Fixes tests, adds notes

src/Umbraco.Core/Persistence/Mappers/BaseMapper.cs

@@ -52,6 +52,7 @@ namespace Umbraco.Core.Persistence.Mappers
             return mappedName;
         }
 
+        // fixme: TSource is used for nothing
         protected void DefineMap<TSource, TTarget>(string sourceName, string targetName)
         {
             if (_sqlSyntax == null)

src/Umbraco.Core/Persistence/Mappers/MapperCollectionBuilder.cs

@@ -35,6 +35,7 @@ namespace Umbraco.Core.Persistence.Mappers
             Add<AuditItemMapper>();
             Add<ContentMapper>();
             Add<ContentTypeMapper>();
+            Add<SimpleContentTypeMapper>();
             Add<DataTypeMapper>();
             Add<DictionaryMapper>();
             Add<DictionaryTranslationMapper>();

src/Umbraco.Core/Persistence/Mappers/SimpleContentTypeMapper.cs

@@ -0,0 +1,37 @@
+using System;
+using System.Collections.Concurrent;
+using Umbraco.Core.Models;
+using Umbraco.Core.Persistence.Dtos;
+
+namespace Umbraco.Core.Persistence.Mappers
+{
+    // TODO: This mapper is actually very useless because the only time it would ever be used is when trying to generate a strongly typed query
+    // on an IContentBase object which is what exposes ISimpleContentType, however the queries that we execute in the content repositories don't actually
+    // join on the content type table. The content type data is resolved outside of the query so the end result of the query that is generated by using
+    // this mapper will either fail or the syntax will target umbracoNode which will be filtering on the actual content NOT the content type.
+    // I'm leaving this here purely because the ExpressionTests rely on this which is fine for testing that the expressions work, but note that this
+    // resulting query will not.
+
+    [MapperFor(typeof(ISimpleContentType))]
+    [MapperFor(typeof(SimpleContentType))]
+    public sealed class SimpleContentTypeMapper : BaseMapper
+    {
+        public SimpleContentTypeMapper(Lazy<ISqlContext> sqlContext, ConcurrentDictionary<Type, ConcurrentDictionary<string, string>> maps)
+            : base(sqlContext, maps)
+        { }
+
+        protected override void DefineMaps()
+        {
+            // There is no reason for using ContentType here instead of SimpleContentType, in fact the underlying DefineMap call does nothing with the first type parameter
+
+            DefineMap<ContentType, NodeDto>(nameof(ContentType.Id), nameof(NodeDto.NodeId));
+            DefineMap<ContentType, NodeDto>(nameof(ContentType.Key), nameof(NodeDto.UniqueId));
+            DefineMap<ContentType, NodeDto>(nameof(ContentType.Name), nameof(NodeDto.Text));
+            DefineMap<ContentType, ContentTypeDto>(nameof(ContentType.Alias), nameof(ContentTypeDto.Alias));
+            DefineMap<ContentType, ContentTypeDto>(nameof(ContentType.Icon), nameof(ContentTypeDto.Icon));
+            DefineMap<ContentType, ContentTypeDto>(nameof(ContentType.IsContainer), nameof(ContentTypeDto.IsContainer));
+            DefineMap<ContentType, ContentTypeDto>(nameof(ContentType.AllowedAsRoot), nameof(ContentTypeDto.AllowAtRoot));
+            DefineMap<ContentType, ContentTypeDto>(nameof(ContentType.IsElement), nameof(ContentTypeDto.IsElement));
+        }
+    }
+}

src/Umbraco.Core/Umbraco.Core.csproj

@@ -217,6 +217,7 @@
     <Compile Include="Models\PublishedContent\VariationContextAccessorExtensions.cs" />
     <Compile Include="Models\Template.cs" />
     <Compile Include="Models\TemplateOnDisk.cs" />
+    <Compile Include="Persistence\Mappers\SimpleContentTypeMapper.cs" />
     <Compile Include="PropertyEditors\Validators\DelimitedValueValidator.cs" />
     <Compile Include="PropertyEditors\Validators\RegexValidator.cs" />
     <Compile Include="PropertyEditors\Validators\RequiredValidator.cs" />