fix empty object emitted for optional property with only null values

xtraplatform-features/src/main/java/de/ii/xtraplatform/features/domain/FeatureTokenTransformerRemoveEmptyOptionals.java

@@ -107,7 +107,7 @@ public void onGeometry(ModifiableContext<FeatureSchema, SchemaMapping> context)
     }
 
     if (Objects.nonNull(context.geometry())
-        || context.schema().get().isRequired()
+        || isEffectivelyRequired(context)
         || !removeNullValues.getOrDefault(context.type(), true)) {
       openIfNecessary(context);
 
@@ -122,14 +122,35 @@ public void onValue(ModifiableContext<FeatureSchema, SchemaMapping> context) {
     }
 
     if (Objects.nonNull(context.value())
-        || context.schema().get().isRequired()
+        || isEffectivelyRequired(context)
         || !removeNullValues.getOrDefault(context.type(), true)) {
       openIfNecessary(context);
 
       super.onValue(context);
     }
   }
 
+  /**
+   * A null value is kept only if its property is effectively required, i.e. the property itself and
+   * every wrapping property up to (but excluding) the feature are required. A required property
+   * inside an optional object must not keep that object alive when all of its values are null: in
+   * that case the optional wrapper, together with the null value, is removed. Without this check
+   * the wrapping property is only ever looked at indirectly, so an optional object whose required
+   * sub-properties are all null is emitted as an empty object instead of being omitted.
+   */
+  private boolean isEffectivelyRequired(ModifiableContext<FeatureSchema, SchemaMapping> context) {
+    if (context.schema().isEmpty() || !context.schema().get().isRequired()) {
+      return false;
+    }
+
+    List<FeatureSchema> parentSchemas = context.parentSchemas();
+
+    return parentSchemas.size() <= 1
+        || parentSchemas.stream()
+            .limit(parentSchemas.size() - 1L)
+            .allMatch(FeatureSchema::isRequired);
+  }
+
   private void openIfNecessary(ModifiableContext<FeatureSchema, SchemaMapping> context) {
     if (!schemaStack.isEmpty()) {
       List<String> previousPath = context.path();

xtraplatform-features/src/test/groovy/de/ii/xtraplatform/features/domain/FeatureTokenTransformerRemoveEmptyOptionalsSpec.groovy

@@ -88,6 +88,34 @@ class FeatureTokenTransformerRemoveEmptyOptionalsSpec extends Specification {
 
     }
 
+    def 'single feature optional object with only null required sub-properties'() {
+
+        given:
+
+        when:
+
+        FeatureTokenFixtures.SINGLE_FEATURE_NESTED_OBJECT_REQUIRED_NULLS.forEach(token -> tokenReader.onToken(token))
+
+        then:
+
+        tokens == FeatureTokenFixtures.SINGLE_FEATURE
+
+    }
+
+    def 'single feature optional object with a non-null and a null required sub-property'() {
+
+        given:
+
+        when:
+
+        FeatureTokenFixtures.SINGLE_FEATURE_NESTED_OBJECT_REQUIRED_NULLS_PARTIAL.forEach(token -> tokenReader.onToken(token))
+
+        then:
+
+        tokens == FeatureTokenFixtures.SINGLE_FEATURE_NESTED_OBJECT_REQUIRED_NULLS_PARTIAL_REDUCED
+
+    }
+
     def 'single feature value array'() {
 
         given:

xtraplatform-features/src/testFixtures/groovy/de/ii/xtraplatform/features/domain/FeatureSchemaFixtures.groovy

