implement support for defaultArg in update expressions

src/FSharp.DynamoDB/Expression/UpdateExpr.fs

@@ -284,6 +284,11 @@ let extractUpdateOps (exprs : IntermediateUpdateExprs) =
         | SpecificCall2 <@ List.append @> (None, _, _, [left; right]) ->
             UpdateValue.EList_Append (extractOperand pickler left) (extractOperand pickler right)
 
+        | SpecificCall2 <@ defaultArg @> (None, _, _, [AttributeGet attr; operand]) ->
+            match extractOperand attr.Pickler operand with
+            | Undefined -> invalidExpr()
+            | op -> If_Not_Exists(attr.Id, op)
+
         | _ -> extractOperand pickler expr |> Operand
 
     let rec extractUpdateOp (parent : QuotedAttribute) (expr : Expr) : UpdateOperation =

src/FSharp.DynamoDB/Picklers/PrimitivePicklers.fs

@@ -187,6 +187,11 @@ type OptionPickler<'T>(tp : Pickler<'T>) =
     override __.DefaultValue = None
     override __.Pickle topt = match topt with None -> None | Some t -> tp.Pickle t
     override __.UnPickle a = if a.NULL then None else Some(tp.UnPickle a)
+    override __.PickleCoerced obj =
+        match obj with
+        | :? 'T as t -> tp.Pickle t
+        | :? ('T option) as topt -> __.Pickle topt
+        | _ -> raise <| new InvalidCastException()
 
 type StringRepresentationPickler<'T>(ep : StringRepresentablePickler<'T>) =
     inherit Pickler<'T> ()

tests/FSharp.DynamoDB.Tests/UpdateExpressionTests.fs

@@ -230,6 +230,20 @@ type ``Update Expression Tests`` () =
         let item' = table.UpdateItem(key, <@ fun (r : R) -> { r with List = 1L :: r.List } @>)
         item'.List |> should equal [1L ; 2L]
 
+    [<Fact>]
+    let ``Update using defaultArg combinator (Some)`` () =
+        let item = { mkItem() with Optional = Some (guid()) }
+        let key = table.PutItem item
+        let item' = table.UpdateItem(key, <@ fun (r : R) -> { r with String = defaultArg r.Optional "<undefined>" } @>)
+        item'.String |> should equal item.Optional.Value
+
+    [<Fact>]
+    let ``Update using defaultArg combinator (None)`` () =
+        let item = { mkItem() with Optional = None }
+        let key = table.PutItem item
+        let item' = table.UpdateItem(key, <@ fun (r : R) -> { r with String = defaultArg r.Optional "<undefined>" } @>)
+        item'.String |> should equal "<undefined>"
+
     [<Fact>]
     let ``Update set with add element`` () =
         let item = { mkItem() with Set = set [1L;2L] }