README.md

@@ -42,8 +42,17 @@ relspec convert --from pgsql --from-conn "postgres://..." --to sqlite --to-path
 
 # Multiple input files merged
 relspec convert --from json --from-list "a.json,b.json" --to yaml --to-path merged.yaml
+
+# Watch mode: regenerate whenever the source file(s) change (Ctrl-C to stop)
+relspec convert --from dbml --from-path schema.dbml --to gorm --to-path models/ --package models --watch
 ```
 
+`--watch` works with `--from-path` and `--from-list` (not live database
+connections or `--dry-run`). Source files are polled every `--watch-interval`
+(default 500ms), a directory source is watched recursively, and the output path
+is ignored so generating into the source tree does not loop. Conversion errors
+are printed and watching continues.
+
 PostgreSQL connections opened by relspec set `application_name` by default to
 `relspecgo/<version>` (with component suffixes internally, e.g. readers/writers).
 If you need a custom value, provide `application_name` explicitly in the connection

cmd/relspec/convert.go

@@ -59,6 +59,8 @@ var (
 	convertContinueOnError bool
 	convertExtraFields     string
 	convertDryRun          bool
+	convertWatch           bool
+	convertWatchInterval   time.Duration
 )
 
 var convertCmd = &cobra.Command{
@@ -167,7 +169,11 @@ Examples:
 
   # Convert SQLite to PostgreSQL SQL
   relspec convert --from sqlite --from-path database.db \
-                  --to pgsql --to-path schema.sql`,
+                  --to pgsql --to-path schema.sql
+
+  # Regenerate GORM models every time the DBML file changes
+  relspec convert --from dbml --from-path schema.dbml \
+                  --to gorm --to-path models/ --package models --watch`,
 	RunE: runConvert,
 }
 
@@ -189,6 +195,9 @@ func init() {
 
 	convertCmd.Flags().BoolVar(&convertDryRun, "dry-run", false, "Read and validate the input and print the plan without writing any output")
 
+	convertCmd.Flags().BoolVar(&convertWatch, "watch", false, "Watch the source files (--from-path or --from-list) and regenerate the output whenever they change")
+	convertCmd.Flags().DurationVar(&convertWatchInterval, "watch-interval", 500*time.Millisecond, "Polling interval used by --watch")
+
 	err := convertCmd.MarkFlagRequired("from")
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "Error marking from flag as required: %v\n", err)
@@ -204,6 +213,13 @@ func init() {
 }
 
 func runConvert(cmd *cobra.Command, args []string) error {
+	if convertWatch {
+		return runConvertWatch(cmd.Context(), os.Stderr, func() error { return runConvertOnce(cmd) })
+	}
+	return runConvertOnce(cmd)
+}
+
+func runConvertOnce(cmd *cobra.Command) error {
 	fmt.Fprintf(os.Stderr, "\n=== RelSpec Schema Converter ===\n")
 	fmt.Fprintf(os.Stderr, "Started at: %s\n\n", getCurrentTimestamp())
 

cmd/relspec/watch.go

@@ -0,0 +1,152 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"io/fs"
+	"os"
+	"os/signal"
+	"path/filepath"
+	"strings"
+	"syscall"
+	"time"
+)
+
+// watchSnapshot maps a file path to its modification time and size.
+type watchSnapshot map[string]string
+
+// takeWatchSnapshot records the state of every file under the given paths.
+// Directories are walked recursively. Anything at or below the excluded path
+// (typically the output path) is skipped so regenerating output does not
+// retrigger the watcher. Missing paths are simply absent from the snapshot, so
+// creating them later counts as a change.
+func takeWatchSnapshot(paths []string, exclude string) watchSnapshot {
+	snap := watchSnapshot{}
+	exclude = absPathOrSelf(exclude)
+	for _, root := range paths {
+		_ = filepath.WalkDir(root, func(p string, d fs.DirEntry, err error) error {
+			if err != nil {
+				return nil
+			}
+			if exclude != "" && isWithin(absPathOrSelf(p), exclude) {
+				if d.IsDir() {
+					return filepath.SkipDir
+				}
+				return nil
+			}
+			if d.IsDir() {
+				return nil
+			}
+			info, err := d.Info()
+			if err != nil {
+				return nil
+			}
+			snap[p] = fmt.Sprintf("%d-%d", info.ModTime().UnixNano(), info.Size())
+			return nil
+		})
+	}
+	return snap
+}
+
+func (s watchSnapshot) equal(o watchSnapshot) bool {
+	if len(s) != len(o) {
+		return false
+	}
+	for k, v := range s {
+		if o[k] != v {
+			return false
+		}
+	}
+	return true
+}
+
+func absPathOrSelf(p string) string {
+	if p == "" {
+		return ""
+	}
+	if abs, err := filepath.Abs(p); err == nil {
+		return abs
+	}
+	return p
+}
+
+// isWithin reports whether path equals dir or is located below it.
+func isWithin(path, dir string) bool {
+	if path == dir {
+		return true
+	}
+	return strings.HasPrefix(path, dir+string(filepath.Separator))
+}
+
+// watchLoop runs fn once immediately and again whenever the watched paths
+// change, until ctx is cancelled. Changes are debounced: fn runs only after
+// the snapshot has stayed unchanged for one poll interval. Errors from fn are
+// reported to w and do not stop the loop.
