fix(mappers): dot-notation field overrides now resolve through collection element types (#92)

Author: j-d-ha

docs/examples/nested-mapping.md

@@ -59,4 +59,34 @@ public class Product
 public static partial class OrderMapper { }
 ```
 
+## Collections with Field Overrides
+
+Dot-notation overrides work through collection properties by targeting the element type:
+
+```csharp
+[DynamoMapper]
+[DynamoField("Items.ProductId", AttributeName = "product_id")]
+[DynamoField("Items.CreatedAt", Format = "yyyy-MM-dd")]
+public static partial class OrderMapper
+{
+    public static partial Dictionary<string, AttributeValue> ToItem(Order source);
+    public static partial Order FromItem(Dictionary<string, AttributeValue> item);
+}
+
+public class Order
+{
+    public string Id { get; set; }
+    public List<LineItem> Items { get; set; }
+}
+
+public class LineItem
+{
+    public string ProductId { get; set; }
+    public DateTime CreatedAt { get; set; }
+}
+```
+
+The same pattern works for arrays (`LineItem[]`) and dictionary values
+(`Dictionary<string, LineItem>`).
+
 See `examples/DynamoMapper.Nested/Program.cs` for the full walkthrough.

docs/usage/field-configuration.md

@@ -38,6 +38,41 @@ Notes:
 - Dot-notation overrides force inline mapping for the nested path.
 - Invalid paths emit `DM0008`.
 
+### Collection Element Members
+
+The same dot-notation syntax works when the intermediate segment is a collection (`List<T>`, `T[]`,
+`Dictionary<string, T>`, etc.). The path traverses into the **element type** of the collection:
+
+```csharp
+[DynamoMapper]
+[DynamoField("Contacts.VerifiedAt", Format = "yyyy-MM-dd")]
+[DynamoField("Contacts.Name", AttributeName = "contact_name")]
+public static partial class CustomerMapper
+{
+    public static partial Dictionary<string, AttributeValue> ToItem(Customer source);
+    public static partial Customer FromItem(Dictionary<string, AttributeValue> item);
+}
+
+public class Customer
+{
+    public string Id { get; set; }
+    public List<CustomerContact> Contacts { get; set; }
+}
+
+public class CustomerContact
+{
+    public string Name { get; set; }
+    public DateTime VerifiedAt { get; set; }
+}
+```
+
+Notes:
+
+- The override applies to every element in the collection — there is no per-index syntax.
+- An invalid property name on the element type still emits `DM0008`.
+- Dictionary value types are also supported: `"ProductMap.CreatedAt"` targets `CreatedAt` on
+  the value type of `Dictionary<string, OrderItem>`.
+
 ## Supported Options
 
 | Option | Description |

skills/dynamo-mapper/references/core-usage.md

@@ -54,7 +54,8 @@ Use `[DynamoIgnore(memberName)]` to skip one or both directions.
 - `FromModel` skips model -> item
 - `ToModel` skips item -> model
 
-Dot notation works for nested members like `"ShippingAddress.Line1"`.
+Dot notation works for nested members like `"ShippingAddress.Line1"` and for collection element
+members like `"Contacts.VerifiedAt"` (where `Contacts` is `List<CustomerContact>`).
 
 ## Constructors
 

skills/dynamo-mapper/references/diagnostics.md

@@ -9,7 +9,9 @@
 - `DM0005` incompatible `DynamoKind` -> remove the override or use a compatible kind
 - `DM0006` nested cycle -> break the cycle, ignore a back-reference, or custom-convert one side
 - `DM0007` unsupported nested member -> fix or ignore that nested member
-- `DM0008` invalid dot path -> fix the path, or include base properties if inheritance is involved
+- `DM0008` invalid dot path -> fix the path; paths can traverse nested objects and collection
+  element types (`"Items.ProductId"` targets `ProductId` on the element of `Items`); include base
+  properties if inheritance is involved
 - `DM0009` helper rendering limit -> likely generator issue
 - `DM0101` no mapper methods found -> add a valid `To*` or `From*` method
 - `DM0102` mismatched model types -> make both directions use the same model type

src/LayeredCraft.DynamoMapper.Generators/Models/ModelClassInfo.cs

@@ -287,6 +287,11 @@ private static DiagnosticInfo[] ValidateDotNotationPaths(
                     propertyType = nullableType.TypeArguments[0];
                 }
 
+                // Unwrap collection element type — dot-notation can target element members
+                var collectionInfo = CollectionTypeAnalyzer.Analyze(propertyType, context);
+                if (collectionInfo is not null)
+                    propertyType = collectionInfo.ElementType;
+
                 currentType = propertyType;
             }
         }

src/LayeredCraft.DynamoMapper.Generators/PropertyMapping/CollectionTypeAnalyzer.cs

@@ -2,7 +2,8 @@
 using LayeredCraft.DynamoMapper.Generator.PropertyMapping.Models;
 using LayeredCraft.DynamoMapper.Runtime;
 using Microsoft.CodeAnalysis;
-using WellKnownType = LayeredCraft.DynamoMapper.Generator.WellKnownTypes.WellKnownTypeData.WellKnownType;
+using WellKnownType =
+    LayeredCraft.DynamoMapper.Generator.WellKnownTypes.WellKnownTypeData.WellKnownType;
 
 namespace LayeredCraft.DynamoMapper.Generator.PropertyMapping;
 
@@ -16,6 +17,7 @@ internal readonly record struct ElementTypeValidationResult(
         NestedMappingInfo? NestedMapping,
         DiagnosticInfo? Error
     );
+
     /// <summary>
     /// Analyzes a type to determine if it's a collection type and returns metadata about it.
