[Fusion] Added pre-merge validation rule "LookupMustNotReturnListRule"

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Logging/LogEntryCodes.cs

@@ -12,6 +12,7 @@ public static class LogEntryCodes
     public const string KeyFieldsSelectInvalidType = "KEY_FIELDS_SELECT_INVALID_TYPE";
     public const string KeyInvalidFields = "KEY_INVALID_FIELDS";
     public const string KeyInvalidSyntax = "KEY_INVALID_SYNTAX";
+    public const string LookupMustNotReturnList = "LOOKUP_MUST_NOT_RETURN_LIST";
     public const string LookupShouldHaveNullableReturnType = "LOOKUP_SHOULD_HAVE_NULLABLE_RETURN_TYPE";
     public const string OutputFieldTypesNotMergeable = "OUTPUT_FIELD_TYPES_NOT_MERGEABLE";
     public const string ProvidesDirectiveInFieldsArg = "PROVIDES_DIRECTIVE_IN_FIELDS_ARG";

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Logging/LogEntryHelper.cs

@@ -257,6 +257,25 @@ public static LogEntry KeyInvalidSyntax(
             schema);
     }
 
+    public static LogEntry LookupMustNotReturnList(
+        OutputFieldDefinition field,
+        INamedTypeDefinition type,
+        SchemaDefinition schema)
+    {
+        var coordinate = new SchemaCoordinate(type.Name, field.Name);
+
+        return new LogEntry(
+            string.Format(
+                LogEntryHelper_LookupMustNotReturnList,
+                coordinate,
+                schema.Name),
+            LogEntryCodes.LookupMustNotReturnList,
+            LogSeverity.Error,
+            coordinate,
+            field,
+            schema);
+    }
+
     public static LogEntry LookupShouldHaveNullableReturnType(
         OutputFieldDefinition field,
         INamedTypeDefinition type,

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/PreMergeValidation/Rules/LookupMustNotReturnListRule.cs

@@ -0,0 +1,27 @@
+using HotChocolate.Fusion.Events;
+using HotChocolate.Skimmed;
+using static HotChocolate.Fusion.Logging.LogEntryHelper;
+
+namespace HotChocolate.Fusion.PreMergeValidation.Rules;
+
+/// <summary>
+/// Fields annotated with the <c>@lookup</c> directive are intended to retrieve a single entity
+/// based on provided arguments. To avoid ambiguity in entity resolution, such fields must return a
+/// single object and not a list. This validation rule enforces that any field annotated with
+/// <c>@lookup</c> must have a return type that is <b>NOT</b> a list.
+/// </summary>
+/// <seealso href="https://graphql.github.io/composite-schemas-spec/draft/#sec--lookup-must-not-return-a-list">
+/// Specification
+/// </seealso>
+internal sealed class LookupMustNotReturnListRule : IEventHandler<OutputFieldEvent>
+{
+    public void Handle(OutputFieldEvent @event, CompositionContext context)
+    {
+        var (field, type, schema) = @event;
+
+        if (ValidationHelper.IsLookup(field) && field.Type.NullableType() is ListTypeDefinition)
+        {
+            context.Log.Write(LookupMustNotReturnList(field, type, schema));
+        }
+    }
+}

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/Properties/CompositionResources.resx

@@ -63,6 +63,9 @@
   <data name="LogEntryHelper_KeyInvalidSyntax" xml:space="preserve">
     <value>A @key directive on type '{0}' in schema '{1}' contains invalid syntax in the 'fields' argument.</value>
   </data>
+  <data name="LogEntryHelper_LookupMustNotReturnList" xml:space="preserve">
+    <value>The lookup field '{0}' in schema '{1}' must not return a list.</value>
+  </data>
   <data name="LogEntryHelper_LookupShouldHaveNullableReturnType" xml:space="preserve">
     <value>The lookup field '{0}' in schema '{1}' should return a nullable type.</value>
   </data>

