feat(api): Add attributeExists query predicate

packages/amplify_core/lib/src/types/query/query_field.dart

@@ -255,6 +255,27 @@ class QueryField<T> {
   QueryPredicateOperation beginsWith(String value) =>
       QueryPredicateOperation(fieldName, BeginsWithQueryOperator(value));
 
+  /// An **attribute exists** operation.
+  ///
+  /// Matches models whether the given field exists or not.
+  ///
+  /// ### Example:
+  /// The example returns Blog where the optional Author attribute exists.
+  ///
+  /// ```dart
+  /// ModelQueries.list(
+  ///  Blog.classType,
+  ///  where: Blog.AUTHOR.attributeExists(),
+  /// );
+  /// ```
+  QueryPredicateOperation attributeExists({bool exists = true}) =>
+      QueryPredicateOperation(
+        fieldName,
+        AttributeExistsQueryOperator(
+          exists: exists,
+        ),
+      );
+
   /// Sorts models by the given field in ascending order
   ///
   /// ### Example:

packages/amplify_core/lib/src/types/query/query_field_operators.dart

@@ -14,7 +14,8 @@ enum QueryFieldOperatorType {
   greater_than,
   contains,
   between,
-  begins_with
+  begins_with,
+  attribute_exists,
 }
 
 extension QueryFieldOperatorTypeExtension on QueryFieldOperatorType {
@@ -257,3 +258,26 @@ class BeginsWithQueryOperator extends QueryFieldOperatorSingleValue<String> {
     return other.startsWith(value);
   }
 }
+
+class AttributeExistsQueryOperator<T> extends QueryFieldOperator<T> {
+  const AttributeExistsQueryOperator({this.exists = true})
+      : super(QueryFieldOperatorType.attribute_exists);
+
+  final bool exists;
+
+  @override
+  bool evaluate(T? other) {
+    if (exists == true) {
+      return other != null;
+    }
+    return other == null;
+  }
+
+  @override
+  Map<String, dynamic> serializeAsMap() {
+    return <String, dynamic>{
+      'operatorName': QueryFieldOperatorType.attribute_exists.toShortString(),
+      'exists': this.exists,
+    };
+  }
+}

packages/amplify_datastore/example/integration_test/observe_test.dart

@@ -124,5 +124,36 @@ void main() {
       await Amplify.DataStore.delete(updatedBlog);
       await Amplify.DataStore.save(otherBlog);
     });
+
+    testWidgets(
+        'observe with attribute exists query predicate filters out non matches',
+        (WidgetTester tester) async {
+      HasOneChild hasAttribute = HasOneChild(name: 'name - ${uuid()}');
+      HasOneChild hasNoAttribute = HasOneChild();
+
+      var hasAttributeStream = Amplify.DataStore.observe(HasOneChild.classType,
+              where: Blog.NAME.attributeExists())
+          .map((event) => event.item);
+      expectLater(
+        hasAttributeStream,
+        emitsInOrder(
+          [hasAttribute],
+        ),
+      );
+
+      var hasNoAttributeStream = Amplify.DataStore.observe(
+              HasOneChild.classType,
+              where: Blog.NAME.attributeExists(exists: false))
+          .map((event) => event.item);
+      expectLater(
+        hasNoAttributeStream,
+        emitsInOrder(
+          [hasNoAttribute],
+        ),
+      );
+
+      await Amplify.DataStore.save(hasAttribute);
+      await Amplify.DataStore.save(hasNoAttribute);
+    });
   });
 }

packages/amplify_datastore/test/query_predicate_test.dart

@@ -315,6 +315,12 @@ void main() {
       expect(testPredicate.evaluate(post2), isTrue);
     });
 
