DATAREST-39 Introduced new property on AttributeMetadata to encapsulate the decision of whether or not a property is nullable. Removed references to JPA-specific classes from everything but the JpaAttributeMetadata class.

spring-data-rest-repository/src/main/java/org/springframework/data/rest/repository/AttributeMetadata.java

@@ -40,6 +40,13 @@ public interface AttributeMetadata {
    */
   Class<?> elementType();
 
+  /**
+   * Whether this attribute can be nulled or not.
+   *
+   * @return
+   */
+  boolean isNullable();
+
   /**
    * Can this attribute look like a {@link Collection}?
    *

spring-data-rest-repository/src/main/java/org/springframework/data/rest/repository/jpa/JpaAttributeMetadata.java

@@ -7,6 +7,8 @@ import java.lang.reflect.Method;
 import java.util.Collection;
 import java.util.Map;
 import java.util.Set;
+import javax.persistence.ManyToOne;
+import javax.persistence.OneToOne;
 import javax.persistence.metamodel.Attribute;
 import javax.persistence.metamodel.EntityType;
 import javax.persistence.metamodel.MapAttribute;
@@ -72,6 +74,18 @@ public class JpaAttributeMetadata implements AttributeMetadata {
             : null);
   }
 
+  @Override public boolean isNullable() {
+    if(hasAnnotation(ManyToOne.class)) {
+      return annotation(ManyToOne.class).optional();
+    }
+
+    if(hasAnnotation(OneToOne.class)) {
+      return annotation(OneToOne.class).optional();
+    }
+
+    return true;
+  }
+
   @Override public boolean isCollectionLike() {
     if(attribute instanceof PluralAttribute) {
       PluralAttribute plattr = (PluralAttribute)attribute;