Support Unique conflict resolution strategy replace.

generator/lib/src/entity_resolver.dart

@@ -257,6 +257,11 @@ class EntityResolver extends Builder {
       }
     }
 
+    // Verify there is at most 1 unique property with REPLACE strategy.
+    ensureSingleUniqueReplace(entity);
+    // If sync enabled, verify all unique properties use REPLACE strategy.
+    ifSyncEnsureAllUniqueAreReplace(entity);
+
     entity.properties.forEach((p) => log.info('  $p'));
 
     return entity;
@@ -301,9 +306,9 @@ class EntityResolver extends Builder {
       FieldElement f, int? fieldType, Element elementBare, ModelProperty prop) {
     IndexType? indexType;
 
-    final hasIndexAnnotation = _indexChecker.hasAnnotationOfExact(f);
-    final hasUniqueAnnotation = _uniqueChecker.hasAnnotationOfExact(f);
-    if (!hasIndexAnnotation && !hasUniqueAnnotation) return null;
+    final indexAnnotation = _indexChecker.firstAnnotationOfExact(f);
+    final uniqueAnnotation = _uniqueChecker.firstAnnotationOfExact(f);
+    if (indexAnnotation == null && uniqueAnnotation == null) return null;
 
     // Throw if property type does not support any index.
     if (fieldType == OBXPropertyType.Float ||
@@ -319,8 +324,6 @@ class EntityResolver extends Builder {
     }
 
     // If available use index type from annotation.
-    final indexAnnotation =
-        hasIndexAnnotation ? _indexChecker.firstAnnotationOfExact(f) : null;
     if (indexAnnotation != null && !indexAnnotation.isNull) {
       final enumValItem = enumValueItem(indexAnnotation.getField('type')!);
       if (enumValItem != null) indexType = IndexType.values[enumValItem];
@@ -343,8 +346,15 @@ class EntityResolver extends Builder {
           "entity ${elementBare.name}: a hash index is not supported for type '${f.type}' of field '${f.name}'");
     }
 
-    if (hasUniqueAnnotation) {
+    if (uniqueAnnotation != null && !uniqueAnnotation.isNull) {
       prop.flags |= OBXPropertyFlags.UNIQUE;
+      // Determine unique conflict resolution.
+      final onConflictVal =
+          enumValueItem(uniqueAnnotation.getField('onConflict')!);
+      if (onConflictVal != null &&
+          ConflictStrategy.values[onConflictVal] == ConflictStrategy.replace) {
+        prop.flags |= OBXPropertyFlags.UNIQUE_ON_CONFLICT_REPLACE;
+      }
     }
 
     switch (indexType) {
@@ -363,6 +373,27 @@ class EntityResolver extends Builder {
     }
   }
 
+  void ensureSingleUniqueReplace(ModelEntity entity) {
+    final uniqueReplaceProps = entity.properties
+        .where((p) => p.hasFlag(OBXPropertyFlags.UNIQUE_ON_CONFLICT_REPLACE));
+    if (uniqueReplaceProps.length > 1) {
+      throw InvalidGenerationSourceError(
+          "ConflictStrategy.replace can only be used on a single property, but found multiple in '${entity.name}':\n  ${uniqueReplaceProps.join('\n  ')}");
+    }
+  }
+
+  void ifSyncEnsureAllUniqueAreReplace(ModelEntity entity) {
+    if (!entity.hasFlag(OBXEntityFlags.SYNC_ENABLED)) return;
+    final uniqueButNotReplaceProps = entity.properties.where((p) {
+      return p.hasFlag(OBXPropertyFlags.UNIQUE) &&
+          !p.hasFlag(OBXPropertyFlags.UNIQUE_ON_CONFLICT_REPLACE);
+    });
+    if (uniqueButNotReplaceProps.isNotEmpty) {
+      throw InvalidGenerationSourceError(
+          "Synced entities must use @Unique(onConflict: ConflictStrategy.replace) on all unique properties, but found others in '${entity.name}':\n  ${uniqueButNotReplaceProps.join('\n  ')}");
+    }
+  }
+
   int? enumValueItem(DartObject typeField) {
     if (!typeField.isNull) {
       final enumValues = (typeField.type as InterfaceType)

objectbox/CHANGELOG.md

@@ -1,3 +1,8 @@
+## latest
+
+* Support annotating a single property with `@Unique(onConflict: ConflictStrategy.replace)` to 
+  replace an existing object if a conflict occurs when doing a put. #297
+
 ## 1.2.1 (2021-11-09)
 
 * Fix Flutter apps crashing on iOS 15 simulator. #313

objectbox/lib/src/annotations.dart

@@ -1,3 +1,5 @@
+import 'package:objectbox/objectbox.dart';
+
 /// Entity annotation is used on a class to let ObjectBox know it should store
 /// it - making the class a "persistable Entity".
 ///
@@ -190,17 +192,31 @@ enum IndexType {
   hash64,
 }
 
