Add include fallback for abstract navigation filters

src/Directory.Build.props

@@ -2,7 +2,7 @@
 <Project>
   <PropertyGroup>
     <NoWarn>CS1591;NU5104;CS1573;CS9107;NU1608;NU1109</NoWarn>
-    <Version>34.1.6</Version>
+    <Version>34.1.7</Version>
     <LangVersion>preview</LangVersion>
     <AssemblyVersion>1.0.0</AssemblyVersion>
     <PackageTags>EntityFrameworkCore, EntityFramework, GraphQL</PackageTags>

src/GraphQL.EntityFramework/GraphApi/EfGraphQLService_First.cs

@@ -166,7 +166,7 @@ FieldType BuildFirstField<TSource, TReturn>(
                 // Get filter-required fields early so we can add filter-required navigations via Include
                 var allFilterFields = fieldContext.Filters?.GetAllRequiredFilterProperties();
 
-                query = includeAppender.AddIncludesWithFiltersAndDetectNavigations(query, context);
+                query = includeAppender.AddIncludesWithFiltersAndDetectNavigations(query, context, allFilterFields);
                 query = query.ApplyGraphQlArguments(context, names, false, omitQueryArguments);
 
                 // Apply column projection based on requested GraphQL fields

src/GraphQL.EntityFramework/GraphApi/EfGraphQLService_Queryable.cs

@@ -112,7 +112,7 @@ FieldType BuildQueryField<TSource, TReturn>(
                     // Get filter-required fields early so we can add filter-required navigations via Include
                     var allFilterFields = fieldContext.Filters?.GetAllRequiredFilterProperties();
 
-                    query = includeAppender.AddIncludesWithFiltersAndDetectNavigations(query, context);
+                    query = includeAppender.AddIncludesWithFiltersAndDetectNavigations(query, context, allFilterFields);
                     if (!omitQueryArguments)
                     {
                         query = query.ApplyGraphQlArguments(context, names, true, omitQueryArguments);

src/GraphQL.EntityFramework/GraphApi/EfGraphQLService_QueryableConnection.cs

@@ -158,7 +158,7 @@ ConnectionBuilder<TSource> AddQueryableConnection<TSource, TGraph, TReturn>(
                     // Get filter-required fields early so we can add filter-required navigations via Include
                     var allFilterFields = fieldContext.Filters?.GetAllRequiredFilterProperties();
 
-                    query = includeAppender.AddIncludesWithFiltersAndDetectNavigations(query, context);
+                    query = includeAppender.AddIncludesWithFiltersAndDetectNavigations(query, context, allFilterFields);
                     query = query.ApplyGraphQlArguments(context, names, true, omitQueryArguments);
 
                     // Apply column projection based on requested GraphQL fields

src/GraphQL.EntityFramework/GraphApi/EfGraphQLService_Single.cs

@@ -166,7 +166,7 @@ FieldType BuildSingleField<TSource, TReturn>(
                     // Get filter-required fields early so we can add filter-required navigations via Include
                     var allFilterFields = fieldContext.Filters?.GetAllRequiredFilterProperties();
 
-                    query = includeAppender.AddIncludesWithFiltersAndDetectNavigations(query, context);
+                    query = includeAppender.AddIncludesWithFiltersAndDetectNavigations(query, context, allFilterFields);
                     query = query.ApplyGraphQlArguments(context, names, false, omitQueryArguments);
 
                     // Apply column projection based on requested GraphQL fields

src/GraphQL.EntityFramework/IncludeAppender.cs

@@ -25,20 +25,94 @@ public IQueryable<TItem> AddIncludesWithFilters<TItem>(
         IResolveFieldContext context,
         IReadOnlyDictionary<Type, IReadOnlySet<string>>? allFilterFields)
         where TItem : class =>
-        AddIncludesWithFiltersAndDetectNavigations(query, context);
+        AddIncludesWithFiltersAndDetectNavigations(query, context, allFilterFields);
 
     internal IQueryable<TItem> AddIncludesWithFiltersAndDetectNavigations<TItem>(
         IQueryable<TItem> query,
-        IResolveFieldContext context)
+        IResolveFieldContext context,
+        IReadOnlyDictionary<Type, IReadOnlySet<string>>? allFilterFields = null)
         where TItem : class
     {
         // Add includes from GraphQL query
         query = AddIncludes(query, context);
 
