Allow empty include query string parameter value

Author: bkoelman

src/JsonApiDotNetCore/Queries/Internal/Parsing/IncludeParser.cs

@@ -30,12 +30,18 @@ public IncludeExpression Parse(string source, ResourceType resourceTypeInScope,
     protected IncludeExpression ParseInclude(ResourceType resourceTypeInScope, int? maximumDepth)
     {
         var treeRoot = IncludeTreeNode.CreateRoot(resourceTypeInScope);
-
-        ParseRelationshipChain(treeRoot);
+        bool isAtStart = true;
 
         while (TokenStack.Any())
         {
-            EatSingleCharacterToken(TokenKind.Comma);
+            if (!isAtStart)
+            {
+                EatSingleCharacterToken(TokenKind.Comma);
+            }
+            else
+            {
+                isAtStart = false;
+            }
 
             ParseRelationshipChain(treeRoot);
         }
@@ -244,7 +250,7 @@ public IncludeExpression ToExpression()
 
             if (element.Relationship is HiddenRootRelationshipAttribute)
             {
-                return new IncludeExpression(element.Children);
+                return element.Children.Any() ? new IncludeExpression(element.Children) : IncludeExpression.Empty;
             }
 
             return new IncludeExpression(ImmutableHashSet.Create(element));

src/JsonApiDotNetCore/QueryStrings/Internal/IncludeQueryStringParameterReader.cs

@@ -18,7 +18,7 @@ public class IncludeQueryStringParameterReader : QueryStringParameterReader, IIn
 
     private IncludeExpression? _includeExpression;
 
-    public bool AllowEmptyValue => false;
+    public bool AllowEmptyValue => true;
 
     public IncludeQueryStringParameterReader(IJsonApiRequest request, IResourceGraph resourceGraph, IJsonApiOptions options)
         : base(request, resourceGraph)

test/JsonApiDotNetCoreTests/IntegrationTests/QueryStrings/Includes/IncludeTests.cs

@@ -828,6 +828,31 @@ await _testContext.RunOnDatabaseAsync(async dbContext =>
         responseDocument.Included[0].Attributes.ShouldContainKey("userName").With(value => value.Should().Be(account.UserName));
     }
 
+    [Fact]
+    public async Task Can_select_empty_includes()
+    {
+        // Arrange
+        WebAccount account = _fakers.WebAccount.Generate();
+
+        await _testContext.RunOnDatabaseAsync(async dbContext =>
+        {
+            dbContext.Accounts.Add(account);
+            await dbContext.SaveChangesAsync();
+        });
+
+        string route = $"/webAccounts/{account.StringId}?include=";
+
+        // Act
+        (HttpResponseMessage httpResponse, Document responseDocument) = await _testContext.ExecuteGetAsync<Document>(route);
+
+        // Assert
+        httpResponse.ShouldHaveStatusCode(HttpStatusCode.OK);
+
+        responseDocument.Data.SingleValue.ShouldNotBeNull();
+
+        responseDocument.Included.Should().BeEmpty();
+    }
+
     [Fact]
     public async Task Cannot_include_unknown_relationship()
     {

test/JsonApiDotNetCoreTests/IntegrationTests/QueryStrings/QueryStringTests.cs

@@ -64,7 +64,6 @@ public async Task Can_use_unknown_query_string_parameter()
     }
 
     [Theory]
-    [InlineData("include")]
     [InlineData("filter")]
     [InlineData("sort")]
     [InlineData("page[size]")]

test/JsonApiDotNetCoreTests/UnitTests/QueryStringParameters/IncludeParseTests.cs

@@ -50,7 +50,6 @@ public void Reader_Is_Enabled(JsonApiQueryStringParameters parametersDisabled, b
     }
 
     [Theory]
-    [InlineData("includes", "", "Relationship name expected.")]
     [InlineData("includes", " ", "Unexpected whitespace.")]
     [InlineData("includes", ",", "Relationship name expected.")]
     [InlineData("includes", "posts,", "Relationship name expected.")]
@@ -85,6 +84,7 @@ public void Reader_Read_Fails(string parameterName, string parameterValue, strin
     }
 
     [Theory]
+    [InlineData("includes", "", "")]
     [InlineData("includes", "owner", "owner")]
     [InlineData("includes", "posts", "posts")]
     [InlineData("includes", "owner.posts", "owner.posts")]