program.cs

using System;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;

namespace Repro
{
    [Table("bar")]
    public sealed class Bar : IThing
    {
        [Key, Column("id")]
        public int Id { get; set; }

        [Column("relation_id")]
        public int RelationId { get; set; }

        [ForeignKey(nameof(RelationId))]
        public Relation Relation { get; set; }
    }


    [Table("foo")]
    public sealed class Foo : IThing
    {
        [Key, Column("id")]
        public int Id { get; set; }

        [Column("relation_id")]
        public int RelationId { get; set; }

        [ForeignKey(nameof(RelationId))]
        public Relation Relation { get; set; }
    }

    public interface IThing
    {
        int Id { get; }

        Relation Relation { get; }
    }

    [Table("relation")]
    public sealed class Relation
    {
        [Key, Column("id")]
        public int Id { get; set; }

        [Column("some_data")]
        public string SomeData { get; set; }
    }

    public class MyDbContext : DbContext
    {
        public MyDbContext(DbContextOptions<MyDbContext> options)
            : base(options)
        {
        }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Bar>();
            modelBuilder.Entity<Foo>();
        }
    }

    public class Program
    {
        static void Main()
        {
            var services = new ServiceCollection();
            var cs = "<redacted>";
            services.AddDbContext<MyDbContext>(ctx => ctx.UseNpgsql(cs), ServiceLifetime.Transient);
            services.AddLogging(l => l.AddConsole());

            var provider = services.BuildServiceProvider();

            using (var dbContext = provider.GetRequiredService<MyDbContext>())
            {
                var bars = dbContext.Set<Bar>().Include(x => x.Relation);
                var foos = dbContext.Set<Foo>().Include(x => x.Relation);

                var all = bars.Cast<IThing>().Concat(foos).OrderBy(x => x.Id);

                Console.WriteLine(all.ToList().Count);
            }
        }
    }
}

project.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp2.0</TargetFramework>
  </PropertyGroup>

  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">
    <LangVersion>7.2</LangVersion>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.EntityFrameworkCore" Version="2.1.0-rc1-final" />
    <PackageReference Include="Microsoft.Extensions.Logging.Console" Version="2.1.0-rc1-final" />
    <PackageReference Include="Npgsql.EntityFrameworkCore.PostgreSQL" Version="2.1.0-rc1" />
  </ItemGroup>

</Project>