feat(reader): 🎉 Add support for multi-file DBML loading

* Implement directory reading for DBML files.
* Merge schemas and tables from multiple files.
* Add tests for multi-file loading and merging behavior.
* Enhance file discovery and sorting logic.

pkg/readers/dbml/reader_test.go

@@ -1,6 +1,7 @@
 package dbml
 
 import (
+	"os"
 	"path/filepath"
 	"testing"
 
@@ -517,3 +518,286 @@ func TestGetForeignKeys(t *testing.T) {
 		t.Error("Expected foreign key constraint type")
 	}
 }
+
+// Tests for multi-file directory loading
+
+func TestReadDirectory_MultipleFiles(t *testing.T) {
+	opts := &readers.ReaderOptions{
+		FilePath: filepath.Join("..", "..", "..", "tests", "assets", "dbml", "multifile"),
+	}
+
+	reader := NewReader(opts)
+	db, err := reader.ReadDatabase()
+	if err != nil {
+		t.Fatalf("ReadDatabase() error = %v", err)
+	}
+
+	if db == nil {
+		t.Fatal("ReadDatabase() returned nil database")
+	}
+
+	// Should have public schema
+	if len(db.Schemas) == 0 {
+		t.Fatal("Expected at least one schema")
+	}
+
+	var publicSchema *models.Schema
+	for _, schema := range db.Schemas {
+		if schema.Name == "public" {
+			publicSchema = schema
+			break
+		}
+	}
+
+	if publicSchema == nil {
+		t.Fatal("Public schema not found")
+	}
+
+	// Should have 3 tables: users, posts, comments
+	if len(publicSchema.Tables) != 3 {
+		t.Fatalf("Expected 3 tables, got %d", len(publicSchema.Tables))
+	}
+
+	// Find tables
+	var usersTable, postsTable, commentsTable *models.Table
+	for _, table := range publicSchema.Tables {
+		switch table.Name {
+		case "users":
+			usersTable = table
+		case "posts":
+			postsTable = table
+		case "comments":
+			commentsTable = table
+		}
+	}
+
+	if usersTable == nil {
+		t.Fatal("Users table not found")
+	}
+	if postsTable == nil {
+		t.Fatal("Posts table not found")
+	}
+	if commentsTable == nil {
+		t.Fatal("Comments table not found")
+	}
+
+	// Verify users table has merged columns from 1_users.dbml and 3_add_columns.dbml
+	expectedUserColumns := []string{"id", "email", "name", "created_at"}
+	if len(usersTable.Columns) != len(expectedUserColumns) {
+		t.Errorf("Expected %d columns in users table, got %d", len(expectedUserColumns), len(usersTable.Columns))
+	}
+
+	for _, colName := range expectedUserColumns {
+		if _, exists := usersTable.Columns[colName]; !exists {
+			t.Errorf("Expected column '%s' in users table", colName)
+		}
+	}
+
+	// Verify posts table columns
+	expectedPostColumns := []string{"id", "user_id", "title", "content", "created_at"}
+	for _, colName := range expectedPostColumns {
+		if _, exists := postsTable.Columns[colName]; !exists {
+			t.Errorf("Expected column '%s' in posts table", colName)
+		}
+	}
+}
+
+func TestReadDirectory_TableMerging(t *testing.T) {
+	opts := &readers.ReaderOptions{
+		FilePath: filepath.Join("..", "..", "..", "tests", "assets", "dbml", "multifile"),
+	}
+
+	reader := NewReader(opts)
+	db, err := reader.ReadDatabase()
+	if err != nil {
+		t.Fatalf("ReadDatabase() error = %v", err)
+	}
+
+	// Find users table
+	var usersTable *models.Table
+	for _, schema := range db.Schemas {
+		for _, table := range schema.Tables {
+			if table.Name == "users" && schema.Name == "public" {
+				usersTable = table
+				break
+			}
+		}
+	}
+
+	if usersTable == nil {
+		t.Fatal("Users table not found")
+	}
+
+	// Verify columns from file 1 (id, email)
+	if _, exists := usersTable.Columns["id"]; !exists {
+		t.Error("Column 'id' from 1_users.dbml not found")
+	}
+	if _, exists := usersTable.Columns["email"]; !exists {
+		t.Error("Column 'email' from 1_users.dbml not found")
+	}
+
+	// Verify columns from file 3 (name, created_at)
+	if _, exists := usersTable.Columns["name"]; !exists {
+		t.Error("Column 'name' from 3_add_columns.dbml not found")
+	}
+	if _, exists := usersTable.Columns["created_at"]; !exists {
+		t.Error("Column 'created_at' from 3_add_columns.dbml not found")
+	}
+
+	// Verify column properties from file 1
+	emailCol := usersTable.Columns["email"]
+	if !emailCol.NotNull {
+		t.Error("Email column should be not null (from 1_users.dbml)")
+	}
+	if emailCol.Type != "varchar(255)" {
+		t.Errorf("Expected email type 'varchar(255)', got '%s'", emailCol.Type)
+	}
+}
+
+func TestReadDirectory_CommentedRefsLast(t *testing.T) {
+	// This test verifies that files with commented refs are processed last
