How to deal with model content polymorphism

Hi

I don't know if it's a real issue, but I can't find a way to deal with polymorphic content in RepresentationModel.

Let's say I have an abstract Shape class, extended by Square and Circle.
My API will expose an EntityModel< Shape > , and the clients should be able to get the concrete content type.
Made some tests, without success..

The JsonSubType field is ignored when the content is serialized.
Any way to make it works ?
Thanks

import com.fasterxml.jackson.annotation.JsonSubTypes;
import com.fasterxml.jackson.annotation.JsonTypeInfo;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
import lombok.Data;
import org.junit.Test;
import org.springframework.hateoas.EntityModel;
import org.springframework.hateoas.IanaLinkRelations;
import org.springframework.hateoas.Link;
import org.springframework.hateoas.mediatype.MessageResolver;
import org.springframework.hateoas.mediatype.hal.CurieProvider;
import org.springframework.hateoas.mediatype.hal.Jackson2HalModule;
import org.springframework.hateoas.server.core.AnnotationLinkRelationProvider;

import static org.junit.Assert.*;

public class TestSpringHateoas {

    @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "@type")
    @JsonSubTypes({
            @JsonSubTypes.Type(value = Circle.class, name = "CIRCLE"),
            @JsonSubTypes.Type(value = Square.class, name = "SQUARE")
    })
    @Data
    public class Shape {
        private float x;
        private float y;
    }

    @Data
    public class Circle extends Shape {
        private float radius;
    }

    public class Square extends Shape {
        private float size;
    }

    @Test
    public void tests() throws Exception {

        var mapper = new ObjectMapper();
        mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
        mapper.setHandlerInstantiator(
                new Jackson2HalModule.HalHandlerInstantiator(new AnnotationLinkRelationProvider(), CurieProvider.NONE, MessageResolver.DEFAULTS_ONLY));


        // create a circle
        var circle = new Circle();
        circle.setX(10f);
        circle.setY(10f);
        circle.setRadius(5f);
        assertEquals("{\"@type\":\"CIRCLE\",\"x\":10.0,\"y\":10.0,\"radius\":5.0}", mapper.writeValueAsString(circle));

        // instantiate and serialize a circle
        var circleModel = EntityModel.of(circle, Link.of("http://host/shape", IanaLinkRelations.SELF_VALUE));
        var circleModelAsJson = mapper.writeValueAsString(circleModel);
        // => missing '@type'
        assertEquals("{\"x\":10.0,\"y\":10.0,\"radius\":5.0,\"links\":[{\"rel\":\"self\",\"href\":\"http://host/shape\"}]}", circleModelAsJson);

        // try to deserialize as a shape (clients do not know the concrete type)
        var shapeModel = mapper.readValue(circleModelAsJson, new TypeReference<EntityModel<Shape>>() {});
        assertNotNull(shapeModel);
        assertTrue(shapeModel.getContent() instanceof Circle);
    }

}

Start by reproducing the Java example around EntityModel, Jackson2HalModule, and HalHandlerInstantiator. Compare direct Circle serialization with serialization through EntityModel, then check deserialization as EntityModel. Done means the polymorphic @type field is retained and the content is restored as Circle.