     /// </summary>
@@ -44,8 +46,10 @@ internal readonly record struct ElementTypeValidationResult(
             return null;
 
         // Check for Dictionary<TKey, TValue> or IDictionary<TKey, TValue>
-        var dictionaryType = context.WellKnownTypes.Get(WellKnownType.System_Collections_Generic_Dictionary_2);
-        var iDictionaryType = context.WellKnownTypes.Get(WellKnownType.System_Collections_Generic_IDictionary_2);
+        var dictionaryType =
+            context.WellKnownTypes.Get(WellKnownType.System_Collections_Generic_Dictionary_2);
+        var iDictionaryType =
+            context.WellKnownTypes.Get(WellKnownType.System_Collections_Generic_IDictionary_2);
 
         if (IsOrImplements(namedType, dictionaryType) || IsOrImplements(namedType, iDictionaryType))
         {
@@ -66,7 +70,8 @@ internal readonly record struct ElementTypeValidationResult(
         }
 
         // Check for HashSet<T> or ISet<T>
-        var hashSetType = context.WellKnownTypes.Get(WellKnownType.System_Collections_Generic_HashSet_1);
+        var hashSetType =
+            context.WellKnownTypes.Get(WellKnownType.System_Collections_Generic_HashSet_1);
         var iSetType = context.WellKnownTypes.Get(WellKnownType.System_Collections_Generic_ISet_1);
 
         if (IsOrImplements(namedType, hashSetType) || IsOrImplements(namedType, iSetType))
@@ -92,14 +97,16 @@ internal readonly record struct ElementTypeValidationResult(
 
         // Check for List<T>, IList<T>, ICollection<T>, or IEnumerable<T>
         var listType = context.WellKnownTypes.Get(WellKnownType.System_Collections_Generic_List_1);
-        var iListType = context.WellKnownTypes.Get(WellKnownType.System_Collections_Generic_IList_1);
-        var iCollectionType = context.WellKnownTypes.Get(WellKnownType.System_Collections_Generic_ICollection_1);
-        var iEnumerableType = context.WellKnownTypes.Get(WellKnownType.System_Collections_Generic_IEnumerable_1);
-
-        if (IsOrImplements(namedType, listType)
-            || IsOrImplements(namedType, iListType)
-            || IsOrImplements(namedType, iCollectionType)
-            || IsOrImplements(namedType, iEnumerableType))
+        var iListType =
+            context.WellKnownTypes.Get(WellKnownType.System_Collections_Generic_IList_1);
+        var iCollectionType =
+            context.WellKnownTypes.Get(WellKnownType.System_Collections_Generic_ICollection_1);
+        var iEnumerableType =
+            context.WellKnownTypes.Get(WellKnownType.System_Collections_Generic_IEnumerable_1);
+
+        if (IsOrImplements(namedType, listType) || IsOrImplements(namedType, iListType) ||
+            IsOrImplements(namedType, iCollectionType) ||
+            IsOrImplements(namedType, iEnumerableType))
         {
             // Extract element type: List<T>
             if (namedType.TypeArguments.Length == 1)
@@ -141,8 +148,7 @@ internal static bool IsValidElementType(ITypeSymbol elementType, GeneratorContex
     /// <param name="context">The generator context.</param>
     /// <returns>A tuple of (isValid, nestedMappingInfo). nestedMappingInfo is null for primitives.</returns>
     internal static ElementTypeValidationResult ValidateElementType(
-        ITypeSymbol elementType,
-        GeneratorContext context
+        ITypeSymbol elementType, GeneratorContext context
     )
     {
         var nestedContext = NestedAnalysisContext.Create(context, context.MapperRegistry);
@@ -161,16 +167,17 @@ GeneratorContext context
     /// <param name="nestedContext">The nested analysis context to preserve ancestor tracking.</param>
     /// <returns>A tuple of (isValid, nestedMappingInfo). nestedMappingInfo is null for primitives.</returns>
     internal static ElementTypeValidationResult ValidateElementType(
-        ITypeSymbol elementType,
-        NestedAnalysisContext nestedContext