-/// Unique annotation forces that the value of a property is unique among all
-/// objects stored for the given entity.
+/// Enforces that the value of a property is unique among all objects in a box
+/// before an object can be put.
 ///
-/// Trying to put an Object with offending values will result in an exception.
+/// Trying to put an object with offending values will result in a
+/// [UniqueViolationException] (see [ConflictStrategy.fail]).
+/// Set [onConflict] to change this strategy.
 ///
-/// Unique properties are based on an [Index], so the same restrictions apply.
+/// Note: Unique properties are based on an [Index], so the same restrictions apply.
 /// It is supported to explicitly add the [Index] annotation to configure the
-/// index type.
+/// index.
 class Unique {
+  /// The strategy to use when a conflict is detected when an object is put.
+  final ConflictStrategy onConflict;
+
   /// Create a Unique annotation.
-  const Unique();
+  const Unique({this.onConflict = ConflictStrategy.fail});
+}
+
+/// Used with [Unique] to specify the conflict resolution strategy.
+enum ConflictStrategy {
+  /// Throws [UniqueViolationException] if any property violates a [Unique] constraint.
+  fail,
+
+  /// Any conflicting objects are deleted before the object is inserted.
+  replace,
 }
 
 /// Backlink annotation specifies a link in a reverse direction of another

objectbox/lib/src/modelinfo/enums.dart

@@ -122,6 +122,9 @@ abstract class OBXPropertyFlags {
   /// ///
   /// /// For Time Series IDs, a companion property of type Date or DateNano represents the exact timestamp.
   static const int ID_COMPANION = 16384;
+
+  /// Unique on-conflict strategy: the object being put replaces any existing conflicting object (deletes it).
+  static const int UNIQUE_ON_CONFLICT_REPLACE = 32768;
 }
 
 abstract class OBXPropertyType {

objectbox/test/box_test.dart

@@ -203,6 +203,43 @@ void main() {
             e.toString().contains('same property value already exists'))));
   });
 
+  test('.put() replaces duplicate values on a unique replace field on insert', () {
+    // insert without conflict
+    box.putMany([
+      TestEntity.uniqueReplace(replaceLong: 1, tString: 'original-1'),
+      TestEntity.uniqueReplace(replaceLong: 2, tString: 'original-2')
+    ]);
+    expect(box.count(), equals(2));
+
+    // insert with conflict, deletes ID 1 and inserts ID 3
+    box.put(TestEntity.uniqueReplace(replaceLong: 1, tString: 'replacement-1'));
+    expect(box.count(), equals(2));
+    final replaced = box.get(3)!;
+    expect(replaced.replaceLong, equals(1));
+    expect(replaced.tString, equals('replacement-1'));
+  });
+
+  test('.put() replaces duplicate values on a unique replace field on update', () {
+    // update without conflict
+    var first = TestEntity.uniqueReplace(replaceLong: 1, tString: 'first');
+    box.put(first);
+    first.replaceLong = 2;
+    box.put(first);
+    expect(box.count(), equals(1));
+    final updated = box.get(1)!;
+    expect(updated.replaceLong, equals(2));
+    expect(updated.tString, 'first');
+
+    // update with conflict, deletes ID 2 and keeps ID 1
+    box.put(TestEntity.uniqueReplace(replaceLong: 1, tString: 'second'));
+    first.replaceLong = 1;
+    box.put(first);
+    expect(box.count(), equals(1));
+    final updated2 = box.get(1)!;
+    expect(updated2.replaceLong, equals(1));
+    expect(updated2.tString, 'first');
+  });
+
   test('.getAll retrieves all items', () {
     final int id1 = box.put(TestEntity(tString: 'One'));
     final int id2 = box.put(TestEntity(tString: 'Two'));

objectbox/test/entity.dart

@@ -137,6 +137,11 @@ class TestEntity {
     this.uChar,
   }) : tString = '';
 
+  @Unique(onConflict: ConflictStrategy.replace)
+  int? replaceLong;
+
+  TestEntity.uniqueReplace({this.replaceLong, this.tString});
+
   @Property(type: PropertyType.byte)
   @Index()
   int? iByte;

objectbox/test/objectbox-model.json

@@ -5,7 +5,7 @@
   "entities": [
     {
       "id": "1:4630700155272683157",
-      "lastPropertyId": "34:3975438751767916074",
+      "lastPropertyId": "35:1724663621433823504",
       "name": "TestEntity",
       "properties": [
         {
@@ -189,6 +189,13 @@
           "id": "34:3975438751767916074",
           "name": "tDateNano",
           "type": 12
+        },
+        {
+          "id": "35:1724663621433823504",
+          "name": "replaceLong",
+          "type": 6,
+          "flags": 32808,
+          "indexId": "20:4846837430056399798"
         }
       ],
       "relations": [
@@ -523,7 +530,7 @@
     }
   ],
   "lastEntityId": "10:8814538095619551454",
-  "lastIndexId": "19:3009172190024929732",
+  "lastIndexId": "20:4846837430056399798",
   "lastRelationId": "1:2155747579134420981",
   "lastSequenceId": "0:0",
   "modelVersion": 5,