Permit querying any npgsql mapped type with custom SQL

docs/documents/querying/linq/sql.md

@@ -18,5 +18,5 @@ public void query_with_matches_sql()
     user.Id.ShouldBe(u.Id);
 }
 ```
-<sup><a href='https://github.com/JasperFx/marten/blob/master/src/DocumentDbTests/Reading/query_by_sql.cs#L259-L274' title='Snippet source file'>snippet source</a> | <a href='#snippet-sample_query_with_matches_sql' title='Start of snippet'>anchor</a></sup>
+<sup><a href='https://github.com/JasperFx/marten/blob/master/src/DocumentDbTests/Reading/query_by_sql.cs#L260-L275' title='Snippet source file'>snippet source</a> | <a href='#snippet-sample_query_with_matches_sql' title='Start of snippet'>anchor</a></sup>
 <!-- endSnippet -->

docs/documents/querying/sql.md

@@ -50,15 +50,25 @@ a document body, but in that case you will need to supply the full SQL statement
 var sumResults = await session
     .QueryAsync<int>("select count(*) from mt_doc_target");
 ```
-<sup><a href='https://github.com/JasperFx/marten/blob/master/src/DocumentDbTests/Reading/query_by_sql.cs#L343-L348' title='Snippet source file'>snippet source</a> | <a href='#snippet-sample_query_by_full_sql' title='Start of snippet'>anchor</a></sup>
+<sup><a href='https://github.com/JasperFx/marten/blob/master/src/DocumentDbTests/Reading/query_by_sql.cs#L358-L363' title='Snippet source file'>snippet source</a> | <a href='#snippet-sample_query_by_full_sql' title='Start of snippet'>anchor</a></sup>
+<!-- endSnippet -->
+
+When querying single JSONB properties into a primitive/value type, you'll need to cast the value to the respective postgres type:
+
+<!-- snippet: sample_using-queryasync-casting -->
+<a id='snippet-sample_using-queryasync-casting'></a>
+```cs
+var times = await session.QueryAsync<DateTimeOffset>("SELECT (data ->> 'ModifiedAt')::timestamptz from mt_doc_user");
+```
+<sup><a href='https://github.com/JasperFx/marten/blob/master/src/DocumentDbTests/Reading/query_by_sql.cs#L324-L328' title='Snippet source file'>snippet source</a> | <a href='#snippet-sample_using-queryasync-casting' title='Start of snippet'>anchor</a></sup>
 <!-- endSnippet -->
 
 The basic rules for how Marten handles user-supplied queries are:
 
 * The `T` argument to `Query<T>()/QueryAsync<T>()` denotes the return value of each item
-* If the `T` is a simple, scalar value like a .Net `int`, the data is handled by reading the first
+* If the `T` is a [npgsql mapped type](https://www.npgsql.org/doc/types/basic.html) like a .Net `int` or `Guid`, the data is handled by reading the first
   field of the returned data
-* If the `T` is not a simple type, Marten will try to read the first field with the JSON serializer
+* If the `T` is not a mapped type, Marten will try to read the first field with the JSON serializer
   for the current `DocumentStore`
 * If the SQL starts with the `SELECT` keyword (and it's not case sensitive), the SQL supplied is used verbatim
 * If the supplied SQL does not start with a `SELECT` keyword, Marten assumes that the `T` is a document

src/DocumentDbTests/Reading/query_by_sql.cs

@@ -1,4 +1,5 @@
-using System.IO;
+using System;
+using System.IO;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
@@ -297,7 +298,8 @@ public void query_with_select_in_query()
     public async Task query_with_select_in_query_async()
     {
         await using var session = theStore.LightweightSession();
-        var u = new User {FirstName = "Jeremy", LastName = "Miller"};
+        var time = DateTimeOffset.UtcNow;
+        var u = new User {FirstName = "Jeremy", LastName = "Miller", ModifiedAt = time};
         session.Store(u);
         await session.SaveChangesAsync();
 
@@ -313,6 +315,19 @@ public async Task query_with_select_in_query_async()
 
         user.LastName.ShouldBe("Miller");
         user.Id.ShouldBe(u.Id);
+
+
+        var ids = await session.QueryAsync<Guid>("SELECT id from mt_doc_user");
+        var id = ids.Single();
+        id.ShouldBe(u.Id);
+
+        #region sample_using-queryasync-casting
+
+        var times = await session.QueryAsync<DateTimeOffset>("SELECT (data ->> 'ModifiedAt')::timestamptz from mt_doc_user");
+
+        #endregion
+        var fetchedTime = times.Single();
+        fetchedTime.Second.ShouldBe(time.Second);
     }
 
     [Fact]

src/Marten.Testing/Documents/User.cs

@@ -50,6 +50,8 @@ public User()
 
     public int Age { get; set; }
 
+    public DateTimeOffset ModifiedAt { get; set; }
+
     public string ToJson()
     {
         return

src/Marten/Linq/QueryHandlers/UserSuppliedQueryHandler.cs

@@ -119,17 +119,18 @@ private ISelectClause GetSelectClause(IMartenSession session)
     {
         if (typeof(T) == typeof(string))
         {
-            return new ScalarStringSelectClause("", "");
+            return new ScalarStringSelectClause(string.Empty, string.Empty);
         }
 
-        if (typeof(T).IsSimple())
+        if (PostgresqlProvider.Instance.HasTypeMapping(typeof(T)))
         {
-            return typeof(ScalarSelectClause<>).CloseAndBuildAs<ISelectClause>("", "", typeof(T));
+            return typeof(ScalarSelectClause<>).CloseAndBuildAs<ISelectClause>(string.Empty, string.Empty, typeof(T));
         }
 
+
         if (SqlContainsCustomSelect)
         {
-            return new DataSelectClause<T>("", "");
+            return new DataSelectClause<T>(string.Empty, string.Empty);
         }
 
         return session.StorageFor(typeof(T));