-        // Note: Filter-required navigations are now handled entirely by the projection system
-        // in TryGetProjectionExpressionWithFilters, which builds projections that include
-        // filter-required fields. Abstract navigation access is prevented by FilterEntry validation.
-        // No additional includes are needed here.
+        // Add includes for filter-required navigations.
+        // While projection handles most cases, abstract navigation types cannot be projected
+        // (can't do "new AbstractType { ... }"). In those cases, Include is needed as fallback.
+        if (allFilterFields is { Count: > 0 })
+        {
+            query = AddFilterNavigationIncludes(query, allFilterFields, typeof(TItem));
+        }
+
+        return query;
+    }
+
+    IQueryable<TItem> AddFilterNavigationIncludes<TItem>(
+        IQueryable<TItem> query,
+        IReadOnlyDictionary<Type, IReadOnlySet<string>> allFilterFields,
+        Type entityType)
+        where TItem : class
+    {
+        // Get filter fields for this entity type and its base types
+        var relevantFilterFields = new List<string>();
+        foreach (var (filterType, filterFields) in allFilterFields)
+        {
+            if (filterType.IsAssignableFrom(entityType))
+            {
+                relevantFilterFields.AddRange(filterFields);
+            }
+        }
+
+        if (relevantFilterFields.Count == 0)
+        {
+            return query;
+        }
+
+        // Get navigation metadata for this entity type
+        if (!navigations.TryGetValue(entityType, out var navigationProperties))
+        {
+            return query;
+        }
+
+        // Extract unique navigation names from filter fields (paths like "TravelRequest.Status" -> "TravelRequest")
+        var navigationNames = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+        foreach (var field in relevantFilterFields)
+        {
+            if (field.Contains('.'))
+            {
+                var navName = field[..field.IndexOf('.')];
+                navigationNames.Add(navName);
+            }
+        }
+
+        // Add Include for each filter-required navigation that has an abstract type
+        // (Non-abstract navigations can be handled by projection)
+        foreach (var navName in navigationNames)
+        {
+            // Find navigation in metadata (case-insensitive)
+            Navigation? navMetadata = null;
+            string? actualNavName = null;
+            foreach (var (key, value) in navigationProperties)
+            {
+                if (string.Equals(key, navName, StringComparison.OrdinalIgnoreCase))
+                {
+                    navMetadata = value;
+                    actualNavName = key;
+                    break;
+                }
+            }
+
+            if (navMetadata == null || actualNavName == null)
+            {
+                continue;
+            }
+
+            // Only add Include for abstract types - concrete types can use projection
+            if (navMetadata.Type.IsAbstract)
+            {
+                query = query.Include(navMetadata.Name);
+            }
+        }
 
         return query;
     }

src/Tests/IntegrationTests/IntegrationTests.Filter_combining_abstract_and_concrete_navigations.verified.txt

@@ -0,0 +1,29 @@
+{
+  target:
+{
+  "data": {
+    "derivedChildEntities": [
+      {
+        "id": "Guid_1",
+        "property": "Child1"
+      }
+    ]
+  }
+},
+  sql: {
+    Text:
+select   d.Id,
+         d.ParentId,
+         d.Property,
+         d.TypedParentId,
+         b.Id,
+         b.Discriminator,
+         b.Property,
+         b.Status
+from     DerivedChildEntities as d
+         left outer join
+         BaseEntities as b
+         on d.ParentId = b.Id
+order by d.Property
+  }
+}

src/Tests/IntegrationTests/IntegrationTests.Filter_with_abstract_navigation_should_correctly_exclude_non_matching.verified.txt

@@ -0,0 +1,29 @@
+{
+  target:
+{
+  "data": {
+    "derivedChildEntities": [
+      {
+        "id": "Guid_1",
+        "property": "Child1-Approved"
+      }
+    ]
+  }
+},
+  sql: {
+    Text:
+select   d.Id,
+         d.ParentId,
+         d.Property,
+         d.TypedParentId,
+         b.Id,
+         b.Discriminator,
+         b.Property,
+         b.Status
+from     DerivedChildEntities as d
+         left outer join
+         BaseEntities as b
+         on d.ParentId = b.Id
+order by d.Property
+  }
+}

src/Tests/IntegrationTests/IntegrationTests.Filter_with_abstract_navigation_should_use_include_fallback.verified.txt

@@ -0,0 +1,29 @@
+{
+  target:
+{
+  "data": {
+    "derivedChildEntities": [
+      {
+        "id": "Guid_1",
+        "property": "Child1"
+      }
+    ]
+  }
+},
+  sql: {
+    Text:
+select   d.Id,
+         d.ParentId,
+         d.Property,
+         d.TypedParentId,
+         b.Id,
+         b.Discriminator,
+         b.Property,
+         b.Status
+from     DerivedChildEntities as d
+         left outer join
+         BaseEntities as b
+         on d.ParentId = b.Id
+order by d.Property
+  }
+}

src/Tests/IntegrationTests/IntegrationTests.Filter_with_multiple_properties_from_abstract_navigation.verified.txt

@@ -0,0 +1,29 @@
+{
+  target:
+{
+  "data": {
+    "derivedChildEntities": [
+      {
+        "id": "Guid_1",
+        "property": "Child1"
+      }
+    ]
+  }
+},
+  sql: {
+    Text:
+select   d.Id,
+         d.ParentId,
+         d.Property,
+         d.TypedParentId,
+         b.Id,
+         b.Discriminator,
+         b.Property,
+         b.Status
+from     DerivedChildEntities as d
+         left outer join
+         BaseEntities as b
+         on d.ParentId = b.Id
+order by d.Property
+  }
+}

src/Tests/IntegrationTests/IntegrationTests.Filter_with_null_abstract_navigation_should_handle_gracefully.verified.txt

@@ -0,0 +1,29 @@
+{
+  target:
+{
+  "data": {
+    "derivedChildEntities": [
+      {
+        "id": "Guid_1",
+        "property": "ChildWithParent"
+      }
+    ]
+  }
+},
+  sql: {
+    Text:
+select   d.Id,
+         d.ParentId,
+         d.Property,
+         d.TypedParentId,
+         b.Id,
+         b.Discriminator,
+         b.Property,
+         b.Status
+from     DerivedChildEntities as d
+         left outer join
+         BaseEntities as b
+         on d.ParentId = b.Id
+order by d.Property
+  }
+}