DATACMNS-1318 - Allow inspection of a reference's ultimate target entity.

We now expose what type or PersistentEntity an association points to by trying to match the association's type to identifier types to entities. In case multiple matches are found, we require the user to explicitly declare the target type via @Reference.

Introduced PersistentEntities.of(…) for convenience.

src/main/java/org/springframework/data/annotation/Reference.java

@@ -21,6 +21,8 @@ import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
 
+import org.springframework.core.annotation.AliasFor;
+
 /**
  * Meta-annotation to be used to annotate annotations that mark references to other objects.
  *
@@ -30,4 +32,22 @@ import java.lang.annotation.Target;
 @Retention(RetentionPolicy.RUNTIME)
 @Target(value = { FIELD, METHOD, ANNOTATION_TYPE })
 public @interface Reference {
+
+	/**
+	 * Explicitly define the target type of the reference. Used in case the annotated property is not the target type but
+	 * rather an identifier and/or if that identifier type is not uniquely identifying the target entity.
+	 * 
+	 * @return
+	 */
+	@AliasFor(attribute = "to")
+	Class<?> value() default Class.class;
+
+	/**
+	 * Explicitly define the target type of the reference. Used in case the annotated property is not the target type but
+	 * rather an identifier and/or if that identifier type is not uniquely identifying the target entity.
+	 * 
+	 * @return
+	 */
+	@AliasFor(attribute = "value")
+	Class<?> to() default Class.class;
 }