Fix copy slice, close #75

copier.go

@@ -122,7 +122,12 @@ func copier(toValue interface{}, fromValue interface{}, opt Option) (err error)
 			slice := reflect.MakeSlice(reflect.SliceOf(to.Type().Elem()), from.Len(), from.Cap())
 			to.Set(slice)
 		}
+
 		for i := 0; i < from.Len(); i++ {
+			if to.Len() < i+1 {
+				to = reflect.Append(to, reflect.New(to.Type().Elem()).Elem())
+			}
+
 			if !set(to.Index(i), from.Index(i), opt.DeepCopy) {
 				err = CopyWithOption(to.Index(i).Addr().Interface(), from.Index(i).Interface(), opt)
 				if err != nil {

copier_test.go

@@ -243,6 +243,29 @@ func TestCopyFromSliceToSlice2(t *testing.T) {
 	}
 }
 
+type CollectionAlias struct {
+	CollectionName string `json:"collection_name"`
+	Name           string `json:"name"`
+}
+
+func createNewCollectionAlias(collectionName string, name string) *CollectionAlias {
+	return &CollectionAlias{
+		CollectionName: collectionName,
+		Name:           name,
+	}
+}
+
+func TestCopyFromSliceToSlice3(t *testing.T) {
+	expectedResult := []*CollectionAlias{
+		createNewCollectionAlias("collection", "collection_alias1"),
+		createNewCollectionAlias("collection", "collection_alias2"),
+		createNewCollectionAlias("collection", "collection_alias3"),
+	}
+
+	mockedResult := []*CollectionAlias{}
+	copier.Copy(&mockedResult, &expectedResult)
+}
+
 func TestEmbeddedAndBase(t *testing.T) {
 	type Base struct {
 		BaseField1 int