implement polymorphism support for DynamoDB entries

generator/.DevConfigs/bc057d5e-a710-458b-ac2e-92e0cf88b741.json

@@ -0,0 +1,11 @@
+{
+  "services": [
+    {
+      "serviceName": "DynamoDBv2",
+      "type": "patch",
+      "changeLogMessages": [
+        "Implement DynamoDBDerivedTypeAttribute to enable polymorphism support for nested items on save and load data."
+      ]
+    }
+  ]
+}

sdk/src/Services/DynamoDBv2/Custom/DataModel/Attributes.cs

@@ -71,6 +71,38 @@ public DynamoDBTableAttribute(string tableName, bool lowerCamelCaseProperties)
         }
     }
 
+    /// <summary>
+    /// DynamoDB attribute that marks a class for polymorphism support.
+    /// 
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Struct, Inherited = true, AllowMultiple = true)]
+    public sealed class DynamoDBPolymorphicTypeAttribute : DynamoDBAttribute
+    {
+        /// <summary>
+        /// Unique name discriminator of the derived type.
+        /// </summary>
+        public string TypeDiscriminator { get; }
+
+        /// <summary>
+        /// Derived type of the Property type.
+        /// Type must be a subclass of the Property type.
+        /// </summary>
+        [DynamicallyAccessedMembers(InternalConstants.DataModelModeledType)]
+        public Type DerivedType { get; }
+
+        /// <summary>
+        /// Construct an instance of DynamoDBPolymorphicTypeAttribute
+        /// </summary>
+        /// <param name="typeDiscriminator"></param>
+        /// <param name="derivedType"></param>
+        public DynamoDBPolymorphicTypeAttribute(string typeDiscriminator,
+            [DynamicallyAccessedMembers(InternalConstants.DataModelModeledType)] Type derivedType)
+        {
+            TypeDiscriminator = typeDiscriminator;
+            DerivedType = derivedType;
+        }
+
+    }
     /// <summary>
     /// DynamoDB attribute that directs the specified attribute not to
     /// be included when saving or loading objects.
@@ -243,6 +275,36 @@ public DynamoDBPropertyAttribute(string attributeName, bool storeAsEpoch)
         public bool StoreAsEpoch { get; set; }
     }
 
+    /// <summary>
+    /// DynamoDB Polymorphic Property Attribute that marks up current member for polymorphism support.
+    /// Specifies the field name to be used as the type discriminator and the derived types.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Property, Inherited = true, AllowMultiple = true)]
+    public sealed class DynamoDBPolymorphicPropertyAttribute : DynamoDBPropertyAttribute
+    {
+        /// <summary>
+        /// Unique name discriminator of the derived type.
+        /// </summary>
+        public string TypeDiscriminator { get; }
+
+        /// <summary>
+        /// Derived type of the Property type.
+        /// Type must be a subclass of the Property type.
+        /// </summary>
+        public Type DerivedType{ get; }
+
+        /// <summary>
+        /// Construct an instance of DynamoDBPolymorphicPropertyAttribute
+        /// </summary>
+        /// <param name="typeDiscriminator">Name of the field to be used as the type discriminator.</param>
+        /// <param name="derivedType">Derived type names and their corresponding types.</param>
+        public DynamoDBPolymorphicPropertyAttribute(string typeDiscriminator, Type derivedType)
+        {
+            TypeDiscriminator = typeDiscriminator;
+            DerivedType = derivedType;
+        }
+    }
+
     /// <summary>
     /// DynamoDB property that marks up current member as a hash key element.
     /// Exactly one member in a class must be marked with this attribute.

sdk/src/Services/DynamoDBv2/Custom/DataModel/Configs.cs

@@ -140,6 +140,11 @@ public DynamoDBContextConfig()
         /// Service calls made via <see cref="AmazonDynamoDBClient"/> will always return 
         /// <see cref="DateTime"/> attributes in UTC.</remarks>
         public bool? RetrieveDateTimeInUtc { get; set; }
+
+        /// <summary>
+        /// Property indicating the name of the attribute used for derived types conversion.
+        /// </summary>
+        public string DerivedTypeAttributeName { get; set; }
     }
 
     /// <summary>
@@ -434,6 +439,9 @@ public DynamoDBFlatConfig(DynamoDBOperationConfig operationConfig, DynamoDBConte
                 !string.IsNullOrEmpty(operationConfig.IndexName) ? operationConfig.IndexName : DefaultIndexName;
             List<ScanCondition> queryFilter = operationConfig.QueryFilter ?? new List<ScanCondition>();
             ConditionalOperatorValues conditionalOperator = operationConfig.ConditionalOperator;
+            string derivedTypeAttributeName =
+                //!string.IsNullOrEmpty(operationConfig.DerivedTypeAttributeName) ? operationConfig.DerivedTypeAttributeName :
+                !string.IsNullOrEmpty(contextConfig.DerivedTypeAttributeName) ? contextConfig.DerivedTypeAttributeName : "$type";
 
             ConsistentRead = consistentRead;
             SkipVersionCheck = skipVersionCheck;
@@ -449,6 +457,7 @@ public DynamoDBFlatConfig(DynamoDBOperationConfig operationConfig, DynamoDBConte
             MetadataCachingMode = metadataCachingMode;
             DisableFetchingTableMetadata = disableFetchingTableMetadata;
             RetrieveDateTimeInUtc = retrieveDateTimeInUtc;
+            DerivedTypeAttributeName = derivedTypeAttributeName;
 
             State = new OperationState();
         }
@@ -550,6 +559,12 @@ public DynamoDBFlatConfig(DynamoDBOperationConfig operationConfig, DynamoDBConte
         // State of the operation using this config
         internal OperationState State { get; private set; }
 
+        /// <summary>
+        /// Property indicating the name of the attribute used for derived types conversion.
+        /// Default value is "$type" if not set in the config.
+        /// </summary>
+        public string DerivedTypeAttributeName { get; set; }
+
         public class OperationState
         {
             private CircularReferenceTracking referenceTracking;