datastore: allow flatten tag on struct pointers

Previously, if a struct had a field marked with the `flatten` tag the
type could not be a pointer to a struct. It would fail with the
following error when loading:

datastore: cannot load field "x.x" into a "y.y": no such struct field

This CL fixes this by checking for a pointer type in loadOneElement.

Relvant issue: #1296

Change-Id: I6de60b76b46bcbde20a0c2ad8574e7ee18758e56
Reviewed-on: https://code-review.googlesource.com/c/38550
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jean de Klerk <deklerk@google.com>

datastore/datastore_test.go

@@ -363,7 +363,8 @@ type OuterFlatten struct {
 	I      []Inner1 `datastore:",flatten"`
 	J      Inner2   `datastore:",flatten,noindex"`
 	Inner3 `datastore:",flatten"`
-	K      Inner4 `datastore:",flatten"`
+	K      Inner4  `datastore:",flatten"`
+	L      *Inner2 `datastore:",flatten"`
 }
 
 type OuterEquivalent struct {
@@ -1116,13 +1117,17 @@ var testCases = []testCase{
 					WW: 12,
 				},
 			},
+			L: &Inner2{
+				Y: 2.71,
+			},
 		},
 		&PropertyList{
 			Property{Name: "A", Value: int64(1), NoIndex: false},
 			Property{Name: "I.W", Value: []interface{}{int64(10), int64(20), int64(30)}, NoIndex: false},
 			Property{Name: "I.X", Value: []interface{}{"ten", "twenty", "thirty"}, NoIndex: false},
 			Property{Name: "J.Y", Value: float64(3.14), NoIndex: true},
 			Property{Name: "K.X.WW", Value: int64(12), NoIndex: false},
+			Property{Name: "L.Y", Value: float64(2.71), NoIndex: false},
 			Property{Name: "Z", Value: true, NoIndex: false},
 		},
 		"",
@@ -1135,6 +1140,7 @@ var testCases = []testCase{
 			Property{Name: "I.W", Value: []interface{}{int64(10), int64(20), int64(30)}, NoIndex: false},
 			Property{Name: "I.X", Value: []interface{}{"ten", "twenty", "thirty"}, NoIndex: false},
 			Property{Name: "J.Y", Value: float64(3.14), NoIndex: true},
+			Property{Name: "L.Y", Value: float64(2.71), NoIndex: false},
 			Property{Name: "Z", Value: true, NoIndex: false},
 		},
 		&OuterFlatten{
@@ -1150,6 +1156,9 @@ var testCases = []testCase{
 			Inner3: Inner3{
 				Z: true,
 			},
+			L: &Inner2{
+				Y: 2.71,
+			},
 		},
 		"",
 		"",

datastore/load.go

@@ -136,6 +136,19 @@ func (l *propertyLoader) loadOneElement(codec fields.List, structValue reflect.V
 			return ""
 		}
 
+		if field.Type.Kind() == reflect.Ptr && field.Type.Elem().Kind() == reflect.Struct {
+			codec, err = structCache.Fields(field.Type.Elem())
+			if err != nil {
+				return err.Error()
+			}
+
+			// Init value if its nil
+			if v.IsNil() {
+				v.Set(reflect.New(field.Type.Elem()))
+			}
+			structValue = v.Elem()
+		}
+
 		if field.Type.Kind() == reflect.Struct {
 			codec, err = structCache.Fields(field.Type)
 			if err != nil {