Missing field and property annotations do not allow conditional conversion [DATAMONGO-889]

[spring-projects-issues]

Emerson Farrugia opened DATAMONGO-889 and commented

This StackOverflow question got me curious about how to control conversion logic using an annotation.

I started digging around ConversionService and MappingMongoConverter code looking for a solution. I eventually landed on ConditionalGenericConverter, which I thought would let me solve conditional conversion using an annotation. The code to convert a Long to a Date based on a @Foo annotation would have looked like this...

public class ConditionalLongToDateConverter implements ConditionalGenericConverter {

    @Override
    public boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType) {

        return sourceType.hasAnnotation(Foo.class);
    }

    @Override
    public Set<ConvertiblePair> getConvertibleTypes() {
        return Collections.singleton(new ConvertiblePair(Long.class, Date.class));
    }

    @Override
    public Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {

        if (source == null) {
            return null;
        }

        return new Date((Long) source);
    }
}

which would have allowed the persistence of a POJO that looked like

public class MyDocument {
   @Id
   private String id;

   @Foo
   private Long timestamp; // gets persisted as a JS Date
}

I assumed the sourceType TypeDescriptor for property timestamp would contain the @Foo annotation, because of this constructor:

/**
 * Create a new type descriptor from a {@link Property}.
 * <p>Use this constructor when a source or target conversion point is a
 * property on a Java class.
 * @param property the property
 */
public TypeDescriptor(Property property) {
     Assert.notNull(property, "Property must not be null");
     this.resolvableType = ResolvableType.forMethodParameter(property.getMethodParameter());
     this.type = this.resolvableType.resolve(property.getType());
     this.annotations = nullSafeAnnotations(property.getAnnotations());
}

I registered the converter using CustomConversions from an AbstractMongoConfiguration subclass and thought I was done. But it all broke.

The problem is that MappingMongoConverter never really gets its hands on a TypeDescriptor for the properties to convert, and calls the GenericConversionService in a way that effectively discards the metadata of those properties. This happens specifically in getPotentiallyConvertedSimpleWrite(), but the code higher up the call stack doesn't help because it doesn't give that method TypeDescriptor arguments to work with either.

Is this intentional? I don't know how rare this use case is, but it seems like there's a lot of capability in Spring Core's conversion code that could be exposed to support smarter conversion if TypeDescriptors are propagated better

---

Affects: 1.4.1 (Codd SR1)

Issue Links:

• DATACMNS-1036 Add API for property specific converters

• DATAMONGO-2328 Add missing type converters for native target type conversion

4 votes, 5 watchers

[christophstrobl]

resolved by #3596