[Core] Aliases fields to reduce fields count

[alexander-schranz]

If we have a document like this:

{
    "id": "1",
    "title": "New Blog",
    "article": "<article><h2>Some Subtitle<\/h2><p>A html field with some content<\/p><\/article>",
    "blocks": [
        {
            "type": "text",
            "title": "Title",
            "description": "<p>Description<\/p>"
        },
        {
            "type": "text",
            "title": "Title 2",
            "description": "<p>Description 2<\/p>"
        },
        {
            "type": "embed",
            "title": "Video"
        },
        {
            "type": "quote",
            "title": "Quote",
            "description": "<p>Some quote and more<\/p>"
        }
    ],
    "footer": {
        "title": "New Footer"
    }
}

Currently we are mapping each field into an own field itself.

[
    'id' => new Field\IdentifierField('id'),
    'title' => new Field\TextField('title'),
    'article' => new Field\TextField('article'),
    'blocks' => new Field\TypedField('blocks', 'type', [
        'text' => [
            'title' => new Field\TextField('title'),
            'description' => new Field\TextField('description'),
        ],
        'embed' => [
            'title' => new Field\TextField('title'),
        ],
        'text' => [
            'title' => new Field\TextField('title'),
            'description' => new Field\TextField('description'),
        ],
    ], multiple: true),
    'footer' => new Field\ObjectField('footer', [
        'title' => new Field\TextField('title'),
    ]),
];

This make that in complex Systems like CMS a lot fields are created, or a lot of converts are required to map all fields in a collected fields. I think it would be a great addition to make this possible out of the box via some alias field type. Lets say we map all content data into an own content field e.g.:

[
    'id' => new Field\IdentifierField('id'),
    'title' => new Field\TextField('title'),
    'block_titles' => new Field\TextField('content', multiple: true),
    'content' => new Field\TextField('content', multiple: true),
    'article' => new Field\AliasField('content'),
    'blocks' => new Field\TypedField('blocks', 'type', [
        'text' => [
            'title' => new Field\AliasField('block_titles'),
            'description' => new Field\AliasField('content'),
        ],
        'embed' => [
            'title' => new Field\AliasField('block_titles'),
        ],
        'text' => [
            'title' => new Field\AliasField('block_titles'),
            'description' => new Field\AliasField('content'),
        ],
    ], multiple: true),
    'footer' => new Field\ObjectField('footer', [
        'title' => new new Field\AliasField('content'),
    ]),
];

This way the Marshaller would be required to create a document like this:

{
    "id": "1",
    "title": "New Blog",
    "block_titles": [
        "Title",
        "Title 2",
        "Video",
        "Quote"
    ],
    "content": [
        "<article><h2>Some Subtitle<\/h2><p>A html field with some content<\/p><\/article>",
        "<p>Description<\/p>",
        "<p>Description 2<\/p>",
        "<p>Some quote and more<\/p>",
        "New Footer"
    ]
}

The question which is coming up here if we always should save the original JSON in a _raw field to so we get it back in the same way we are setting it. This would also mean that the documents inside search indexes get a little bit bigger as e.g. elasticsearch will save whole doc already in _source but with additonal _raw the _source will twice as big as without _raw. Example:

Current JSON Size: 494
Squashed JSON File: 265
Squashed with Raw JSON Size: 767

/c @chirimoya @wachterjohannes

This is also related to #200