feat: adding flatten formatter middleware

Author: samber

README.md

@@ -25,6 +25,7 @@ Common formatters for [slog](https://pkg.go.dev/golang.org/x/exp/slog) library +
 - [HTTPResponseFormatter](#HTTPRequestFormatter-and-HTTPResponseFormatter): transforms a *http.Response into a readable object
 - [PIIFormatter](#PIIFormatter): hide private Personal Identifiable Information (PII)
 - [IPAddressFormatter](#IPAddressFormatter): hide ip address from logs
+- [FlattenFormatterMiddlewareOptions](#FlattenFormatterMiddlewareOptions): returns a formatter middleware that flatten attributes recursively
 
 **Custom formatter:**
 - [Format](#Format): pass any attribute into a formatter
@@ -370,6 +371,45 @@ logger.
 // }
 ```
 
+### FlattenFormatterMiddlewareOptions
+
+A formatter middleware that flatten attributes recursively.
+
+```go
+import (
+	slogformatter "github.com/samber/slog-formatter"
+	slogmulti "github.com/samber/slog-multi"
+	"golang.org/x/exp/slog"
+)
+
+logger := slog.New(
+    slogmulti.
+        Pipe(slogformatter.FlattenFormatterMiddlewareOptions{Separator: ".", Prefix: "attrs", IgnorePath: false}.NewFlattenFormatterMiddlewareOptions()).
+        Handler(slog.NewJSONHandler(os.Stdout)),
+)
+
+logger.
+    With("email", "samuel@acme.org").
+    With("environment", "dev").
+    WithGroup("group1").
+    With("hello", "world").
+    WithGroup("group2").
+    With("hello", "world").
+    Error("A message", "foo", "bar")
+
+// outputs:
+// {
+//   "time": "2023-05-20T22:14:55.857065+02:00",
+//   "level": "ERROR",
+//   "msg": "A message",
+//   "attrs.email": "samuel@acme.org",
+//   "attrs.environment": "dev",
+//   "attrs.group1.hello": "world",
+//   "attrs.group1.group2.hello": "world",
+//   "foo": "bar"
+// }
+```
+
 ### Format
 
 Pass every attributes into a formatter.

example/example.go

@@ -5,6 +5,7 @@ import (
 	"os"
 
 	slogformatter "github.com/samber/slog-formatter"
+	slogmulti "github.com/samber/slog-multi"
 	"golang.org/x/exp/slog"
 )
 
@@ -104,6 +105,24 @@ func example() {
 		Error("A message")
 }
 
+func exampleFlatten() {
+	logger := slog.New(
+		slogmulti.
+			Pipe(slogformatter.FlattenFormatterMiddlewareOptions{Separator: ".", Prefix: "attrs", IgnorePath: false}.NewFlattenFormatterMiddlewareOptions()).
+			Handler(slog.NewJSONHandler(os.Stdout)),
+	)
+
+	logger.
+		With("email", "samuel@acme.org").
+		With("environment", "dev").
+		WithGroup("group1").
+		With("hello", "world").
+		WithGroup("group2").
+		With("hello", "world").
+		Error("A message", "foo", "bar")
+}
+
 func main() {
 	example()
+	// exampleFlatten()
 }

formatter.go

@@ -55,7 +55,6 @@ func FormatByKey(key string, formatter func(slog.Value) slog.Value) Formatter {
 		}
 
 		return formatter(value), true
-
 	}
 }
 

middleware.go middleware_formatter.go

@@ -0,0 +1,146 @@
+package slogformatter
+
+import (
+	"context"
+
+	"github.com/samber/lo"
+	slogmulti "github.com/samber/slog-multi"
+	"golang.org/x/exp/slog"
+)
+
+type FlattenFormatterMiddlewareOptions struct {
+	// Ignore attribute path and therefore ignore attribute key prefix.
+	// Some attribute keys may collide.
+	IgnorePath bool
+	// Separator between prefix and key.
+	Separator string
+	// Attribute key prefix.
+	Prefix string
+}
+
+// NewFlattenFormatterMiddlewareOptions returns a formatter middleware that flatten attributes recursively.
+func (o FlattenFormatterMiddlewareOptions) NewFlattenFormatterMiddlewareOptions() slogmulti.Middleware {
+	return func(next slog.Handler) slog.Handler {
+		return &FlattenFormatterMiddleware{
+			next: next,
+			option: FlattenFormatterMiddlewareOptions{
+				IgnorePath: o.IgnorePath,
+				Separator:  lo.Ternary(o.Separator == "", ".", o.Separator),
