fix: fix wrong accumulator return type for initializeModel function

[zxl629]

Related issues:

• initializeModel function breaks accumulator return type  amplify-js#13958

Description of changes:

This change adds a defined-behavior integration test for initializeModel function. The test fails since the function has a wrong return type when it accumulates sort keys from data.

The change then fixes the return type and passes the test.

Example of why we need this:

If customers have two data models Order and Customer, where Order has reference fields that match Customer's identifier, and a belongsTo relationship field with Customer :

Order: a.model({
    id: a.id(),
    customerFirstName: a.string().required(),
    customerLastName: a.string().required(),
    customer: a.belongsTo('Customer', ['customerFirstName','customerLastName']),
  }).authorization(allow => [allow.publicApiKey()])

 Customer: a.model({
    customerFirstName: a.string().required(),
    customerLastName: a.string().required(),
    orders: a.hasMany('Order', ['customerFirstName','customerLastName']),
  }).identifier(['customerFirstName', 'customerLastName']).authorization(allow => [allow.publicApiKey()]),

when they try to lazy load the customer data from an order:

  const { data: order } = await client.models.Order.get({
    id: 'order-id',
  });
  const { data: customer } = await order!.customer();

they will get the following error: Uncaught (in promise) TypeError

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[stocaaro]

Looks good overall. One nit

[stocaaro]

describe blocks are helpful when grouping tests together, especially when we need to do some setup or teardown related to some shared grouping/testcase relationship.

In cases like this, I might just use a test without the wrapper.

[zxl629]

That makes sense, I have removed the describe() wrapper.

[svidgen]

Just want to confirm: This test fails in the absence of the change?