schema: add encoder.SetOmitEmptyDefault() method to properly encode ptrs

encoder.go

@@ -11,8 +11,9 @@ type encoderFunc func(reflect.Value) string
 
 // Encoder encodes values from a struct into url.Values.
 type Encoder struct {
-	cache  *cache
-	regenc map[reflect.Type]encoderFunc
+	cache     *cache
+	regenc    map[reflect.Type]encoderFunc
+	omitEmpty bool
 }
 
 // NewEncoder returns a new Encoder with defaults.
@@ -40,6 +41,16 @@ func (e *Encoder) SetAliasTag(tag string) {
 	e.cache.tag = tag
 }
 
+// SetOmitEmptyDefault allows to set omit empty behaviour by default.
+// When pointers to values are used in fields it is required to use omitempty to avoid
+// encoding them as 'null' (which are not decodable by Decoder). This function allows
+// to make omitempty behaviour to be a default, rather than requiring to specify
+// per field tag.
+// The default is false, i.e. one should specify 'omitempty' in tags as usual.
+func (e *Encoder) SetOmitEmptyDefault(omit bool) {
+	e.omitEmpty = omit
+}
+
 // isValidStructPointer test if input value is a valid struct pointer.
 func isValidStructPointer(v reflect.Value) bool {
 	return v.Type().Kind() == reflect.Ptr && v.Elem().IsValid() && v.Elem().Type().Kind() == reflect.Struct
@@ -106,7 +117,7 @@ func (e *Encoder) encode(v reflect.Value, dst map[string][]string) error {
 		// Encode non-slice types and custom implementations immediately.
 		if encFunc != nil {
 			value := encFunc(v.Field(i))
-			if opts.Contains("omitempty") && isZero(v.Field(i)) {
+			if (e.omitEmpty || opts.Contains("omitempty")) && isZero(v.Field(i)) {
 				continue
 			}
 
@@ -132,7 +143,7 @@ func (e *Encoder) encode(v reflect.Value, dst map[string][]string) error {
 		}
 
 		// Encode a slice.
-		if v.Field(i).Len() == 0 && opts.Contains("omitempty") {
+		if v.Field(i).Len() == 0 && (e.omitEmpty || opts.Contains("omitempty")) {
 			continue
 		}
 

encoder_test.go

@@ -523,3 +523,32 @@ func TestRegisterEncoderWithPtrType(t *testing.T) {
 	valExists(t, "DateStart", ss.DateStart.time.String(), vals)
 	valExists(t, "DateEnd", "", vals)
 }
+
+func TestPtrsWithDefaultOmitEmpty(t *testing.T) {
+	type S1 struct {
+		S *string `schema:"s"`
+		B *bool   `schema:"b"`
+	}
+
+	ss := S1{} // all fields are nil
+
+	// default encoder encodes ptrs as "null", which are not decodable
+	// so one has to use omitempty tags
+	encoder := NewEncoder()
+
+	vals := map[string][]string{}
+	err := encoder.Encode(ss, vals)
+
+	noError(t, err)
+	valsLength(t, 2, vals)
+	valExists(t, "s", "null", vals)
+	valExists(t, "b", "null", vals)
+
+	// however, if encoder.SetOmitEmptyDefault(true) is used such fields should be ignored
+	vals = map[string][]string{}
+	encoder.SetOmitEmptyDefault(true)
+	err = encoder.Encode(ss, vals)
+
+	noError(t, err)
+	valsLength(t, 0, vals)
+}