Ability to use hash partitioning or really any partitioning on the id…

src/CoreTests/Partitioning/partitioning_by_hash_on_id.cs

@@ -0,0 +1,22 @@
+using System.Threading.Tasks;
+using Marten.Testing.Documents;
+using Marten.Testing.Harness;
+using Xunit;
+
+namespace CoreTests.Partitioning;
+
+// Spawned by https://github.com/JasperFx/marten/issues/4025
+public class partitioning_by_hash_on_id : OneOffConfigurationsContext
+{
+    [Fact]
+    public async Task try_it_out()
+    {
+        StoreOptions(opts =>
+        {
+            opts.Schema.For<LongDoc>()
+                .PartitionOn(x => x.Id, x => x.ByHash("one", "two", "three"));
+        });
+
+        await theStore.Storage.ApplyAllConfiguredChangesToDatabaseAsync();
+    }
+}

src/Marten/MartenRegistry.cs

@@ -126,8 +126,16 @@ public DocumentMappingExpression<T> PartitionOn(Expression<Func<T, object>> memb
             _builder.Alter = m =>
             {
                 var members = FindMembers.Determine(memberExpression);
-                var field = m.DuplicateField(members);
-                var columnName = field.ColumnName;
+
+                var identity = m.IdMember;
+
+                var columnName = "id";
+
+                if (!members.Contains(identity))
+                {
+                    var field = m.DuplicateField(members);
+                    columnName = field.ColumnName;
+                }
 
                 var expression = new PartitioningExpression(m, [columnName]);
                 partitioning(expression);