fix: array decorator

[jannyHou]

Fixes #4754

To define a nested array property with @property.array(), you must provide a json schema to describe the sub array item:

@model()
class TestModel {
  // alternatively use @property.array('array')
  @property.array(Array, {
    jsonSchema: {
      type: 'array',
      items: {type: 'string'},
    },
  })
  nestedArr: Array<Array<string>>;
}

A few issues this PR addresses:

• We didn't allow nested array property before due to
  https://github.com/strongloop/loopback-next/blob/ef155eeddf11895bac8ddc491cb87c1e5b7398d0/packages/repository-json-schema/src/build-schema.ts#L260
  which is not correct. I tested with todo app, when decorate the nested array property well, creating & retrieving an instance works properly.

• It's not possible to infer the sub type of nested array property, that's why the json schema field is needed(Thank you @deepakrkris for your suggestion in defining a nested array model property using @property.array decorator crashes the app #4754 (comment)). I added a new check for that field to avoid triggering the subsequent error '"items" property must be present if "type" is an array' instead.

Checklist

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• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• API Documentation in code was updated
• Documentation in /docs/site was updated
• Affected artifact templates in packages/cli were updated
• Affected example projects in examples/* were updated

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[agnes512]

Do we have any tests on querying/filtering the nested arrays?

[jannyHou]

@agnes512 @loopback/repository-json-schema doesn't have it. If we add tests, need to do them in the acceptance tests for connectors.

[agnes512]

I mentioned it is because when I was triaging loopbackio/loopback-connector-postgresql#441, I realized that not all connectors support arrays/nested arrays. It'd be good if we can have them documented and tested. Just some thoughts 😄

[jannyHou]

I realized that not all connectors support arrays/nested arrays.

Yeah that's the problem, the original issue #4754 is not trying to support the full feature, it's a fix for the misuse of decorator.

It'd be good if we can have them documented and tested.

It's not a trivial thing to test across connectors, let's do it in a separate story.

[agnes512]

I have another question of the design. Do we need to specify these two fields? if so, why do we need type: 'array' as we already use the decorator?

[jannyHou]

@agnes512 the jsonSchema here is for the sub array property,
e.g. For Array<Array<string>>, the json schema describes Array<string>

If the current doc confuses you:

To define a nested array property, you must provide the jsonSchema field to describe the sub-array property.

Any suggestion on how to make it more clear?

[agnes512]

Ah I see, then I am good with it 👍

[agnes512]

Hopefully we can have some tests for different connectors in the future :D

[jannyHou]

@agnes512 sure 👍 created story in #5570