fixed SelectMany().LongCount() querying. Closes GH-3017

src/LinqTests/Bugs/Bug_3017_SelectMany_with_LongCount.cs

@@ -0,0 +1,41 @@
+using System.Linq;
+using System.Threading.Tasks;
+using Marten;
+using Marten.Testing.Documents;
+using Marten.Testing.Harness;
+using Shouldly;
+
+namespace LinqTests.Bugs;
+
+public class Bug_3017_SelectMany_with_LongCount : BugIntegrationContext
+{
+    [Fact]
+    public async Task can_use_select_many_with_long_count()
+    {
+        var targets = Target.GenerateRandomData(1000).ToArray();
+
+        await theStore.BulkInsertDocumentsAsync(targets);
+
+        var expected = targets.SelectMany(x => x.Children).LongCount();
+
+        var actual = await theSession.Query<Target>().SelectMany(x => x.Children)
+            .LongCountAsync();
+
+        actual.ShouldBe(expected);
+    }
+
+    [Fact]
+    public async Task can_use_select_many_with_integer_count()
+    {
+        var targets = Target.GenerateRandomData(1000).ToArray();
+
+        await theStore.BulkInsertDocumentsAsync(targets);
+
+        var expected = targets.SelectMany(x => x.Children).Count();
+
+        var actual = await theSession.Query<Target>().SelectMany(x => x.Children)
+            .CountAsync();
+
+        actual.ShouldBe(expected);
+    }
+}

src/Marten/Linq/CollectionUsage.Compilation.cs

@@ -216,6 +216,13 @@ private Statement compileNext(IMartenSession session, IQueryableMemberCollection
 
                     selection.ApplyAggregateOperator("SUM");
                 }
+                else if (SingleValueMode == Marten.Linq.Parsing.SingleValueMode.LongCount)
+                {
+                    selection.SelectClause = new NewScalarSelectClause<long>(
+                        $"jsonb_array_length({collectionMember.JSONBLocator})", selection.SelectClause.FromObject);
+
+                    selection.ApplyAggregateOperator("SUM");
+                }
                 else
                 {
                     var next = new CollectionUsage(_options, collectionMember.MemberType);