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Currently the mapFromStorageRecords() / mapToStorageRecord() methods only allow direct mapping of db column names to field names.
This works because, on load,
new $entity_class(array('field_name' => 'value')) automatically assumes "delta 0, first property".
But this forbids using field types with several properties for base fields, which in turn blocks "single set of field types for base fields & configurable fields".
Discussed with @fago and @plach in Vienna:
We should support both:
- the current default mapping - if "column name = a field name" -> the value gets assigned to delta 0, first property
- a mapping with property names - if "column name = [a field name]__[a property name]" -> the value gets assigned to delta 0, the right property