Better exception for unusable LINQ combination. Closes GH-2649

docs/documents/full-text.md

@@ -317,7 +317,7 @@ var result = await session
     .Where(x => x.UserName.NgramSearch(term))
     .ToListAsync();
 ```
-<sup><a href='https://github.com/JasperFx/marten/blob/master/src/DocumentDbTests/Indexes/NgramSearchTests.cs#L52-L57' title='Snippet source file'>snippet source</a> | <a href='#snippet-sample_ngram_search' title='Start of snippet'>anchor</a></sup>
+<sup><a href='https://github.com/JasperFx/marten/blob/master/src/DocumentDbTests/Indexes/NgramSearchTests.cs#L67-L72' title='Snippet source file'>snippet source</a> | <a href='#snippet-sample_ngram_search' title='Start of snippet'>anchor</a></sup>
 <a id='snippet-sample_ngram_search-1'></a>
 ```cs
 var store = DocumentStore.For(_ =>
@@ -350,5 +350,13 @@ var result = await session
     .Where(x => x.UserName.NgramSearch(term))
     .ToListAsync();
 ```
-<sup><a href='https://github.com/JasperFx/marten/blob/master/src/DocumentDbTests/Indexes/NgramSearchTests.cs#L67-L97' title='Snippet source file'>snippet source</a> | <a href='#snippet-sample_ngram_search-1' title='Start of snippet'>anchor</a></sup>
+<sup><a href='https://github.com/JasperFx/marten/blob/master/src/DocumentDbTests/Indexes/NgramSearchTests.cs#L82-L112' title='Snippet source file'>snippet source</a> | <a href='#snippet-sample_ngram_search-1' title='Start of snippet'>anchor</a></sup>
+<a id='snippet-sample_ngram_search-2'></a>
+```cs
+var result = await session
+    .Query<User>()
+    .Where(x => x.Address.Line1.NgramSearch(term))
+    .ToListAsync();
+```
+<sup><a href='https://github.com/JasperFx/marten/blob/master/src/DocumentDbTests/Indexes/NgramSearchTests.cs#L145-L150' title='Snippet source file'>snippet source</a> | <a href='#snippet-sample_ngram_search-2' title='Start of snippet'>anchor</a></sup>
 <!-- endSnippet -->

docs/documents/indexing/computed-indexes.md

@@ -92,7 +92,17 @@ CREATE INDEX mt_doc_user_idx_first_namelast_name ON public.mt_doc_user USING btr
 
 As of Marten V7, you can specify multi-field computed indexes through anonymous types like so:
 
-snippet: sample_multi_column_index
+<!-- snippet: sample_multi_column_index -->
+<a id='snippet-sample_multi_column_index'></a>
+```cs
+var store = DocumentStore.For(opts =>
+{
+    // This creates a single index against both FirstName and ListName
+    opts.Schema.For<User>().Index(x => new { x.FirstName, x.LastName });
+});
+```
+<sup><a href='https://github.com/JasperFx/marten/blob/master/src/DocumentDbTests/Indexes/computed_indexes.cs#L153-L161' title='Snippet source file'>snippet source</a> | <a href='#snippet-sample_multi_column_index' title='Start of snippet'>anchor</a></sup>
+<!-- endSnippet -->
 
 ## Customizing a Calculated Index
 

src/LinqTests/Bugs/Bug_2649_select_and_distinct_string_with_ordering.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Linq;
+using System.Threading.Tasks;
+using Marten;
+using Marten.Exceptions;
+using Marten.Testing.Documents;
+using Marten.Testing.Harness;
+using Shouldly;
+
+namespace LinqTests.Bugs;
+
+public class Bug_2649_select_and_distinct_string_with_ordering : BugIntegrationContext
+{
+    [Fact]
+    public async Task can_query_with_this_combination()
+    {
+        await theStore.BulkInsertAsync(Target.GenerateRandomData(100).ToArray());
+
+        var ex = await Should.ThrowAsync<BadLinqExpressionException>(async () =>
+        {
+            var query = await theSession.Query<Target>()
+                .OrderBy(x=> x.String, StringComparer.InvariantCultureIgnoreCase)
+                .Select(x => x.String)
+                .Distinct()
+                .ToListAsync();
+        });
+
+
+    }
+}

src/Marten/Linq/CollectionUsage.Compilation.cs

@@ -2,6 +2,7 @@
 
 using System;
 using System.Diagnostics;
+using System.Linq;
 using JasperFx.Core.Reflection;
 using Marten.Exceptions;
 using Marten.Internal;
@@ -51,6 +52,12 @@ public Statement BuildTopStatement(IMartenSession session, IQueryableMemberColle
 
         if (IsDistinct)
         {
+            if (SelectExpression != null && OrderingExpressions.Any(x => x.IsTransformed))
+            {
+                throw new BadLinqExpressionException(
+                    "Marten is unable to build a query with a Distinct() + Select() + a 'transformed' OrderBy(). You will have to resort to SQL for this query");
+            }
+
             statement.ApplySqlOperator("DISTINCT");
         }
 

src/Marten/Linq/Parsing/Operators/Ordering.cs

@@ -17,6 +17,12 @@ public Ordering(Expression expression, OrderingDirection direction)
 
     public CasingRule CasingRule { get; set; } = CasingRule.CaseSensitive;
 
+    /// <summary>
+    /// Refers to whether or not this ordering is transformed such that it cannot
+    /// be combined with a Distinct(Select()) usage
+    /// </summary>
+    public bool IsTransformed { get; set; }
+
     public string BuildExpression(IQueryableMemberCollection collection)
     {
         var member = collection.MemberFor(Expression, "Invalid OrderBy() expression");

src/Marten/Linq/Parsing/Operators/OrderingOperator.cs

@@ -15,18 +15,20 @@ public OrderingOperator(string methodName, OrderingDirection direction): base(me
     public override void Apply(ILinqQuery query, MethodCallExpression expression)
     {
         var usage = query.CollectionUsageFor(expression);
-        var newOrdering = new Ordering(expression.Arguments[1], Direction);
+        var ordering = new Ordering(expression.Arguments[1], Direction);
         if (expression.Arguments.Count == 3)
         {
             var comparer = expression.Arguments[2].Value() as StringComparer;
 
             if (comparer == StringComparer.OrdinalIgnoreCase || comparer == StringComparer.CurrentCultureIgnoreCase ||
                 comparer == StringComparer.InvariantCultureIgnoreCase)
             {
-                newOrdering.CasingRule = CasingRule.CaseInsensitive;
+                ordering.CasingRule = CasingRule.CaseInsensitive;
             }
+
+            ordering.IsTransformed = true;
         }
 
-        usage.OrderingExpressions.Insert(0, newOrdering);
+        usage.OrderingExpressions.Insert(0, ordering);
     }
 }