no-mistakes(review): Fix reverse-hop message, doc and Java isAbstract dup

Author: dmealing

docs/features/relationships.md

@@ -173,10 +173,12 @@ reverse-engineered model can `@via` its reference directly:
 ```
 
 A reference hop is inherently **to-one** (a child names the parent it points at), so it
-is valid in a `passthrough` and rejected in an `aggregate`. Inverse navigation
-(parent → many children) still needs a `relationship.composition` on the parent — a bare
-FK has no inverse edge. Explicit `@via` resolves either kind; single-hop-unique
-inference stays relationship-only.
+is valid in a `passthrough` and rejected in an `aggregate`. An origin `@via` hop may
+also walk an `identity.reference` **backwards** (parent → many children), named by the
+holder's collection name in lowerCamel (`Patient.consents`). A reverse hop is to-many, so
+it is valid in an `aggregate` and rejected in a `passthrough` (`ERR_ORIGIN_CARDINALITY`);
+two candidate holders give `ERR_AMBIGUOUS_PATH`, and a declared relationship or reference
+of the same name wins. Single-hop-unique inference stays relationship-only.
 
 ## When one entity has two references to the same target
 

server/csharp/MetaObjects/Loader/ValidationPasses.cs

@@ -1368,7 +1368,10 @@ private static void CheckPassthroughCardinality(
             {
                 errors.Add(new MetaError(
                     $"origin.passthrough on {obj.Name}.{fieldName}: @via hop \"{rel.Name}\" is to-many " +
-                    $"(@cardinality \"{CARDINALITY_MANY}\") — a row-multiplying passthrough — you meant aggregate (ADR-0029).",
+                    (reverse?.Contains(rel) == true
+                        ? $"(reverse hop over identity.reference \"{rel.Name}\")"
+                        : $"(@cardinality \"{CARDINALITY_MANY}\")") +
+                    " — a row-multiplying passthrough — you meant aggregate (ADR-0029).",
                     ErrorCode.ERR_ORIGIN_CARDINALITY,
                     Envelope: originSource));
                 return;

server/java/metadata/src/main/java/com/metaobjects/loader/ReverseReferenceHops.java

@@ -39,7 +39,7 @@ public static String hopName(MetaObject holder) {
     public static List<Hop> find(MetaObject target, String hopName, List<MetaObject> objects) {
         List<Hop> found = new ArrayList<>();
         for (MetaObject holder : objects) {
-            if (isAbstract(holder) || !hopName.equals(hopName(holder))) continue;
+            if (ValidationPhase.isAbstract(holder) || !hopName.equals(hopName(holder))) continue;
             // ADR-0039: resolving — a reference may be inherited through extends.
             for (MetaData c : holder.getChildren(MetaData.class, true)) {
                 if (!(c instanceof MetaIdentity) || !MetaIdentity.SUBTYPE_REFERENCE.equals(c.getSubType())) continue;
@@ -51,11 +51,4 @@ public static List<Hop> find(MetaObject target, String hopName, List<MetaObject>
         }
         return found;
     }
-
-    private static boolean isAbstract(MetaData node) {
-        // ADR-0039: own — abstractness is a declaration-layer marker.
-        if (!node.hasMetaAttr(MetaData.ATTR_IS_ABSTRACT, false)) return false;
-        Object v = node.getMetaAttr(MetaData.ATTR_IS_ABSTRACT, false).getValue();
-        return v instanceof Boolean ? (Boolean) v : v instanceof String && "true".equalsIgnoreCase((String) v);
-    }
 }

server/java/metadata/src/main/java/com/metaobjects/loader/ValidationPhase.java

@@ -4040,7 +4040,7 @@ private static boolean subtypeSupportsFiltering(String st) {
      * boolean-true. Reads only the own attribute (not effective) — matches the
      * own-only validation contract used throughout this class.
      */
-    private static boolean isAbstract(MetaData node) {
+    static boolean isAbstract(MetaData node) {
         if (!node.hasMetaAttr(MetaData.ATTR_IS_ABSTRACT, false)) {
             return false;
         }
@@ -4924,7 +4924,10 @@ private static void checkPassthroughCardinality(java.util.List<MetaData> hops,
                 throw new MetaDataException(
                     "ERR_ORIGIN_CARDINALITY"
                         + ": origin.passthrough on " + obj.getName() + "." + fieldName
-                        + ": @via hop \"" + rel.getName() + "\" is to-many — you meant aggregate (ADR-0029).",
+                        + ": @via hop \"" + rel.getName() + "\" is to-many"
+                        + (reverse != null && reverse.contains(rel)
+                            ? " (reverse hop over identity.reference \"" + rel.getName() + "\")" : "")
+                        + " — you meant aggregate (ADR-0029).",
                     ErrorCode.ERR_ORIGIN_CARDINALITY, originSource);
             }
         }

server/python/src/metaobjects/loader/validation_passes.py

@@ -1993,8 +1993,12 @@ def _check_passthrough_cardinality(
             errors.append(
                 MetaError(
                     f"origin.passthrough on {field_name}: @via hop '{rel.name}' is to-many "
-                    f'(@cardinality "{CARDINALITY_MANY}") — a row-multiplying passthrough — '
-                    f"you meant aggregate (ADR-0029).",
+                    + (
+                        f'(reverse hop over identity.reference "{rel.name}")'
+                        if id(rel) in reverse
+                        else f'(@cardinality "{CARDINALITY_MANY}")'
+                    )
+                    + " — a row-multiplying passthrough — you meant aggregate (ADR-0029).",
                     ErrorCode.ERR_ORIGIN_CARDINALITY,
                     envelope=origin_source,
                 )

server/typescript/packages/metadata/src/loader/validation-passes.ts

@@ -1063,7 +1063,10 @@ function _checkPassthroughCardinality(
       errors.push(
         new ParseError(
           `origin.passthrough on ${obj.name}.${fieldName}: @via hop "${rel.name}" is to-many ` +
-            `(@cardinality "${CARDINALITY_MANY}") — a row-multiplying passthrough — you meant aggregate (ADR-0029).`,
+            (reverse?.has(rel)
+              ? `(reverse hop over identity.reference "${rel.name}")`
+              : `(@cardinality "${CARDINALITY_MANY}")`) +
+            ` — a row-multiplying passthrough — you meant aggregate (ADR-0029).`,
           { code: "ERR_ORIGIN_CARDINALITY", source: originSource },
         ),
       );