Add default cms pages to products and categories

INFO

This document represents an architecture decision record (ADR) and has been mirrored from the ADR section in our Shopware 6 repository. You can find the original version here

Context

We want to provide a way to set a default cms page.
Currently, if a product has no cms page assigned to it, there is a hardcoded fallback twig-template but no default. This does not apply to categories.

Products and categories should both get a default cms page if none is assigned which can be configured by the user.
The default should only be used when the corresponding entity is marked to use a default.

Decision

In order to implement this functionality, we needed a way to mark a product or category, which should use the default. This affects following entities:

• category
  • category.cmsPageId will be set to null if the defined default is given,
  • this only applies to cms pages of type procut_list
• product
  • product.cmsPageId will be set to null if the defined default \Shopware\Core\Defaults::CMS_PRODUCT_DETAIL_PAGE is given.

In order to set the default cms page, we simply provide the corresponding cms page id in the system config.

• \Shopware\Core\Content\Product\ProductDefinition::CONFIG_KEY_DEFAULT_CMS_PAGE_PRODUCT for the default cms page for products
• \Shopware\Core\Content\Category\CategoryDefinition::CONFIG_KEY_DEFAULT_CMS_PAGE_CATEGORY for the default cms page for categories of type product_list

This fallback is loaded at the entity.loaded event of the corresponding entity, where we check if the foreign key is set to NULL and inject the corresponding system config as default.

Consequences

For all related entities, the cmsPageId will be set to null if the default is given. The corresponding cms page id will then be set by subscribers. Therefor the cms page id, which is stored in the database, might differ from the cms page id which can be received via repository.
To use the default functionality, an entity must not have a cms page id assigned to it.