Add fields order (#550)

console.go

@@ -69,6 +69,11 @@ type ConsoleWriter struct {
 	// PartsExclude defines parts to not display in output.
 	PartsExclude []string
 
+	// FieldsOrder defines the order of contextual fields in output.
+	FieldsOrder []string
+
+	fieldIsOrdered map[string]int
+
 	// FieldsExclude defines contextual fields to not display in output.
 	FieldsExclude []string
 
@@ -191,7 +196,12 @@ func (w ConsoleWriter) writeFields(evt map[string]interface{}, buf *bytes.Buffer
 		}
 		fields = append(fields, field)
 	}
-	sort.Strings(fields)
+
+	if len(w.FieldsOrder) > 0 {
+		w.orderFields(fields)
+	} else {
+		sort.Strings(fields)
+	}
 
 	// Write space only if something has already been written to the buffer, and if there are fields.
 	if buf.Len() > 0 && len(fields) > 0 {
@@ -324,6 +334,32 @@ func (w ConsoleWriter) writePart(buf *bytes.Buffer, evt map[string]interface{},
 	}
 }
 
+// orderFields takes an array of field names and an array representing field order
+// and returns an array with any ordered fields at the beginning, in order,
+// and the remaining fields after in their original order.
+func (w ConsoleWriter) orderFields(fields []string) {
+	if w.fieldIsOrdered == nil {
+		w.fieldIsOrdered = make(map[string]int)
+		for i, fieldName := range w.FieldsOrder {
+			w.fieldIsOrdered[fieldName] = i
+		}
+	}
+	sort.Slice(fields, func(i, j int) bool {
+		ii, iOrdered := w.fieldIsOrdered[fields[i]]
+		jj, jOrdered := w.fieldIsOrdered[fields[j]]
+		if iOrdered && jOrdered {
+			return ii < jj
+		}
+		if iOrdered {
+			return true
+		}
+		if jOrdered {
+			return false
+		}
+		return fields[i] < fields[j]
+	})
+}
+
 // needsQuote returns true when the string s should be quoted in output.
 func needsQuote(s string) bool {
 	for i := range s {

console_test.go

@@ -486,6 +486,23 @@ func TestConsoleWriterConfiguration(t *testing.T) {
 		}
 	})
 
+	t.Run("Sets FieldsOrder", func(t *testing.T) {
+		buf := &bytes.Buffer{}
+		w := zerolog.ConsoleWriter{Out: buf, NoColor: true, FieldsOrder: []string{"zebra", "aardvark"}}
+
+		evt := `{"level": "info", "message": "Zoo", "aardvark": "Able", "mussel": "Mountain", "zebra": "Zulu"}`
+		_, err := w.Write([]byte(evt))
+		if err != nil {
+			t.Errorf("Unexpected error when writing output: %s", err)
+		}
+
+		expectedOutput := "<nil> INF Zoo zebra=Zulu aardvark=Able mussel=Mountain\n"
+		actualOutput := buf.String()
+		if actualOutput != expectedOutput {
+			t.Errorf("Unexpected output %q, want: %q", actualOutput, expectedOutput)
+		}
+	})
+
 	t.Run("Sets FieldsExclude", func(t *testing.T) {
 		buf := &bytes.Buffer{}
 		w := zerolog.ConsoleWriter{Out: buf, NoColor: true, FieldsExclude: []string{"foo"}}