Query: Include for derived types fails when source does not contain any of the derived type

Consider a model with a base type and two different derived types. “Include for derived types” works well when the source of the query contains only the first derived type or a mix of both derived types. It fails when the source only does not contain any of the first derived type. Please look at the code example below:

using Microsoft.EntityFrameworkCore;
using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations.Schema;
using System.Linq;

namespace DerivedIncludeSelectiveTest
{
	class Program
	{
		static void Main(string[] args)
		{
			using (var myContext = new MyContext())
			{
				myContext.Database.EnsureDeleted();
				myContext.Database.EnsureCreated();
				GetSchoolsWithInclude(myContext.Schools);
				GetSchoolsWithInclude(myContext.PrimarySchools);
				GetSchoolsWithInclude(myContext.Colleges); // this line throws an exception despite the "include on derived types" being supposed to apply only to the specified derived type
			}
			Console.ReadLine();
		}

		static School[] GetSchoolsWithInclude(IQueryable<School> source) =>
					source
					.Include(s => ((PrimarySchool)s).Students) // loading students only for PrimarySchools
					.ToArray();
	}

	public class MyContext : DbContext
	{
		public DbSet<PrimaryStudent> PrimaryStudents { get; set; }
		public DbSet<School> Schools { get; set; }
		public DbSet<PrimarySchool> PrimarySchools { get; set; }
		public DbSet<College> Colleges { get; set; }
		protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
		{
			optionsBuilder.UseSqlServer(@"Data Source=" + System.Environment.MachineName +
				";Initial Catalog=derivedIncludeSelectiveTest;Integrated Security=True");
			optionsBuilder.EnableSensitiveDataLogging();
			optionsBuilder.ConfigureWarnings(w => w.Log(
				Microsoft.EntityFrameworkCore.Diagnostics.CoreEventId.SensitiveDataLoggingEnabledWarning));
		}
		protected override void OnModelCreating(ModelBuilder modelBuilder)
		{
			modelBuilder.Entity<PrimarySchool>().HasMany(s => s.Students).WithOne(s => s.School);
			modelBuilder.Entity<PrimarySchool>().HasData(new PrimarySchool { Id = 1 });
			modelBuilder.Entity<College>().HasData(new College { Id = 2 });
		}
	}

	public class PrimaryStudent
	{
		public int Id { get; set; }
		public PrimarySchool School { get; set; }
	}

	public abstract class School // base type
	{
		public int Id { get; set; }
	}

	public class PrimarySchool : School // first derived type
	{
		public List<PrimaryStudent> Students { get; set; }
	}

	public class College : School // second derived type
	{
		// does not have an association to students
	}
}

In EF 3.0, it throws:

System.InvalidOperationException
  HResult=0x80131509
  Message=Invalid include.
  Source=Microsoft.EntityFrameworkCore
  StackTrace:
   at Microsoft.EntityFrameworkCore.Query.Internal.NavigationExpandingExpressionVisitor.PopulateIncludeTree(IncludeTreeNode includeTreeNode, Expression expression)
   at Microsoft.EntityFrameworkCore.Query.Internal.NavigationExpandingExpressionVisitor.ProcessInclude(NavigationExpansionExpression source, Expression expression, Boolean thenInclude)
   at Microsoft.EntityFrameworkCore.Query.Internal.NavigationExpandingExpressionVisitor.VisitMethodCall(MethodCallExpression methodCallExpression)
   at System.Linq.Expressions.MethodCallExpression.Accept(ExpressionVisitor visitor)
   at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)
   at Microsoft.EntityFrameworkCore.Query.Internal.NavigationExpandingExpressionVisitor.ExpandAndReduce(Expression query, Boolean applyInclude)
   at Microsoft.EntityFrameworkCore.Query.Internal.NavigationExpandingExpressionVisitor.Expand(Expression query)
   at Microsoft.EntityFrameworkCore.Query.QueryTranslationPreprocessor.Process(Expression query)
   at Microsoft.EntityFrameworkCore.Query.QueryCompilationContext.CreateQueryExecutor[TResult](Expression query)
   at Microsoft.EntityFrameworkCore.Storage.Database.CompileQuery[TResult](Expression query, Boolean async)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileQueryCore[TResult](IDatabase database, Expression query, IModel model, Boolean async)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass9_0`1.<Execute>b__0()
   at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQueryCore[TFunc](Object cacheKey, Func`1 compiler)
   at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQuery[TResult](Object cacheKey, Func`1 compiler)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult](Expression query)
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.Execute[TResult](Expression expression)
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable`1.GetEnumerator()
   at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.IncludableQueryable`2.GetEnumerator()
   at System.Collections.Generic.LargeArrayBuilder`1.AddRange(IEnumerable`1 items)
   at System.Collections.Generic.EnumerableHelpers.ToArray[T](IEnumerable`1 source)
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at DerivedIncludeSelectiveTest.Program.GetSchoolsWithInclude(IQueryable`1 schools) 

