The FieldValue trait (and its sister abstract class, FieldEnumeration) are meant to help
clean up the use of Projection when dealing with field enumeration classes like
ADAMRecordField.
Projection is a class for turning fields from an Enumeration into a projected Avro Schema object.
In the old way of doing this, we used a "normal" Enumeration. Projection would receive a
collection of Read.Value objects, and use their names (plus the Read.SCHEMA$
static field) to turn them into a Schema for ADAMRecords. This worked fine for Read,
and not at all for generalizing to other Schemas over other field enumerations.
In the new system, we embed the Avro Schema object as an argument *in* each enumeration Value.
We do this in two steps:
(1) instead of ADAMRecordField (e.g.) extending Enumeration, it extends FieldEnumeration and
provides the appropriate (static) Schema object as an argument.
(2) instead of using the (final, non-overrideable) Value method within Enumeration to provide
each enum value, it calls FieldEnumeration.SchemaValue instead, which embeds the corresponding
Schema in each value.
Finally, Projection will extract the Schema value from the first FieldValue that is given to it
and produce the corresponding (projected) Schema.
This means, of course, that Projection can't handle empty field lists -- but that was always
going to be an error-filled edge-case anyway (why would you want to project to zero fields?)
The FieldValue trait (and its sister abstract class, FieldEnumeration) are meant to help clean up the use of Projection when dealing with field enumeration classes like ADAMRecordField.
Projection is a class for turning fields from an Enumeration into a projected Avro Schema object.
In the old way of doing this, we used a "normal" Enumeration. Projection would receive a collection of Read.Value objects, and use their names (plus the Read.SCHEMA$ static field) to turn them into a Schema for ADAMRecords. This worked fine for Read, and not at all for generalizing to other Schemas over other field enumerations.
In the new system, we embed the Avro Schema object as an argument *in* each enumeration Value. We do this in two steps: (1) instead of ADAMRecordField (e.g.) extending Enumeration, it extends FieldEnumeration and provides the appropriate (static) Schema object as an argument. (2) instead of using the (final, non-overrideable) Value method within Enumeration to provide each enum value, it calls FieldEnumeration.SchemaValue instead, which embeds the corresponding Schema in each value.
Finally, Projection will extract the Schema value from the first FieldValue that is given to it and produce the corresponding (projected) Schema.
This means, of course, that Projection can't handle empty field lists -- but that was always going to be an error-filled edge-case anyway (why would you want to project to zero fields?)