+        ITypeSymbol elementType, NestedAnalysisContext nestedContext
     )
     {
         var context = nestedContext.Context;
 
         // Unwrap Nullable<T> - nullable elements are allowed
         var underlyingType = elementType;
-        if (elementType is INamedTypeSymbol { OriginalDefinition.SpecialType: SpecialType.System_Nullable_T } nullableType
-            && nullableType.TypeArguments.Length == 1)
+        if (elementType is INamedTypeSymbol
+            {
+                OriginalDefinition.SpecialType: SpecialType.System_Nullable_T,
+            } nullableType && nullableType.TypeArguments.Length == 1)
         {
             underlyingType = nullableType.TypeArguments[0];
         }
@@ -200,7 +207,8 @@ NestedAnalysisContext nestedContext
                 return new ElementTypeValidationResult(true, null, null);
 
             // DateTimeOffset
-            var dateTimeOffsetType = context.WellKnownTypes.Get(WellKnownType.System_DateTimeOffset);
+            var dateTimeOffsetType =
+                context.WellKnownTypes.Get(WellKnownType.System_DateTimeOffset);
             if (SymbolEqualityComparer.Default.Equals(namedType, dateTimeOffsetType))
                 return new ElementTypeValidationResult(true, null, null);
 
@@ -215,8 +223,8 @@ NestedAnalysisContext nestedContext
         }
 
         // Check for byte[] (valid for BS - Binary Set)
-        if (underlyingType is IArrayTypeSymbol arrayType
-            && arrayType.ElementType.SpecialType == SpecialType.System_Byte)
+        if (underlyingType is IArrayTypeSymbol arrayType &&
+            arrayType.ElementType.SpecialType == SpecialType.System_Byte)
         {
             return new ElementTypeValidationResult(true, null, null);
         }
@@ -225,12 +233,10 @@ NestedAnalysisContext nestedContext
         if (Analyze(underlyingType, context) is not null)
             return new ElementTypeValidationResult(false, null, null);
 
-        // Try to analyze as a nested object
-        var nestedResult = NestedObjectTypeAnalyzer.Analyze(
-            underlyingType,
-            "element", // property name doesn't matter for element type analysis
-            nestedContext
-        );
+        // Try to analyze as a nested object. Use AnalyzeElementType instead of Analyze so the
+        // caller-supplied path prefix is not further modified by a dummy property name.
+        var nestedResult =
+            NestedObjectTypeAnalyzer.AnalyzeElementType(underlyingType, nestedContext);
 
         if (!nestedResult.IsSuccess)
             return new ElementTypeValidationResult(false, null, nestedResult.Error);
@@ -254,8 +260,10 @@ NestedAnalysisContext nestedContext
     {
         // Unwrap Nullable<T> if present
         var underlyingType = elementType;
-        if (elementType is INamedTypeSymbol { OriginalDefinition.SpecialType: SpecialType.System_Nullable_T } nullableType
-            && nullableType.TypeArguments.Length == 1)
+        if (elementType is INamedTypeSymbol
+            {
+                OriginalDefinition.SpecialType: SpecialType.System_Nullable_T,
+            } nullableType && nullableType.TypeArguments.Length == 1)
         {
             underlyingType = nullableType.TypeArguments[0];
         }
@@ -278,8 +286,8 @@ NestedAnalysisContext nestedContext
         }
 
         // byte[] → BS
-        if (underlyingType is IArrayTypeSymbol arrayType
-            && arrayType.ElementType.SpecialType == SpecialType.System_Byte)
+        if (underlyingType is IArrayTypeSymbol arrayType &&
+            arrayType.ElementType.SpecialType == SpecialType.System_Byte)
         {
             return DynamoKind.BS;
         }
@@ -291,7 +299,9 @@ NestedAnalysisContext nestedContext
     /// <summary>
     /// Checks if a type matches or implements a generic type definition.