+				Prefix:     o.Prefix,
+			},
+		}
+	}
+}
+
+type FlattenFormatterMiddleware struct {
+	next   slog.Handler
+	option FlattenFormatterMiddlewareOptions
+}
+
+// Implements slog.Handler
+func (h *FlattenFormatterMiddleware) Enabled(ctx context.Context, level slog.Level) bool {
+	return h.next.Enabled(ctx, level)
+}
+
+// Implements slog.Handler
+func (h *FlattenFormatterMiddleware) Handle(ctx context.Context, record slog.Record) error {
+	// newRecord := slog.NewRecord(record.Time, record.Level, record.Message, record.PC)
+
+	// record.Attrs(func(attr slog.Attr) bool {
+	// 	if !h.option.IgnorePath {
+	// 		newRecord.AddAttrs(
+	// 			PrefixAttrKeys(
+	// 				lo.Ternary(h.option.Prefix == "" || h.option.IgnorePath, "", h.option.Prefix+h.option.Separator),
+	// 				FlattenAttrsWithPrefix(h.option.Separator, h.option.Prefix, []slog.Attr{attr}),
+	// 			)...,
+	// 		)
+	// 	} else {
+	// 		newRecord.AddAttrs(FlattenAttrs([]slog.Attr{attr})...)
+	// 	}
+	// 	return true
+	// })
+
+	// return h.next.Handle(ctx, newRecord)
+	return h.next.Handle(ctx, record)
+}
+
+// Implements slog.Handler
+func (h *FlattenFormatterMiddleware) WithAttrs(attrs []slog.Attr) slog.Handler {
+	if !h.option.IgnorePath {
+		attrs = PrefixAttrKeys(
+			lo.Ternary(h.option.Prefix == "" || h.option.IgnorePath, "", h.option.Prefix+h.option.Separator),
+			FlattenAttrsWithPrefix(h.option.Separator, h.option.Prefix, attrs),
+		)
+	} else {
+		attrs = FlattenAttrs(attrs)
+	}
+
+	return &FlattenFormatterMiddleware{
+		next:   h.next.WithAttrs(attrs),
+		option: h.option,
+	}
+}
+
+// Implements slog.Handler
+func (h *FlattenFormatterMiddleware) WithGroup(name string) slog.Handler {
+	prefix := h.option.Prefix + h.option.Separator
+	if h.option.IgnorePath || h.option.Prefix == "" {
+		prefix = ""
+	}
+
+	return &FlattenFormatterMiddleware{
+		next: h.next,
+		option: FlattenFormatterMiddlewareOptions{
+			IgnorePath: h.option.IgnorePath,
+			Separator:  h.option.Separator,
+			Prefix:     prefix + name,
+		},
+	}
+}
+
+// PrefixAttrKeys prefix attribute keys.
+func PrefixAttrKeys(prefix string, attrs []slog.Attr) []slog.Attr {
+	return lo.Map(attrs, func(item slog.Attr, _ int) slog.Attr {
+		return slog.Attr{
+			Key:   prefix + item.Key,
+			Value: item.Value,
+		}
+	})
+}
+
+// FlattenAttrs flatten attributes recursively.
+func FlattenAttrs(attrs []slog.Attr) []slog.Attr {
+	output := []slog.Attr{}
+
+	for _, attr := range attrs {
+		switch attr.Value.Kind() {
+		case slog.KindAny, slog.KindBool, slog.KindDuration, slog.KindFloat64, slog.KindInt64, slog.KindUint64, slog.KindString, slog.KindTime:
+			output = append(output, attr)
+		case slog.KindGroup:
+			output = append(output, attr.Value.Group()...)
+		case slog.KindLogValuer:
+			output = append(output, slog.Any(attr.Key, attr.Value.Resolve().Any()))
+		}
+	}
+
+	return output
+}
+
+// FlattenAttrsWithPrefix flatten attributes recursively, with prefix.
+func FlattenAttrsWithPrefix(separator string, prefix string, attrs []slog.Attr) []slog.Attr {
+	output := []slog.Attr{}
+
+	for _, attr := range attrs {
+		switch attr.Value.Kind() {
+		case slog.KindAny, slog.KindBool, slog.KindDuration, slog.KindFloat64, slog.KindInt64, slog.KindUint64, slog.KindString, slog.KindTime:
+			output = append(output, attr)
+		case slog.KindGroup:
+			output = append(output, PrefixAttrKeys(prefix+separator+attr.Key+separator, FlattenAttrsWithPrefix(separator, "", attr.Value.Group()))...)
+		case slog.KindLogValuer:
+			attr := slog.Any(attr.Key, attr.Value.Resolve().Any())
+			output = append(output, FlattenAttrsWithPrefix(separator, prefix, []slog.Attr{attr})...)
+		}
+	}
+
+	return output
+}