src/HotChocolate/Fusion-vnext/src/Fusion.Composition/SourceSchemaMerger.cs

@@ -56,6 +56,7 @@ private CompositionResult<SchemaDefinition> MergeSchemaDefinitions(CompositionCo
         new KeyFieldsSelectInvalidTypeRule(),
         new KeyInvalidFieldsRule(),
         new KeyInvalidSyntaxRule(),
+        new LookupMustNotReturnListRule(),
         new LookupShouldHaveNullableReturnTypeRule(),
         new OutputFieldTypesMergeableRule(),
         new ProvidesDirectiveInFieldsArgumentRule(),

src/HotChocolate/Fusion-vnext/test/Fusion.Composition.Tests/PreMergeValidation/Rules/LookupMustNotReturnListRuleTests.cs

@@ -0,0 +1,109 @@
+using HotChocolate.Fusion.Logging;
+using HotChocolate.Fusion.PreMergeValidation;
+using HotChocolate.Fusion.PreMergeValidation.Rules;
+
+namespace HotChocolate.Composition.PreMergeValidation.Rules;
+
+public sealed class LookupMustNotReturnListRuleTests : CompositionTestBase
+{
+    private readonly PreMergeValidator _preMergeValidator =
+        new([new LookupMustNotReturnListRule()]);
+
+    [Theory]
+    [MemberData(nameof(ValidExamplesData))]
+    public void Examples_Valid(string[] sdl)
+    {
+        // arrange
+        var context = CreateCompositionContext(sdl);
+
+        // act
+        var result = _preMergeValidator.Validate(context);
+
+        // assert
+        Assert.True(result.IsSuccess);
+        Assert.True(context.Log.IsEmpty);
+    }
+
+    [Theory]
+    [MemberData(nameof(InvalidExamplesData))]
+    public void Examples_Invalid(string[] sdl, string[] errorMessages)
+    {
+        // arrange
+        var context = CreateCompositionContext(sdl);
+
+        // act
+        var result = _preMergeValidator.Validate(context);
+
+        // assert
+        Assert.True(result.IsFailure);
+        Assert.Equal(errorMessages, context.Log.Select(e => e.Message).ToArray());
+        Assert.True(context.Log.All(e => e.Code == "LOOKUP_MUST_NOT_RETURN_LIST"));
+        Assert.True(context.Log.All(e => e.Severity == LogSeverity.Error));
+    }
+
+    public static TheoryData<string[]> ValidExamplesData()
+    {
+        return new TheoryData<string[]>
+        {
+            // In this example, "userById" returns a "User" object, satisfying the requirement.
+            {
+                [
+                    """
+                    type Query {
+                        userById(id: ID!): User @lookup
+                    }
+
+                    type User {
+                        id: ID!
+                        name: String
+                    }
+                    """
+                ]
+            }
+        };
+    }
+
+    public static TheoryData<string[], string[]> InvalidExamplesData()
+    {
+        return new TheoryData<string[], string[]>
+        {
+            // Here, "usersByIds" returns a list of "User" objects, which violates the requirement
+            // that a @lookup field must return a single object.
+            {
+                [
+                    """
+                    type Query {
+                        usersByIds(ids: [ID!]!): [User!] @lookup
+                    }
+
+                    type User {
+                        id: ID!
+                        name: String
+                    }
+                    """
+                ],
+                [
+                    "The lookup field 'Query.usersByIds' in schema 'A' must not return a list."
+                ]
+            },
+            // Non-null list.
+            {
+                [
+                    """
+                    type Query {
+                        usersByIds(ids: [ID!]!): [User!]! @lookup
+                    }
+
+                    type User {
+                        id: ID!
+                        name: String
+                    }
+                    """
+                ],
+                [
+                    "The lookup field 'Query.usersByIds' in schema 'A' must not return a list."
+                ]
+            }
+        };
+    }
+}