Generator: for @Backlink support specifying ToOne target ID property #160

To support backlinks to a ToOne with a renamed target ID property, also
search for target ID ("relation") properties without adding the "Id"
suffix.

Also limit search to target ID ("relation") properties only. And improve
the error message with details and suggestions.

Author: greenrobot-team

generator/lib/src/code_builder.dart

@@ -429,11 +429,23 @@ class CodeBuilder extends Builder {
         srcRel = matchingRels.first;
       }
     } else {
-      srcProp = srcEntity.findPropertyByName('${bl.srcField}Id');
+      // For backwards compatibility, expect the name of the ToOne field, so add
+      // an "Id" suffix to find the target ID property.
+      srcProp = srcEntity.properties.firstWhereOrNull(
+        (p) => p.isRelation && p.name == '${bl.srcField}Id',
+      );
+      // Otherwise, and to support ToOne with renamed target ID properties
+      // (using @TargetIdProperty), expect the name of the target ID property.
+      srcProp ??= srcEntity.properties.firstWhereOrNull(
+        (p) => p.isRelation && p.name == bl.srcField,
+      );
+      // For ToMany, always expect the name of the ToMany field/relation
       srcRel = srcEntity.relations.firstWhereOrNull(
         (r) => r.name == bl.srcField,
       );
 
+      // This should be impossible as it would mean a ToOne and a ToMany field
+      // share the same name, but check just in case.
       if (srcProp != null && srcRel != null) {
         throwAmbiguousError([srcProp], [srcRel]);
       }
@@ -445,7 +457,11 @@ class CodeBuilder extends Builder {
       return BacklinkSourceProperty(srcProp);
     } else {
       throw InvalidGenerationSourceError(
-        "Unknown relation backlink source for '${entity.name}.${bl.name}'",
+        'Failed to find backlink source for "${entity.name}.${bl.name}" in "${srcEntity.name}", '
+        'make sure a matching ToOne or ToMany relation exists.'
+        ' If the ToOne target ID property is renamed with @TargetIdProperty,'
+        ' then make sure to specify the name of the target ID property instead of the ToOne,'
+        " like @Backlink('<property>').",
       );
     }
   }

generator/test/code_builder_test.dart

@@ -367,6 +367,94 @@ void main() {
       );
     });
 
+    test('Errors if backlink source does not exist', () async {
+      final source = r'''
+      library example;     
+      import 'package:objectbox/objectbox.dart';
+      
+      @Entity()
+      class Example {
+        @Id()
+        int id = 0;
+      }
+       
+      @Entity()
+      class A {
+        @Id()
+        int id = 0;
+        
+        @Backlink()
+        final backRel = ToMany<Example>();
+      }
+      ''';
+
+      final testEnv = GeneratorTestEnv();
+      final result = await testEnv.run(source, ignoreOutput: true);
+
+      expect(result.builderResult.succeeded, false);
+      expect(
+        result.logs,
+        contains(
+          isA<LogRecord>()
+              .having((r) => r.level, 'level', Level.SEVERE)
+              .having(
+                (r) => r.message,
+                'message',
+                contains(
+                  'Failed to find backlink source for "A.backRel" in "Example"',
+                ),
+              ),
+        ),
+      );
+    });
+
+    test(
+      'Does not pick implicit backlink source if explicit one does not exist',
+      () async {
+        final source = r'''
+      library example;     
+      import 'package:objectbox/objectbox.dart';
+      
+      @Entity()
+      class Example {
+        @Id()
+        int id = 0;
+        
+        final relA1 = ToOne<A>();
+        final relA2 = ToMany<A>();
+      }
+       
+      @Entity()
+      class A {
+        @Id()
+        int id = 0;
+        
+        @Backlink('doesnotexist')
+        final backRel = ToMany<Example>();
+      }
+      ''';
+
+        final testEnv = GeneratorTestEnv();
+        final result = await testEnv.run(source, ignoreOutput: true);
+
+        expect(result.builderResult.succeeded, false);
+        expect(
+          result.logs,
+          contains(
+            isA<LogRecord>()
+                .having((r) => r.level, 'level', Level.SEVERE)
+                .having(
+                  (r) => r.message,
+                  'message',
+                  contains(
+                    'Failed to find backlink source for "A.backRel" in "Example"',
+                  ),
+                ),
+          ),
+        );
+      },
+    );
+
     test('@TargetIdProperty ToOne annotation', () async {
       final source = r'''
       library example;     
@@ -394,6 +482,45 @@ void main() {
       expect(renamedRelationProperty!.type, OBXPropertyType.Relation);
     });
 
+    test('Explicit backlink to renamed ToOne target ID property', () async {
+      final source = r'''
+      library example;     
+      import 'package:objectbox/objectbox.dart';
+      
+      @Entity()
+      class Example {
+        @Id()
+        int id = 0;
+        
+        @TargetIdProperty('customerRef')
+        final customer = ToOne<Customer>();
+      }
+      
+      @Entity()
+      class Customer {
+        @Id()
+        int id = 0;
+        
+        // Must specify the target ID property instead of the ToOne field
+        @Backlink('customerRef')
+        final backRel = ToMany<Example>();
+      }
+      ''';
+
+      final testEnv = GeneratorTestEnv();
+      await testEnv.run(source);
+
+      final customerEntity = testEnv.model.entities.firstWhere(
+        (e) => e.name == 'Customer',
+      );
+      var backlinkSource = customerEntity.backlinks.first.source;
+      expect(backlinkSource, isA<BacklinkSourceProperty>());
+      expect(
+        (backlinkSource as BacklinkSourceProperty).srcProp.relationField,
+        'customer',
+      );
+    });
+
     test('ToOne target ID property name conflict', () async {
       // Note: unlike in Java, for Dart it's also not supported to "expose" the
       // target ID (relation) property.