('city', {inverse: 'country'})
});

and:

// mirage/models/city.js
import { Model, belongsTo } from 'miragejs';

export default Model.extend({
    name: '',
    country: belongsTo('country', {inverse: 'cities'})
});

and the serializer:

// mirage/serializers/application.js
import { camelize, capitalize, underscore } from '@ember/string';
import { JSONAPISerializer } from 'miragejs';

export default class ApplicationSerializer extends JSONAPISerializer
{
    alwaysIncludeLinkageData = true;

    keyForAttribute(attr) {
        return underscore(attr);
    };
    keyForRelationship(modelName) {
        return underscore(modelName);
    };
    typeKeyForModel(model) {
        return capitalize(camelize(model.modelName));
    };
};

When I run the tests:

import { module, test } from 'qunit';
import { setupTest } from 'ember-qunit';
import { setupMirage } from 'ember-cli-mirage/test-support';

module('Unit | Mirage | mirage models', function (hooks) {
  setupTest(hooks);
  setupMirage(hooks);

  test('it retrieves the country', async function (assert) {
    const server = this.server;
    let city = server.create('city', { id: '1', name: 'Paris' });

    server.create(
        'country',
        {
            id: 'FR',
            name: 'France',
            iso3166_1_alpha3: 'FRA',
            capitol_city: city
        }
    );

    let response = await fetch('/api/countries')
    assert.strictEqual(response.status, 200, "Should have created the model");
    let json = await response.json();
    assert.deepEqual(
        json,
        {
            data: [
                {
                    type: 'Country',
                    id: 'FR',
                    attributes: {
                        name: 'France',
                        iso3166_1_alpha3: 'FRA',
                    },
                    relationships: {
                        capitol_city: {data: {type: 'City', id: '1'}},
                        cities: {data: []},
                    }
                }
            ]
        }
    )
  });

  test('it creates the country', async function (assert) {
    const server = this.server;
    server.create('city', { id: '1', name: 'Paris' });

    let response = await fetch(
        '/api/countries',
        {
            method: 'POST',
            headers: {'Countent-Type': 'application/json'},
            body: JSON.stringify(
                {
                    data: {
                        id: 'FR',
                        type: 'Country',
                        attributes: {
                            iso3166_1_alpha3: 'FRA',
                            name: 'France',

                        },
                        relationships: {
                            capitol_city: { data: { type: 'City', id: '1'} },
                            cities: { data: [{ type: 'City', id: '1'}] }
                        }
                    }
                }
            )
        }
    );

    console.log((await response.json()).message);
    assert.strictEqual(response.status, 201, "Should have created the model");
  });
});

The first one passes and the second one fails with the message:

> status
> Mirage: You're passing the relationship 'capitol_city' to the 'country' model via a POST to '/api/countries', but you did not define the 'capitol_city' association on the 'country' model.
>

How can I get Mirage to recognise the capitol_city attribute on the model?

Mirage is opinionated with regards to the format of attributes and expects the attributes to be in camelCase (and not snake_case).

Unfortunately the Ember CLI Mirage model relationships documentation does not mention this expectation and all the examples use single-word attributes. Even more unfortunately, Mirage will work with snake_case attributes for simple GET requests and when directly creating models through the API; it is only when you make a request to POST/PUT/PATCH a model into the server that it fails and the message will (confusingly) refer to the snake case attribute which has been defined. (See the Mirage source code for where it fails.)

To solve it, convert the attributes to camel case:

// mirage/models/country.js
import { Model, belongsTo, hasMany } from 'miragejs';

export default Model.extend({
    name: '',
    iso31661Alpha3: 0,
    capitolCity: belongsTo('city', {inverse: null}),
    cities: hasMany('city', {inverse: 'country'})
});

and change it in the tests as well:

import { module, test } from 'qunit';
import { setupTest } from 'ember-qunit';
import { setupMirage } from 'ember-cli-mirage/test-support';

module('Unit | Mirage | mirage models', function (hooks) {
  setupTest(hooks);
  setupMirage(hooks);

  test('it retrieves the country', async function (assert) {
    const server = (this as any).server;
    let city = server.create('city', { id: '1', name: 'Paris' });

    server.create(
        'country',
        {
            id: 'FR',
            name: 'France',
            iso31661Alpha3: 'FRA',
            capitolCity: city
        }
    );

    let response = await fetch('/api/countries')
    assert.strictEqual(response.status, 200, "Should have created the model");
    let json = await response.json();
    console.log(JSON.stringify(json));
    assert.deepEqual(
        json,
        {
            data: [
                {
                    type: 'Country',
                    id: 'FR',
                    attributes: {
                        name: 'France',
                        iso3166_1_alpha3: 'FRA',
                    },
                    relationships: {
                        capitol_city: {data: {type: 'City', id: '1'}},
                        cities: {data: []},
                    }
                }
            ]
        }
    )
  });

  test('it creates the country', async function (assert) {
    const server = (this as any).server;
    let city = server.create('city', { id: '1', name: 'Paris' });

    let response = await fetch(
        '/api/countries',
        {
            method: 'POST',
            headers: {'Countent-Type': 'application/json'},
            body: JSON.stringify(
                {
                    data: {
                        id: 'FR',
                        type: 'Country',
                        attributes: {
                            iso3166_1_alpha3: 'FRA',
                            name: 'France',

                        },
                        relationships: {
                            capitol_city: { data: { type: 'City', id: '1'} },
                            cities: { data: [{ type: 'City', id: '1'}] }
                        }
                    }
                }
            )
        }
    );

    console.log((await response.json()).message);
    assert.strictEqual(response.status, 201, "Should have created the model");
  });
});

However, once you convert it to camel case then the attribute iso31661Alpha3 does not get formatted correctly in the output so you have to manually change the serializer for the country model:

// mirage/serializers/country.js
import ApplicationSerializer from './application';

export default class CountrySerializer extends ApplicationSerializer
{
    keyForAttribute(attr: string) {
        switch(attr)
        {
            case 'iso31661Alpha3': return 'iso3166_1_alpha3';
            default: return super.keyForAttribute(attr);
        }
    };
};

Once the attributes are in the correct case then it will work.