Always emit included array in response when query string contains include parameter

src/JsonApiDotNetCore/Serialization/Building/IncludedResourceObjectBuilder.cs

@@ -6,6 +6,7 @@
 using JsonApiDotNetCore.Configuration;
 using JsonApiDotNetCore.Queries;
 using JsonApiDotNetCore.Queries.Internal;
+using JsonApiDotNetCore.QueryStrings;
 using JsonApiDotNetCore.Resources;
 using JsonApiDotNetCore.Resources.Annotations;
 using JsonApiDotNetCore.Serialization.Objects;
@@ -19,22 +20,25 @@ public class IncludedResourceObjectBuilder : ResourceObjectBuilder, IIncludedRes
         private readonly IFieldsToSerialize _fieldsToSerialize;
         private readonly ILinkBuilder _linkBuilder;
         private readonly IResourceDefinitionAccessor _resourceDefinitionAccessor;
+        private readonly IRequestQueryStringAccessor _queryStringAccessor;
         private readonly SparseFieldSetCache _sparseFieldSetCache;
 
         public IncludedResourceObjectBuilder(IFieldsToSerialize fieldsToSerialize, ILinkBuilder linkBuilder, IResourceContextProvider resourceContextProvider,
             IEnumerable<IQueryConstraintProvider> constraintProviders, IResourceDefinitionAccessor resourceDefinitionAccessor,
-            IResourceObjectBuilderSettingsProvider settingsProvider)
+            IRequestQueryStringAccessor queryStringAccessor, IResourceObjectBuilderSettingsProvider settingsProvider)
             : base(resourceContextProvider, settingsProvider.Get())
         {
             ArgumentGuard.NotNull(fieldsToSerialize, nameof(fieldsToSerialize));
             ArgumentGuard.NotNull(linkBuilder, nameof(linkBuilder));
             ArgumentGuard.NotNull(constraintProviders, nameof(constraintProviders));
             ArgumentGuard.NotNull(resourceDefinitionAccessor, nameof(resourceDefinitionAccessor));
+            ArgumentGuard.NotNull(queryStringAccessor, nameof(queryStringAccessor));
 
             _included = new HashSet<ResourceObject>(ResourceIdentifierObjectComparer.Instance);
             _fieldsToSerialize = fieldsToSerialize;
             _linkBuilder = linkBuilder;
             _resourceDefinitionAccessor = resourceDefinitionAccessor;
+            _queryStringAccessor = queryStringAccessor;
             _sparseFieldSetCache = new SparseFieldSetCache(constraintProviders, resourceDefinitionAccessor);
         }
 
@@ -43,7 +47,7 @@ public IList<ResourceObject> Build()
         {
             if (_included.Any())
             {
-                // cleans relationship dictionaries and adds links of resources.
+                // Cleans relationship dictionaries and adds links of resources.
                 foreach (ResourceObject resourceObject in _included)
                 {
                     if (resourceObject.Relationships != null)
@@ -57,7 +61,7 @@ public IList<ResourceObject> Build()
                 return _included.ToArray();
             }
 
-            return null;
+            return _queryStringAccessor.Query.ContainsKey("include") ? Array.Empty<ResourceObject>() : null;
         }
 
         private void UpdateRelationships(ResourceObject resourceObject)

test/JsonApiDotNetCoreExampleTests/IntegrationTests/Serialization/SerializationTests.cs

@@ -160,6 +160,63 @@ await _testContext.RunOnDatabaseAsync(async dbContext =>
 }");
         }
 
+        [Fact]
+        public async Task Can_get_primary_resources_with_empty_include()
+        {
+            // Arrange
+            List<Meeting> meetings = _fakers.Meeting.Generate(1);
+
+            await _testContext.RunOnDatabaseAsync(async dbContext =>
+            {
+                await dbContext.ClearTableAsync<Meeting>();
+                dbContext.Meetings.AddRange(meetings);
+                await dbContext.SaveChangesAsync();
+            });
+
+            const string route = "/meetings/?include=attendees";
+
+            // Act
+            (HttpResponseMessage httpResponse, string responseDocument) = await _testContext.ExecuteGetAsync<string>(route);
+
+            // Assert
+            httpResponse.Should().HaveStatusCode(HttpStatusCode.OK);
+
+            responseDocument.Should().BeJson(@"{
+  ""links"": {
+    ""self"": ""http://localhost/meetings/?include=attendees"",
+    ""first"": ""http://localhost/meetings/?include=attendees""
+  },
+  ""data"": [
+    {
+      ""type"": ""meetings"",
+      ""id"": """ + meetings[0].StringId + @""",
+      ""attributes"": {
+        ""title"": """ + meetings[0].Title + @""",
+        ""startTime"": """ + meetings[0].StartTime.ToString("O") + @""",
+        ""duration"": """ + meetings[0].Duration + @""",
+        ""location"": {
+          ""lat"": " + meetings[0].Location.Latitude.ToString(CultureInfo.InvariantCulture) + @",
+          ""lng"": " + meetings[0].Location.Longitude.ToString(CultureInfo.InvariantCulture) + @"
+        }
+      },
+      ""relationships"": {
+        ""attendees"": {
+          ""links"": {
+            ""self"": ""http://localhost/meetings/" + meetings[0].StringId + @"/relationships/attendees"",
+            ""related"": ""http://localhost/meetings/" + meetings[0].StringId + @"/attendees""
+          },
+          ""data"": []
+        }
+      },
+      ""links"": {
+        ""self"": ""http://localhost/meetings/" + meetings[0].StringId + @"""
+      }
+    }
+  ],
+  ""included"": []
+}");
+        }
+
         [Fact]
         public async Task Can_get_primary_resource_by_ID()
         {