+func watchLoop(ctx context.Context, w io.Writer, paths []string, exclude string, interval time.Duration, fn func() error) {
+	run := func() {
+		if err := fn(); err != nil {
+			fmt.Fprintf(w, "Error: %v\n", err)
+		}
+		fmt.Fprintf(w, "Watching for changes (Ctrl-C to stop)...\n")
+	}
+
+	last := takeWatchSnapshot(paths, exclude)
+	run()
+
+	ticker := time.NewTicker(interval)
+	defer ticker.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+		}
+		cur := takeWatchSnapshot(paths, exclude)
+		if cur.equal(last) {
+			continue
+		}
+		// Debounce: wait until writes settle.
+		for settled := false; !settled; {
+			select {
+			case <-ctx.Done():
+				return
+			case <-time.After(interval):
+			}
+			next := takeWatchSnapshot(paths, exclude)
+			settled = next.equal(cur)
+			cur = next
+		}
+		fmt.Fprintf(w, "\nChange detected, regenerating...\n")
+		last = cur
+		run()
+	}
+}
+
+// runConvertWatch runs the conversion once and then again whenever the source
+// files change, until interrupted.
+func runConvertWatch(parent context.Context, w io.Writer, run func() error) error {
+	var paths []string
+	switch {
+	case len(convertFromList) > 0:
+		paths = convertFromList
+	case convertSourcePath != "":
+		paths = []string{convertSourcePath}
+	default:
+		return fmt.Errorf("--watch requires --from-path or --from-list (live database connections cannot be watched)")
+	}
+	if convertDryRun {
+		return fmt.Errorf("--watch cannot be combined with --dry-run")
+	}
+	if convertWatchInterval <= 0 {
+		return fmt.Errorf("--watch-interval must be positive")
+	}
+	if parent == nil {
+		parent = context.Background()
+	}
+
+	ctx, stop := signal.NotifyContext(parent, os.Interrupt, syscall.SIGTERM)
+	defer stop()
+	watchLoop(ctx, w, paths, convertTargetPath, convertWatchInterval, run)
+	return nil
+}

cmd/relspec/watch_test.go

@@ -0,0 +1,92 @@
+package main
+
+import (
+	"bytes"
+	"context"
+	"io"
+	"os"
+	"path/filepath"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestWatchSnapshotExcludesOutput(t *testing.T) {
+	dir := t.TempDir()
+	out := filepath.Join(dir, "out")
+	if err := os.MkdirAll(out, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	src := filepath.Join(dir, "schema.dbml")
+	if err := os.WriteFile(src, []byte("a"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	before := takeWatchSnapshot([]string{dir}, out)
+	if err := os.WriteFile(filepath.Join(out, "gen.go"), []byte("x"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if after := takeWatchSnapshot([]string{dir}, out); !before.equal(after) {
+		t.Errorf("writing into the excluded output path changed the snapshot")
+	}
+	if err := os.WriteFile(src, []byte("changed"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if after := takeWatchSnapshot([]string{dir}, out); before.equal(after) {
+		t.Errorf("modifying a source file did not change the snapshot")
+	}
+}
+
+func TestWatchLoopRerunsOnChange(t *testing.T) {
+	dir := t.TempDir()
+	src := filepath.Join(dir, "schema.dbml")
+	if err := os.WriteFile(src, []byte("a"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	var runs atomic.Int32
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		watchLoop(ctx, io.Discard, []string{src}, "", 10*time.Millisecond, func() error {
+			runs.Add(1)
+			return nil
+		})
+	}()
+
+	waitFor(t, func() bool { return runs.Load() == 1 })
+	if err := os.WriteFile(src, []byte("changed content"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	waitFor(t, func() bool { return runs.Load() == 2 })
+	cancel()
+	<-done
+}
+
+func TestRunConvertWatchValidation(t *testing.T) {
+	oldPath, oldList, oldDry, oldInt := convertSourcePath, convertFromList, convertDryRun, convertWatchInterval
+	defer func() {
+		convertSourcePath, convertFromList, convertDryRun, convertWatchInterval = oldPath, oldList, oldDry, oldInt
+	}()
+	convertSourcePath, convertFromList, convertDryRun, convertWatchInterval = "", nil, false, time.Second
+	if err := runConvertWatch(context.Background(), &bytes.Buffer{}, nil); err == nil {
+		t.Error("expected error without --from-path/--from-list")
+	}
+	convertSourcePath, convertDryRun = "x.dbml", true
+	if err := runConvertWatch(context.Background(), &bytes.Buffer{}, nil); err == nil {
+		t.Error("expected error combining --watch with --dry-run")
+	}
+}
+
+func waitFor(t *testing.T, cond func() bool) {
+	t.Helper()
+	deadline := time.Now().Add(5 * time.Second)
+	for time.Now().Before(deadline) {
+		if cond() {
+			return
+		}
+		time.Sleep(5 * time.Millisecond)
+	}
+	t.Fatal("condition not met in time")
+}