feat(models): add support for generated and identity columns

* extend Column struct with Generated, GenerationExpression, Identity, and IdentityGeneration fields
* update queryColumns to handle new column properties
* modify migration writer to avoid altering generated and identity columns
* add tests to ensure correct handling of generated and identity columns

pkg/writers/pgsql/migration_writer.go

@@ -433,6 +433,16 @@ func (w *MigrationWriter) generateAlterTableScripts(schema *models.Schema, model
 				Body:       sql,
 			}
 			scripts = append(scripts, script)
+		} else if currentCol.Generated || currentCol.Identity {
+			// GENERATED ALWAYS ... STORED columns and GENERATED ... AS IDENTITY
+			// columns are both driven by Postgres-side machinery (an expression, or
+			// a backing sequence + attidentity flag) that DBML cannot express, so
+			// the model side always looks like drift (missing type modifiers, no
+			// default, NOT NULL mismatches, etc). None of their properties are
+			// alterable the normal way (e.g. dropping a generated column's default
+			// requires DROP EXPRESSION, not DROP DEFAULT; identity columns have no
+			// default to alter at all), so these DB-side-only columns are left
+			// untouched rather than diffed.
 		} else if !columnsEqual(modelCol, currentCol) {
 			// Column exists but properties changed
 			if !columnTypesEqual(modelCol, currentCol) {

pkg/writers/pgsql/migration_writer_test.go

@@ -219,6 +219,91 @@ func TestWriteMigration_AltersColumnNullabilityWhenNotNullDiffers(t *testing.T)
 	}
 }
 
+func TestWriteMigration_DoesNotDropDefaultOnGeneratedColumn(t *testing.T) {
+	current := models.InitDatabase("testdb")
+	currentSchema := models.InitSchema("public")
+	currentTable := models.InitTable("info_city", "public")
+	currentGeom := models.InitColumn("geom", "info_city", "public")
+	currentGeom.Type = "text"
+	currentGeom.Generated = true
+	currentGeom.GenerationExpression = "st_makepoint(lon, lat)"
+	currentTable.Columns["geom"] = currentGeom
+	currentSchema.Tables = append(currentSchema.Tables, currentTable)
+	current.Schemas = append(current.Schemas, currentSchema)
+
+	model := models.InitDatabase("testdb")
+	modelSchema := models.InitSchema("public")
+	modelTable := models.InitTable("info_city", "public")
+	modelGeom := models.InitColumn("geom", "info_city", "public")
+	modelGeom.Type = "text"
+	// DBML cannot express GENERATED ALWAYS columns, so the model declares
+	// this as a plain column with no default — it must not be treated as a
+	// default-value drift against the DB-side generated column.
+	modelTable.Columns["geom"] = modelGeom
+	modelSchema.Tables = append(modelSchema.Tables, modelTable)
+	model.Schemas = append(model.Schemas, modelSchema)
+
+	var buf bytes.Buffer
+	writer, err := NewMigrationWriter(&writers.WriterOptions{})
+	if err != nil {
+		t.Fatalf("Failed to create writer: %v", err)
+	}
+	writer.writer = &buf
+
+	if err := writer.WriteMigration(model, current); err != nil {
+		t.Fatalf("WriteMigration failed: %v", err)
+	}
+
+	output := buf.String()
+	if strings.Contains(output, "DROP DEFAULT") {
+		t.Fatalf("did not expect DROP DEFAULT against a generated column, got:\n%s", output)
+	}
+}
+
+func TestWriteMigration_DoesNotAlterIdentityColumn(t *testing.T) {
+	current := models.InitDatabase("testdb")
+	currentSchema := models.InitSchema("public")
+	currentTable := models.InitTable("users", "public")
+	currentID := models.InitColumn("id", "users", "public")
+	currentID.Type = "bigint"
+	currentID.NotNull = true
+	currentID.Identity = true
+	currentID.IdentityGeneration = "ALWAYS"
+	currentTable.Columns["id"] = currentID
+	currentSchema.Tables = append(currentSchema.Tables, currentTable)
+	current.Schemas = append(current.Schemas, currentSchema)
+
+	model := models.InitDatabase("testdb")
+	modelSchema := models.InitSchema("public")
+	modelTable := models.InitTable("users", "public")
+	modelID := models.InitColumn("id", "users", "public")
+	modelID.Type = "bigint"
+	// DBML cannot express GENERATED ... AS IDENTITY, so the model declares this
+	// as a plain nullable column — it must not be diffed as NOT NULL drift
+	// against the DB-side identity column (Postgres rejects DROP NOT NULL on
+	// an identity column).
+	modelID.NotNull = false
+	modelTable.Columns["id"] = modelID
+	modelSchema.Tables = append(modelSchema.Tables, modelTable)
+	model.Schemas = append(model.Schemas, modelSchema)
+
+	var buf bytes.Buffer
+	writer, err := NewMigrationWriter(&writers.WriterOptions{})
+	if err != nil {
+		t.Fatalf("Failed to create writer: %v", err)
+	}
+	writer.writer = &buf
+
+	if err := writer.WriteMigration(model, current); err != nil {
+		t.Fatalf("WriteMigration failed: %v", err)
+	}
+
+	output := buf.String()
+	if strings.Contains(output, "DROP NOT NULL") {
+		t.Fatalf("did not expect DROP NOT NULL against an identity column, got:\n%s", output)
+	}
+}
+
 func TestWriteMigration_UsesStorageTypeForSerialAlterStatements(t *testing.T) {
 	current := models.InitDatabase("testdb")
 	currentSchema := models.InitSchema("public")

