Updated: Comment #41
Problem/Motivation
- Mapping field item properties to DB storage record is hardcoded in \Drupal\Core\Entity\Sql\SqlContentEntityStorage.
- It contains special cases for the serialize key of FieldItemInterface::schema, and a special case of this special case for MapItem field (where not a single property but all properties are serialized).
- Issues like #2563843: MapItem is broken for configurable fields demand to add more special cases
- In the context of the JSON storage initiatives (like #3276818: [META] Add support for JSON field queries in entity queries), a proper JSON map field adds more requirements to that.
Time to add an API so that fields can specify how their properties are mapped to their schema columns.
Proposed resolution
- Allow field type classes to define how their values should be (un)serialized.
- Remove the serialize key from MapItem's schema and use this mechanism instead.
- Deprecate the serialize key in a followup.
Remaining tasks
- Decide on how this API should work, code, review, commit.
User interface changes
None.
API changes
Addition: Field items can specify how they are mapped to storage and possibly serialized or JSON encoded.