+    test('attributeExists', () {
+      QueryPredicate testPredicate = Post.LIKECOUNT.attributeExists();
+      expect(testPredicate.evaluate(post4), isFalse);
+      expect(testPredicate.evaluate(post2), isTrue);
+    });
+
     test('Temporal type', () {
       QueryPredicate testPredicate = Post.CREATED.lt(TemporalDateTime(
         DateTime(2020, 01, 01, 12, 00),

packages/api/amplify_api/example/amplify/backend/api/apiintegmultiauth/schema.graphql

@@ -1,21 +1,29 @@
-type Blog @model @auth(rules: [
-  { allow: public, operations: [read], provider: apiKey},
-  { allow: public, operations: [read], provider: iam},
-  { allow: private, operations: [read], provider: iam},
-  { allow: private, operations: [read], provider: userPools},
-  { allow: owner, operations: [create, read, update, delete] }
-]) {
+type Blog
+  @model
+  @auth(
+    rules: [
+      { allow: public, operations: [read], provider: apiKey }
+      { allow: public, operations: [read], provider: iam }
+      { allow: private, operations: [read], provider: iam }
+      { allow: private, operations: [read], provider: userPools }
+      { allow: owner, operations: [create, read, update, delete] }
+    ]
+  ) {
   id: ID!
   name: String!
   posts: [Post] @hasMany(indexName: "byBlog", fields: ["id"])
 }
 
-type Post @model @auth(rules: [
-  { allow: public, operations: [read], provider: iam},
-  { allow: private, operations: [read], provider: iam},
-  { allow: private, operations: [read], provider: userPools},
-  { allow: owner, operations: [create, read, update, delete] }
-]) {
+type Post
+  @model
+  @auth(
+    rules: [
+      { allow: public, operations: [read], provider: iam }
+      { allow: private, operations: [read], provider: iam }
+      { allow: private, operations: [read], provider: userPools }
+      { allow: owner, operations: [create, read, update, delete] }
+    ]
+  ) {
   id: ID!
   title: String!
   rating: Int!
@@ -24,37 +32,41 @@ type Post @model @auth(rules: [
   comments: [Comment] @hasMany(indexName: "byPost", fields: ["id"])
 }
 
-type Comment @model @auth(rules: [
-  { allow: private, operations: [read], provider: iam},
-  { allow: private, operations: [read], provider: userPools},
-  { allow: owner, operations: [create, read, update, delete] }
-]) {
+type Comment
+  @model
+  @auth(
+    rules: [
+      { allow: private, operations: [read], provider: iam }
+      { allow: private, operations: [read], provider: userPools }
+      { allow: owner, operations: [create, read, update, delete] }
+    ]
+  ) {
   id: ID!
   postID: ID! @index(name: "byPost")
   post: Post @belongsTo(fields: ["postID"])
   content: String!
 }
 
-type CpkOneToOneBidirectionalParentCD @model @auth(rules: [
-  { allow: private, provider: iam}
-]) {
+type CpkOneToOneBidirectionalParentCD
+  @model
+  @auth(rules: [{ allow: private, provider: iam }]) {
   customId: ID! @primaryKey(sortKeyFields: ["name"])
   name: String!
   implicitChild: CpkOneToOneBidirectionalChildImplicitCD @hasOne
   explicitChild: CpkOneToOneBidirectionalChildExplicitCD @hasOne
 }
 
-type CpkOneToOneBidirectionalChildImplicitCD @model @auth(rules: [
-  { allow: private, provider: iam}
-]) {
+type CpkOneToOneBidirectionalChildImplicitCD
+  @model
+  @auth(rules: [{ allow: private, provider: iam }]) {
   id: ID! @primaryKey(sortKeyFields: ["name"])
   name: String!
   belongsToParent: CpkOneToOneBidirectionalParentCD @belongsTo
 }
 
-type CpkOneToOneBidirectionalChildExplicitCD @model @auth(rules: [
-  { allow: private, provider: iam}
-]) {
+type CpkOneToOneBidirectionalChildExplicitCD
+  @model
+  @auth(rules: [{ allow: private, provider: iam }]) {
   id: ID! @primaryKey(sortKeyFields: ["name"])
   name: String!
   belongsToParentID: ID
@@ -63,22 +75,41 @@ type CpkOneToOneBidirectionalChildExplicitCD @model @auth(rules: [
     @belongsTo(fields: ["belongsToParentID", "belongsToParentName"])
 }
 