+	// by checking that the file discovery returns them in the correct order
+	dirPath := filepath.Join("..", "..", "..", "tests", "assets", "dbml", "multifile")
+
+	opts := &readers.ReaderOptions{
+		FilePath: dirPath,
+	}
+
+	reader := NewReader(opts)
+	files, err := reader.discoverDBMLFiles(dirPath)
+	if err != nil {
+		t.Fatalf("discoverDBMLFiles() error = %v", err)
+	}
+
+	if len(files) < 2 {
+		t.Skip("Not enough files to test ordering")
+	}
+
+	// Check that 9_refs.dbml (which has commented refs) comes last
+	lastFile := filepath.Base(files[len(files)-1])
+	if lastFile != "9_refs.dbml" {
+		t.Errorf("Expected last file to be '9_refs.dbml' (has commented refs), got '%s'", lastFile)
+	}
+
+	// Check that numbered files without commented refs come first
+	firstFile := filepath.Base(files[0])
+	if firstFile != "1_users.dbml" {
+		t.Errorf("Expected first file to be '1_users.dbml', got '%s'", firstFile)
+	}
+}
+
+func TestReadDirectory_EmptyDirectory(t *testing.T) {
+	// Create a temporary empty directory
+	tmpDir := filepath.Join("..", "..", "..", "tests", "assets", "dbml", "empty_test_dir")
+	err := os.MkdirAll(tmpDir, 0755)
+	if err != nil {
+		t.Fatalf("Failed to create temp directory: %v", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	opts := &readers.ReaderOptions{
+		FilePath: tmpDir,
+	}
+
+	reader := NewReader(opts)
+	db, err := reader.ReadDatabase()
+	if err != nil {
+		t.Fatalf("ReadDatabase() should not error on empty directory, got: %v", err)
+	}
+
+	if db == nil {
+		t.Fatal("ReadDatabase() returned nil database")
+	}
+
+	// Empty directory should return empty database
+	if len(db.Schemas) != 0 {
+		t.Errorf("Expected 0 schemas for empty directory, got %d", len(db.Schemas))
+	}
+}
+
+func TestReadDatabase_BackwardCompat(t *testing.T) {
+	// Test that single file loading still works
+	opts := &readers.ReaderOptions{
+		FilePath: filepath.Join("..", "..", "..", "tests", "assets", "dbml", "simple.dbml"),
+	}
+
+	reader := NewReader(opts)
+	db, err := reader.ReadDatabase()
+	if err != nil {
+		t.Fatalf("ReadDatabase() error = %v", err)
+	}
+
+	if db == nil {
+		t.Fatal("ReadDatabase() returned nil database")
+	}
+
+	if len(db.Schemas) == 0 {
+		t.Fatal("Expected at least one schema")
+	}
+
+	schema := db.Schemas[0]
+	if len(schema.Tables) != 1 {
+		t.Fatalf("Expected 1 table, got %d", len(schema.Tables))
+	}
+
+	table := schema.Tables[0]
+	if table.Name != "users" {
+		t.Errorf("Expected table name 'users', got '%s'", table.Name)
+	}
+}
+
+func TestParseFilePrefix(t *testing.T) {
+	tests := []struct {
+		filename   string
+		wantPrefix int
+		wantHas    bool
+	}{
+		{"1_schema.dbml", 1, true},
+		{"2_tables.dbml", 2, true},
+		{"10_relationships.dbml", 10, true},
+		{"99_data.dbml", 99, true},
+		{"schema.dbml", 0, false},
+		{"tables_no_prefix.dbml", 0, false},
+		{"/path/to/1_file.dbml", 1, true},
+		{"/path/to/file.dbml", 0, false},
+		{"1-file.dbml", 1, true},
+		{"2-another.dbml", 2, true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.filename, func(t *testing.T) {
+			gotPrefix, gotHas := parseFilePrefix(tt.filename)
+			if gotPrefix != tt.wantPrefix {
+				t.Errorf("parseFilePrefix(%s) prefix = %d, want %d", tt.filename, gotPrefix, tt.wantPrefix)
+			}
+			if gotHas != tt.wantHas {
+				t.Errorf("parseFilePrefix(%s) hasPrefix = %v, want %v", tt.filename, gotHas, tt.wantHas)
+			}
+		})
+	}
+}
+
+func TestHasCommentedRefs(t *testing.T) {
+	// Test with the actual multifile test fixtures
+	tests := []struct {
+		filename string
+		wantHas  bool
+	}{
+		{filepath.Join("..", "..", "..", "tests", "assets", "dbml", "multifile", "1_users.dbml"), false},
+		{filepath.Join("..", "..", "..", "tests", "assets", "dbml", "multifile", "2_posts.dbml"), false},
+		{filepath.Join("..", "..", "..", "tests", "assets", "dbml", "multifile", "3_add_columns.dbml"), false},
+		{filepath.Join("..", "..", "..", "tests", "assets", "dbml", "multifile", "9_refs.dbml"), true},
+	}
+
+	for _, tt := range tests {
+		t.Run(filepath.Base(tt.filename), func(t *testing.T) {
+			gotHas, err := hasCommentedRefs(tt.filename)
+			if err != nil {
+				t.Fatalf("hasCommentedRefs() error = %v", err)
+			}
+			if gotHas != tt.wantHas {
+				t.Errorf("hasCommentedRefs(%s) = %v, want %v", filepath.Base(tt.filename), gotHas, tt.wantHas)
+			}
+		})
+	}
+}