test/UnitTests/Serialization/FakeRequestQueryStringAccessor.cs

@@ -0,0 +1,21 @@
+using JsonApiDotNetCore.QueryStrings;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.WebUtilities;
+
+namespace UnitTests.Serialization
+{
+    internal sealed class FakeRequestQueryStringAccessor : IRequestQueryStringAccessor
+    {
+        public IQueryCollection Query { get; }
+
+        public FakeRequestQueryStringAccessor()
+            : this(null)
+        {
+        }
+
+        public FakeRequestQueryStringAccessor(string queryString)
+        {
+            Query = string.IsNullOrEmpty(queryString) ? QueryCollection.Empty : new QueryCollection(QueryHelpers.ParseQuery(queryString));
+        }
+    }
+}

test/UnitTests/Serialization/SerializerTestsSetup.cs

@@ -6,6 +6,7 @@
 using JsonApiDotNetCore.Queries;
 using JsonApiDotNetCore.Queries.Expressions;
 using JsonApiDotNetCore.Queries.Internal;
+using JsonApiDotNetCore.QueryStrings;
 using JsonApiDotNetCore.Resources;
 using JsonApiDotNetCore.Resources.Annotations;
 using JsonApiDotNetCore.Serialization;
@@ -56,7 +57,7 @@ protected ResponseSerializer<T> GetResponseSerializer<T>(IEnumerable<IEnumerable
             IMetaBuilder meta = GetMetaBuilder(metaDict);
             ILinkBuilder link = GetLinkBuilder(topLinks, resourceLinks, relationshipLinks);
             IEnumerable<IQueryConstraintProvider> includeConstraints = GetIncludeConstraints(inclusionChainArray);
-            IIncludedResourceObjectBuilder includedBuilder = GetIncludedBuilder();
+            IIncludedResourceObjectBuilder includedBuilder = GetIncludedBuilder(inclusionChainArray != null);
             IFieldsToSerialize fieldsToSerialize = GetSerializableFields();
             IResourceDefinitionAccessor resourceDefinitionAccessor = GetResourceDefinitionAccessor();
             IResourceObjectBuilderSettingsProvider settingsProvider = GetSerializerSettingsProvider();
@@ -77,17 +78,23 @@ protected ResponseResourceObjectBuilder GetResponseResourceObjectBuilder(IEnumer
 
             ILinkBuilder link = GetLinkBuilder(null, resourceLinks, relationshipLinks);
             IEnumerable<IQueryConstraintProvider> includeConstraints = GetIncludeConstraints(inclusionChainArray);
-            IIncludedResourceObjectBuilder includedBuilder = GetIncludedBuilder();
+            IIncludedResourceObjectBuilder includedBuilder = GetIncludedBuilder(inclusionChains != null);
             IEvaluatedIncludeCache evaluatedIncludeCache = GetEvaluatedIncludeCache(inclusionChainArray);
 
             return new ResponseResourceObjectBuilder(link, includedBuilder, includeConstraints, ResourceGraph, GetResourceDefinitionAccessor(),
                 GetSerializerSettingsProvider(), evaluatedIncludeCache);
         }
 
-        private IIncludedResourceObjectBuilder GetIncludedBuilder()
+        private IIncludedResourceObjectBuilder GetIncludedBuilder(bool hasIncludeQueryString)
         {
-            return new IncludedResourceObjectBuilder(GetSerializableFields(), GetLinkBuilder(), ResourceGraph, Enumerable.Empty<IQueryConstraintProvider>(),
-                GetResourceDefinitionAccessor(), GetSerializerSettingsProvider());
+            IFieldsToSerialize fieldsToSerialize = GetSerializableFields();
+            ILinkBuilder linkBuilder = GetLinkBuilder();
+            IResourceDefinitionAccessor resourceDefinitionAccessor = GetResourceDefinitionAccessor();
+            IRequestQueryStringAccessor queryStringAccessor = new FakeRequestQueryStringAccessor(hasIncludeQueryString ? "include=" : null);
+            IResourceObjectBuilderSettingsProvider resourceObjectBuilderSettingsProvider = GetSerializerSettingsProvider();
+
+            return new IncludedResourceObjectBuilder(fieldsToSerialize, linkBuilder, ResourceGraph, Enumerable.Empty<IQueryConstraintProvider>(),
+                resourceDefinitionAccessor, queryStringAccessor, resourceObjectBuilderSettingsProvider);
         }
 
         protected IResourceObjectBuilderSettingsProvider GetSerializerSettingsProvider()

test/UnitTests/Serialization/Server/IncludedResourceObjectBuilderTests.cs

@@ -180,11 +180,12 @@ private IncludedResourceObjectBuilder GetBuilder()
         {
             IFieldsToSerialize fields = GetSerializableFields();
             ILinkBuilder links = GetLinkBuilder();
+            IResourceDefinitionAccessor resourceDefinitionAccessor = new Mock<IResourceDefinitionAccessor>().Object;
+            var queryStringAccessor = new FakeRequestQueryStringAccessor();
+            IResourceObjectBuilderSettingsProvider resourceObjectBuilderSettingsProvider = GetSerializerSettingsProvider();
 
-            IResourceDefinitionAccessor accessor = new Mock<IResourceDefinitionAccessor>().Object;
-
-            return new IncludedResourceObjectBuilder(fields, links, ResourceGraph, Enumerable.Empty<IQueryConstraintProvider>(), accessor,
-                GetSerializerSettingsProvider());
+            return new IncludedResourceObjectBuilder(fields, links, ResourceGraph, Enumerable.Empty<IQueryConstraintProvider>(), resourceDefinitionAccessor,
+                queryStringAccessor, resourceObjectBuilderSettingsProvider);
         }
 
         private sealed class AuthorChainInstances