pkg/writers/pgsql/writer.go

@@ -462,6 +462,12 @@ func (w *Writer) GenerateAlterColumnTypeStatements(schema *models.Schema) ([]str
 	for _, table := range schema.Tables {
 		columns := getSortedColumns(table.Columns)
 		for _, col := range columns {
+			if col.Generated || col.Identity {
+				// Type is derived from the generation expression (generated columns) or
+				// tied to the backing sequence (identity columns); leave both alone here
+				// rather than risk an ALTER COLUMN TYPE Postgres won't accept cleanly.
+				continue
+			}
 			targetType := effectiveAlterColumnSQLType(col)
 			stmt, err := w.executor.ExecuteAlterColumnTypeWithCheck(AlterColumnTypeWithCheckData{
 				SchemaName:      schema.Name,
@@ -492,6 +498,14 @@ func (w *Writer) GenerateAlterColumnDefaultStatements(schema *models.Schema) ([]
 	for _, table := range schema.Tables {
 		columns := getSortedColumns(table.Columns)
 		for _, col := range columns {
+			if col.Generated || col.Identity {
+				// Generated columns: Postgres stores the generation expression in
+				// pg_attrdef just like a real default, so the guard template would see
+				// it as a stray default and emit DROP DEFAULT, which Postgres rejects.
+				// Identity columns never carry a DEFAULT at all (they're driven by a
+				// backing sequence), so there is nothing for this generator to manage.
+				continue
+			}
 			setDefault, defaultVal := formatColumnDefaultSQL(col)
 			stmt, err := w.executor.ExecuteAlterColumnDefaultWithCheck(AlterColumnDefaultWithCheckData{
 				SchemaName:   schema.Name,
@@ -605,13 +619,25 @@ func (w *Writer) generateColumnDefinition(col *models.Column) string {
 
 	parts = append(parts, effectiveColumnSQLType(col))
 
+	// GENERATED ALWAYS ... STORED columns are computed from an expression and
+	// cannot also carry a DEFAULT, so this takes the place of the DEFAULT clause.
+	if col.Generated && col.GenerationExpression != "" {
+		parts = append(parts, fmt.Sprintf("GENERATED ALWAYS AS (%s) STORED", col.GenerationExpression))
+	}
+
+	// GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY columns are driven by an
+	// internal sequence, not a literal DEFAULT, and cannot carry one either.
+	if col.Identity && col.IdentityGeneration != "" {
+		parts = append(parts, fmt.Sprintf("GENERATED %s AS IDENTITY", col.IdentityGeneration))
+	}
+
 	// NOT NULL
 	if col.NotNull {
 		parts = append(parts, "NOT NULL")
 	}
 
 	// DEFAULT
-	if col.Default != nil {
+	if !col.Generated && !col.Identity && col.Default != nil {
 		switch v := col.Default.(type) {
 		case string:
 			parts = append(parts, fmt.Sprintf("DEFAULT %s", writers.QuoteDefaultValue(stripBackticks(v), col.Type)))

pkg/writers/pgsql/writer_test.go

@@ -1510,3 +1510,42 @@ func TestIndexStorageParameters(t *testing.T) {
 		})
 	}
 }
+
+func TestGenerateColumnDefinition_GeneratedColumnEmitsGeneratedClauseNotDefault(t *testing.T) {
+	w := &Writer{}
+
+	col := models.InitColumn("geom", "info_city", "public")
+	col.Type = "text"
+	col.NotNull = true
+	col.Generated = true
+	col.GenerationExpression = "st_makepoint(lon, lat)"
+	// A generated column's introspected "default" is actually its generation
+	// expression surfaced via pg_attrdef; it must never be rendered as DEFAULT.
+	col.Default = "st_makepoint(lon, lat)"
+
+	got := w.generateColumnDefinition(col)
+
+	want := "geom text GENERATED ALWAYS AS (st_makepoint(lon, lat)) STORED NOT NULL"
+	if got != want {
+		t.Fatalf("generateColumnDefinition() = %q, want %q", got, want)
+	}
+}
+
+func TestGenerateColumnDefinition_IdentityColumnEmitsIdentityClauseNotDefault(t *testing.T) {
+	w := &Writer{}
+
+	col := models.InitColumn("id", "users", "public")
+	col.Type = "bigint"
+	col.NotNull = true
+	col.Identity = true
+	col.IdentityGeneration = "ALWAYS"
+	// Identity columns never carry a literal default; this must never surface.
+	col.Default = "some-stray-value"
+
+	got := w.generateColumnDefinition(col)
+
+	want := "id bigint GENERATED ALWAYS AS IDENTITY NOT NULL"
+	if got != want {
+		t.Fatalf("generateColumnDefinition() = %q, want %q", got, want)
+	}
+}