     /// </summary>
-    private static bool IsOrImplements(INamedTypeSymbol type, INamedTypeSymbol? genericTypeDefinition)
+    private static bool IsOrImplements(
+        INamedTypeSymbol type, INamedTypeSymbol? genericTypeDefinition
+    )
     {
         if (genericTypeDefinition == null)
             return false;
@@ -301,7 +311,8 @@ private static bool IsOrImplements(INamedTypeSymbol type, INamedTypeSymbol? gene
             return true;
 
         // Check if any interface matches
-        return type.AllInterfaces.Any(i =>
-            SymbolEqualityComparer.Default.Equals(i.OriginalDefinition, genericTypeDefinition));
+        return type.AllInterfaces.Any(
+            i => SymbolEqualityComparer.Default.Equals(i.OriginalDefinition, genericTypeDefinition)
+        );
     }
 }

src/LayeredCraft.DynamoMapper.Generators/PropertyMapping/NestedObjectTypeAnalyzer.cs

@@ -14,6 +14,52 @@ namespace LayeredCraft.DynamoMapper.Generator.PropertyMapping;
 /// </summary>
 internal static class NestedObjectTypeAnalyzer
 {
+    /// <summary>
+    ///     Analyzes a collection element type to determine if it's a nested object and how it should
+    ///     be mapped. Unlike <see cref="Analyze" />, this method does not append a property name to the
+    ///     context path, so that the caller can pre-set the correct path prefix (e.g. "Contacts") and have
+    ///     field overrides like "Contacts.VerifiedAt" resolve correctly.
+    /// </summary>
+    /// <param name="type">The element type to analyze.</param>
+    /// <param name="nestedContext">
+    ///     The nested analysis context, with CurrentPath already set to the
+    ///     collection property's path.
+    /// </param>
+    internal static DiagnosticResult<NestedMappingInfo?> AnalyzeElementType(
+        ITypeSymbol type, NestedAnalysisContext nestedContext
+    )
+    {
+        nestedContext.Context.ThrowIfCancellationRequested();
+
+        if (!IsNestedObjectType(type, nestedContext.Context))
+            return DiagnosticResult<NestedMappingInfo?>.Success(null);
+
+        if (nestedContext.WouldCreateCycle(type))
+            return DiagnosticResult<NestedMappingInfo?>.Failure(
+                DiagnosticDescriptors.CycleDetectedInNestedType,
+                type.Locations.FirstOrDefault()?.CreateLocationInfo(),
+                "element",
+                type.ToDisplayString(SymbolDisplayFormat.FullyQualifiedFormat)
+            );
+
+        if (nestedContext.HasOverridesForCurrentPath())
+            return AnalyzeForInline(type, nestedContext);
+
+        if (nestedContext.Registry.TryGetMapper(type, out var mapperReference) &&
+            mapperReference != null)
+        {
+            var requiresTo = nestedContext.Context.HasToItemMethod;
+            var requiresFrom = nestedContext.Context.HasFromItemMethod;
+            if ((!requiresTo || mapperReference.HasToItemMethod) &&
+                (!requiresFrom || mapperReference.HasFromItemMethod))
+                return DiagnosticResult<NestedMappingInfo?>.Success(
+                    new MapperBasedNesting(mapperReference)
+                );
+        }
+
+        return AnalyzeForInline(type, nestedContext);
+    }
+
     /// <summary>
     ///     Analyzes a type to determine if it's a nested object and how it should be mapped.
     /// </summary>
@@ -135,14 +181,13 @@ private static bool IsWellKnownNonNestedType(INamedTypeSymbol type, GeneratorCon
     /// </summary>
     private static IPropertySymbol[] GetMappableProperties(
         INamedTypeSymbol type, GeneratorContext context
-    ) =>
-        PropertySymbolLookup.GetProperties(
-            type,
-            context.MapperOptions.IncludeBaseClassProperties,
-            static (p, declaringType) =>
-                !p.IsStatic && !p.IsIndexer && (p.GetMethod != null || p.SetMethod != null) &&
-                !(declaringType.IsRecord && p.Name == "EqualityContract")
-        );
+    ) => PropertySymbolLookup.GetProperties(
+        type,
+        context.MapperOptions.IncludeBaseClassProperties,
+        static (p, declaringType) =>
+            !p.IsStatic && !p.IsIndexer && (p.GetMethod != null || p.SetMethod != null) &&
+            !(declaringType.IsRecord && p.Name == "EqualityContract")
+    );
 
     /// <summary>
     ///     Analyzes a type for inline code generation, recursively building property specs.
@@ -225,10 +270,12 @@ private static IPropertySymbol[] GetMappableProperties(
                 CollectionTypeAnalyzer.Analyze(underlyingType, nestedContext.Context);
             if (collectionInfo != null)
             {
+                // Include the collection property's name in the context path so that
+                // element-level overrides (e.g. "Contacts.VerifiedAt") resolve correctly.
                 var validation =
                     CollectionTypeAnalyzer.ValidateElementType(
                         collectionInfo.ElementType,
-                        contextWithAncestor
+                        contextWithAncestor.WithPath(property.Name)
                     );
 
                 if (validation.Error is not null)