@@ -429,6 +429,23 @@ class FeatureSchemaFixtures {
                                     .required(true)
                                     .build())
                             .sourcePath("name"))
+            .putProperties2("optional_with_required",
+                    new ImmutableFeatureSchema.Builder()
+                            .type(Type.OBJECT)
+                            .putProperties2("numerator",
+                                    new ImmutableFeatureSchema.Builder()
+                                            .type(Type.FLOAT)
+                                            .constraints(new ImmutableSchemaConstraints.Builder()
+                                                    .required(true)
+                                                    .build())
+                                            .sourcePath("numerator"))
+                            .putProperties2("denominator",
+                                    new ImmutableFeatureSchema.Builder()
+                                            .type(Type.FLOAT)
+                                            .constraints(new ImmutableSchemaConstraints.Builder()
+                                                    .required(true)
+                                                    .build())
+                                            .sourcePath("denominator")))
             .build()
 
     public static final SchemaMapping BIOTOP_MAPPING = new ImmutableSchemaMapping.Builder()

xtraplatform-features/src/testFixtures/groovy/de/ii/xtraplatform/features/domain/FeatureTokenFixtures.groovy

@@ -227,6 +227,86 @@ class FeatureTokenFixtures {
             FeatureTokenType.INPUT_END
     ]
 
+    public static final List<Object> SINGLE_FEATURE_NESTED_OBJECT_REQUIRED_NULLS = [
+            FeatureTokenType.INPUT,
+            true,
+            FeatureTokenType.FEATURE,
+            FeatureTokenType.VALUE,
+            ["id"],
+            "24",
+            Type.STRING,
+            FeatureTokenType.OBJECT,
+            ["optional_with_required"],
+            FeatureTokenType.VALUE,
+            ["optional_with_required", "numerator"],
+            null,
+            Type.FLOAT,
+            FeatureTokenType.VALUE,
+            ["optional_with_required", "denominator"],
+            null,
+            Type.FLOAT,
+            FeatureTokenType.OBJECT_END,
+            ["optional_with_required"],
+            FeatureTokenType.VALUE,
+            ["kennung"],
+            "611320001-1",
+            Type.STRING,
+            FeatureTokenType.FEATURE_END,
+            FeatureTokenType.INPUT_END
+    ]
+
+    public static final List<Object> SINGLE_FEATURE_NESTED_OBJECT_REQUIRED_NULLS_PARTIAL = [
+            FeatureTokenType.INPUT,
+            true,
+            FeatureTokenType.FEATURE,
+            FeatureTokenType.VALUE,
+            ["id"],
+            "24",
+            Type.STRING,
+            FeatureTokenType.OBJECT,
+            ["optional_with_required"],
+            FeatureTokenType.VALUE,
+            ["optional_with_required", "numerator"],
+            "0.5",
+            Type.FLOAT,
+            FeatureTokenType.VALUE,
+            ["optional_with_required", "denominator"],
+            null,
+            Type.FLOAT,
+            FeatureTokenType.OBJECT_END,
+            ["optional_with_required"],
+            FeatureTokenType.VALUE,
+            ["kennung"],
+            "611320001-1",
+            Type.STRING,
+            FeatureTokenType.FEATURE_END,
+            FeatureTokenType.INPUT_END
+    ]
+
+    public static final List<Object> SINGLE_FEATURE_NESTED_OBJECT_REQUIRED_NULLS_PARTIAL_REDUCED = [
+            FeatureTokenType.INPUT,
+            true,
+            FeatureTokenType.FEATURE,
+            FeatureTokenType.VALUE,
+            ["id"],
+            "24",
+            Type.STRING,
+            FeatureTokenType.OBJECT,
+            ["optional_with_required"],
+            FeatureTokenType.VALUE,
+            ["optional_with_required", "numerator"],
+            "0.5",
+            Type.FLOAT,
+            FeatureTokenType.OBJECT_END,
+            ["optional_with_required"],
+            FeatureTokenType.VALUE,
+            ["kennung"],
+            "611320001-1",
+            Type.STRING,
+            FeatureTokenType.FEATURE_END,
+            FeatureTokenType.INPUT_END
+    ]
+
     public static final List<Object> SINGLE_FEATURE_VALUE_ARRAY = [
             FeatureTokenType.INPUT,
             true,