-type OwnerOnly @model @auth(rules: [{allow: owner}]) {
+type OwnerOnly @model @auth(rules: [{ allow: owner }]) {
   id: ID!
   name: String!
-} 
+}
 
 type lowerCase
   @model
   @auth(
     rules: [
-      { allow: public, operations: [read], provider: apiKey },
-      { allow: public, operations: [read], provider: iam },
-      { allow: private, operations: [read], provider: iam },
-      { allow: private, operations: [read], provider: userPools },
+      { allow: public, operations: [read], provider: apiKey }
+      { allow: public, operations: [read], provider: iam }
+      { allow: private, operations: [read], provider: iam }
+      { allow: private, operations: [read], provider: userPools }
       { allow: owner, operations: [create, read, update, delete] }
     ]
   ) {
   id: ID!
   name: String!
-}
+}
+
+type Sample
+  @model
+  @auth(
+    rules: [
+      { allow: public, operations: [read], provider: apiKey }
+      { allow: public, operations: [read], provider: iam }
+      { allow: private, operations: [read], provider: iam }
+      { allow: private, operations: [read], provider: userPools }
+      { allow: owner, operations: [create, read, update, delete] }
+    ]
+  ) {
+  id: ID!
+  name: String
+  number: Int
+  flag: Boolean
+  date: AWSTime
+  rootbeer: Float
+}

packages/api/amplify_api/example/integration_test/graphql/iam_test.dart

@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: Apache-2.0
 import 'dart:async';
 import 'dart:convert';
+import 'dart:math';
 
 import 'package:amplify_api/amplify_api.dart';
 import 'package:amplify_api_example/models/ModelProvider.dart';
@@ -20,6 +21,8 @@ import '../util.dart';
 /// increase past the default limit.
 const _limit = 10000;
 
+const _max = 10000;
+
 void main({bool useExistingTestUser = false}) {
   IntegrationTestWidgetsFlutterBinding.ensureInitialized();
 
@@ -188,6 +191,51 @@ void main({bool useExistingTestUser = false}) {
         expect(postFromResponse?.title, title);
       });
 
+      testWidgets('should return model if attribute exists',
+          (WidgetTester tester) async {
+        // Use same name to scope the query to the created model.
+        final name = 'Lorem Ipsum Test Sample: ${uuid()}';
+        final number = Random().nextInt(_max);
+        await addSamplePartial(
+          name,
+          number: number,
+        );
+        await addSamplePartial(name);
+
+        final existsRequest = ModelQueries.list(
+          Sample.classType,
+          where: Sample.NUMBER.attributeExists().and(Sample.NAME.eq(name)),
+          limit: _limit,
+        );
+
+        final existsResponse = await Amplify.API
+            .query(
+              request: existsRequest,
+            )
+            .response;
+
+        final existsData = existsResponse.data;
+        expect(existsData?.items.length, 1);
+        expect(existsData?.items[0]?.number, number);
+
+        final doesNotExistRequest = ModelQueries.list(
+          Sample.classType,
+          where: Sample.NUMBER
+              .attributeExists(exists: false)
+              .and(Sample.NAME.eq(name)),
+          limit: _limit,
+        );
+        final doesNotExistResponse = await Amplify.API
+            .query(
+              request: doesNotExistRequest,
+            )
+            .response;
+
+        final doesNotExistData = doesNotExistResponse.data;
+        expect(doesNotExistData?.items.length, 1);
+        expect(doesNotExistData?.items[0]?.number, null);
+      });
+
       testWidgets('should copyWith request', (WidgetTester tester) async {
         final title = 'Lorem Ipsum Test Post: ${uuid()}';
         const rating = 0;