added ?fields query parameter support to limit the returned api fields

tools/rest/json_serializer.go

@@ -0,0 +1,121 @@
+package rest
+
+import (
+	"encoding/json"
+	"strings"
+
+	"github.com/labstack/echo/v5"
+	"github.com/pocketbase/pocketbase/tools/search"
+)
+
+// Serializer represents custom REST JSON serializer based on echo.DefaultJSONSerializer,
+// with support for additional generic response data transformation (eg. fields picker).
+type Serializer struct {
+	echo.DefaultJSONSerializer
+
+	FieldsParam string
+}
+
+// Serialize converts an interface into a json and writes it to the response.
+//
+// It also provides a generic response data fields picker via the FieldsParam query parameter (default to "fields").
+func (s *Serializer) Serialize(c echo.Context, i any, indent string) error {
+	fieldsParam := s.FieldsParam
+	if fieldsParam == "" {
+		fieldsParam = "fields"
+	}
+
+	param := c.QueryParam(fieldsParam)
+	if param == "" {
+		return s.DefaultJSONSerializer.Serialize(c, i, indent)
+	}
+
+	fields := strings.Split(param, ",")
+	for i, f := range fields {
+		fields[i] = strings.TrimSpace(f)
+	}
+
+	encoded, err := json.Marshal(i)
+	if err != nil {
+		return err
+	}
+
+	var decoded any
+
+	if err := json.Unmarshal(encoded, &decoded); err != nil {
+		return err
+	}
+
+	var isSearchResult bool
+
+	switch i.(type) {
+	case search.Result, *search.Result:
+		isSearchResult = true
+	}
+
+	if isSearchResult {
+		if decodedMap, ok := decoded.(map[string]any); ok {
+			pickFields(decodedMap["items"], fields)
+		}
+	} else {
+		pickFields(decoded, fields)
+	}
+
+	return s.DefaultJSONSerializer.Serialize(c, decoded, indent)
+}
+
+func pickFields(data any, fields []string) {
+	switch v := data.(type) {
+	case map[string]any:
+		pickMapFields(v, fields)
+	case []map[string]any:
+		for _, item := range v {
+			pickMapFields(item, fields)
+		}
+	case []any:
+		if len(v) == 0 {
+			return // nothing to pick
+		}
+
+		if _, ok := v[0].(map[string]any); !ok {
+			return // for now ignore non-map values
+		}
+
+		for _, item := range v {
+			pickMapFields(item.(map[string]any), fields)
+		}
+	}
+}
+
+func pickMapFields(data map[string]any, fields []string) {
+	if len(fields) == 0 {
+		return // nothing to pick
+	}
+
+DataLoop:
+	for k := range data {
+		matchingFields := make([]string, 0, len(fields))
+		for _, f := range fields {
+			if strings.HasPrefix(f+".", k+".") {
+				matchingFields = append(matchingFields, f)
+				continue
+			}
+		}
+
+		if len(matchingFields) == 0 {
+			delete(data, k)
+			continue DataLoop
+		}
+
+		// trim the key from the fields
+		for i, v := range matchingFields {
+			trimmed := strings.TrimSuffix(strings.TrimPrefix(v+".", k+"."), ".")
+			if trimmed == "" {
+				continue DataLoop
+			}
+			matchingFields[i] = trimmed
+		}
+
+		pickFields(data[k], matchingFields)
+	}
+}