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_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")