In EF 2.2.6, it throws:

System.InvalidOperationException
  HResult=0x80131509
  Message=The property 'Students' is not a navigation property of entity type 'College'. The 'Include(string)' method can only be used with a '.' separated list of navigation property names.
  Source=Microsoft.EntityFrameworkCore
  StackTrace:
   at Microsoft.EntityFrameworkCore.Query.Internal.IncludeCompiler.WalkNavigations(IEntityType entityType, IReadOnlyList`1 navigationPropertyPaths, IncludeLoadTree includeLoadTree, Boolean shouldThrow)
   at Microsoft.EntityFrameworkCore.Query.Internal.IncludeCompiler.TryPopulateIncludeLoadTree(IncludeResultOperator includeResultOperator, IncludeLoadTree includeLoadTree, Boolean shouldThrow)
   at Microsoft.EntityFrameworkCore.Query.Internal.IncludeCompiler.CreateIncludeLoadTrees(QueryModel queryModel, Boolean shouldThrow)
   at Microsoft.EntityFrameworkCore.Query.Internal.IncludeCompiler.CompileIncludes(QueryModel queryModel, Boolean trackingQuery, Boolean asyncQuery, Boolean shouldThrow)
   at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.OptimizeQueryModel(QueryModel queryModel, Boolean asyncQuery)
   at Microsoft.EntityFrameworkCore.Query.RelationalQueryModelVisitor.OptimizeQueryModel(QueryModel queryModel, Boolean asyncQuery)
   at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.CreateQueryExecutor[TResult](QueryModel queryModel)
   at Microsoft.EntityFrameworkCore.Storage.Database.CompileQuery[TResult](QueryModel queryModel)
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.CompileQueryCore[TResult](Expression query, IQueryModelGenerator queryModelGenerator, IDatabase database, IDiagnosticsLogger`1 logger, Type contextType)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass13_0`1.<Execute>b__0()
   at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQueryCore[TFunc](Object cacheKey, Func`1 compiler)
   at Microsoft.EntityFrameworkCore.Query.Internal.CompiledQueryCache.GetOrAddQuery[TResult](Object cacheKey, Func`1 compiler)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult](Expression query)
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.Execute[TResult](Expression expression)
   at Remotion.Linq.QueryableBase`1.GetEnumerator()
   at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.IncludableQueryable`2.GetEnumerator()
   at System.Collections.Generic.LargeArrayBuilder`1.AddRange(IEnumerable`1 items)
   at System.Collections.Generic.EnumerableHelpers.ToArray[T](IEnumerable`1 source)
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at DerivedIncludeSelectiveTest.Program.GetSchoolsWithInclude(IQueryable`1 schools) 

Further technical details

EF Core version: 2.2.6 and 3.0 (different errors in each) Database provider: Microsoft.EntityFrameworkCore.SqlServer Target framework: NET Core 3.0 Operating system: Windows 10 x64 Pro 1903 IDE: Visual Studio 2019 16.3.4

[smitpatel]

static School[] GetSchoolsWithInclude<T>(IQueryable<T> source)
            where T : School
        {
            if (typeof(T) == typeof(School)
                || typeof(T) == typeof(PrimarySchool))
            {
                source = source.Include(e => (e as PrimarySchool).Students);
            }

            return source.ToArray();
        }

// Use as 
GetSchoolsWithInclude(db.Schools);
                GetSchoolsWithInclude(db.PrimarySchools);
                GetSchoolsWithInclude(db.Colleges);

[vslee]

Great, thanks for the guidance