Support mixed types in HasMany relationship

[hpawe01]

A HasMany relationship (e.g. books: Book[]) does not allow mixed types (e.g. CrimeBook, NovelBook), even if they extend the same expected HasMany type (Book).

Problem
In this line, the type of the first element in the relationship is checked against all other types.

angular2-jsonapi/src/services/json-api-datastore.service.ts

Line 249 in a736c0f

const relationshipType: string = isJsonApiModel ? objects[0].modelConfig.type : '';

Example
I have

@HasMany()
books: Book[];

and the models

@JsonApiModelConfig({
  type: 'crimeBooks',
  modelEndpointUrl: 'books'
})
export class CrimeBook extends Book {...}

@JsonApiModelConfig({
  type: 'novelBooks',
  modelEndpointUrl: 'books'
})
export class NovelBook extends Book {...}

Possible solution 1
Collect modelEndpointUrl or type of all relationship items and check if there is only one unique. This way, even if the type of the items differ (crimeBooks or novelBooks), if the modelEndpointUrl is the same (books), all should be fine.

Possible solution 2
Check, if all relationship items are instances of the model, that the HasMany attribute expects (Book).

I will create a pull request for solution 1, but I am not sure if solution 2 would be more clean (Although only type and modelEndpointUrl are relevant, when talking to the server, and not the Typescript type of the HasMany attribute).

[hpawe01]

I think solution 1 would also solve a problem, that would occur if we use polymorphic relationships as mentions in issue #201

[hpawe01]

Regarding solution 2: To realize this, TypeScript needs to expose the array element type (see microsoft/TypeScript#7169, e.g. Book for Book[]), so that we can check, that all items are instances of this array element type.

[safo6m]

Correct, the solution 2 is technically the correct one, but for now we can go with the solution 1 due to its simplicity.

[hpawe01]

Closing because #216 was merged.