#3905 Support querying on subclass-specific properties in hierarchical documents

src/LinqTests/Acceptance/query_with_inheritance.cs

@@ -1,8 +1,10 @@
 using System;
 using System.Linq;
+using System.Linq.Expressions;
 using System.Threading.Tasks;
 using JasperFx;
 using Marten;
+using Marten.Linq;
 using Marten.Testing.Harness;
 using Shouldly;
 using Weasel.Core;
@@ -26,14 +28,19 @@ public class Smurf: ISmurf
 
 public interface IPapaSmurf: ISmurf
 {
+    bool IsVillageLeader { get; set; }
 }
 
 public class PapaSmurf: Smurf, IPapaSmurf
 {
+    public bool IsVillageLeader { get; set; }
+
+    public bool IsPapa { get; set; } = true;
 }
 
 public class PapySmurf: Smurf, IPapaSmurf
 {
+    public bool IsVillageLeader { get; set; }
 }
 
 public class BrainySmurf: PapaSmurf
@@ -240,5 +247,36 @@ public async Task get_all_subclasses_of_an_interface()
         theSession.Query<IPapaSmurf>().Count().ShouldBe(3);
     }
 
+    [Fact]
+    public async Task search_on_property_of_subclass()
+    {
+        var smurf = new Smurf {Ability = "Follow the herd"};
+        var papa = new PapaSmurf {Ability = "Lead", IsVillageLeader = true };
+        var papy = new PapySmurf {Ability = "Lead"};
+        var brainy = new BrainySmurf {Ability = "Invent"};
+        theSession.Store(smurf, papa, brainy, papy);
+
+        await theSession.SaveChangesAsync();
+
+        (await theSession.Query<Smurf>().WhereSub<PapaSmurf>(x => x.IsVillageLeader).CountAsync()).ShouldBe(1);
+    }
+
+    [Fact]
+    public async Task search_on_property_of_subclass_and_parent()
+    {
+        var smurf = new Smurf {Ability = "Follow the herd"};
+        var papa = new PapaSmurf {Ability = "Lead" };
+        var papy = new PapySmurf {Ability = "Lead"};
+        var brainy = new BrainySmurf {Ability = "Invent"};
+        theSession.Store(smurf, papa, brainy, papy);
+
+        await theSession.SaveChangesAsync();
+
+        (await theSession.Query<Smurf>()
+            .WhereSub<PapaSmurf>(x => x.IsPapa)
+            .Where(x => x.Ability == "Invent")
+            .CountAsync()).ShouldBe(1);
+    }
+
     #endregion
 }

src/Marten/Linq/IMartenQueryable.cs

@@ -143,4 +143,6 @@ IMartenQueryableIncludeBuilder<T, TKey, TInclude> Include<TKey, TInclude>(
     /// <returns></returns>
     IMartenQueryableIncludeBuilder<T, TKey, TInclude> Include<TKey, TInclude>(
         IDictionary<TKey, List<TInclude>> dictionary) where TInclude : notnull where TKey : notnull;
+
+    IMartenQueryable<T> WhereSub<TSub>(Expression<Func<TSub, bool>> predicate) where TSub : T;
 }

src/Marten/Linq/MartenLinqQueryable.cs

@@ -9,6 +9,7 @@
 using System.Threading;
 using System.Threading.Tasks;
 using JasperFx.Core;
+using JasperFx.Core.Reflection;
 using Marten.Internal.Sessions;
 using Marten.Linq.Includes;
 using Marten.Linq.Parsing;
@@ -125,6 +126,11 @@ public IMartenQueryableIncludeBuilder<T, TKey, TInclude> Include<TKey, TInclude>
         return new MartenQueryableIncludeBuilder<T, TKey, TInclude>(this, dictionary);
     }
 
+    public IMartenQueryable<T> WhereSub<TSub>(Expression<Func<TSub, bool>> predicate) where TSub : T =>
+        (IMartenQueryable<T>)this.Where(
+            Expression.Lambda<Func<T, bool>>(predicate.Body, Expression.Parameter(typeof(T), "x"))
+        );
+
     public IEnumerator<T> GetEnumerator()
     {
         return Provider.Execute<IEnumerable<T>>(Expression).GetEnumerator();