fix(schemas): don't use schema to attribute mapping on singular array schemas

[fpipita]

Problem

Denormalization does not seem to produce the correct result when a singular Array schema owns an Object schema whose instances own an id property.

For example:

const cats = new schema.Entity('cats');
// catRecord instances own an `id` property
const catRecord = new schema.Object({
   cat: cats
});
const catList = new schema.Array(catRecord);

The problem is in the the denormalizeValue method of the Polymorphic class.

When the value to be unvisited owns a property named id, then this method uses the value of the id property as the input value for the call to unvisit in place of the value itself, which is not the case on singular Array schema, where the value itself should always be used instead.

Solution

I've added a guard which prevents the value of the id property to become the input to unvisit when the current schema is a singular schema.

TODO

• [ x] Add & update tests
• [ x] Ensure CI is passing (lint, tests, flow)
• Update relevant documentation

[coveralls]

Coverage remained the same at 100.0% when pulling c593cc3 on fpipita:master into b5f570a on paularmstrong:master.

[paularmstrong]

To avoid confusion for others, since void 0 isn't common, can we use a canonical undefined here?

[fpipita]

Sure, I've just updated the patch and rebased it to upstream/master.