feat(migration): add support for altering column nullability

* Implement ExecuteAlterColumnNullability function
* Create alter_column_nullability template
* Add test for altering column nullability behavior

pkg/writers/pgsql/migration_writer.go

@@ -497,6 +497,29 @@ func (w *MigrationWriter) generateAlterTableScripts(schema *models.Schema, model
 				}
 				scripts = append(scripts, script)
 			}
+
+			// Check nullability changes
+			if modelCol.NotNull != currentCol.NotNull {
+				sql, err := w.executor.ExecuteAlterColumnNullability(AlterColumnNullabilityData{
+					SchemaName: schema.Name,
+					TableName:  modelTable.Name,
+					ColumnName: modelCol.Name,
+					NotNull:    modelCol.NotNull,
+				})
+				if err != nil {
+					return nil, err
+				}
+
+				script := MigrationScript{
+					ObjectName: fmt.Sprintf("%s.%s.%s", schema.Name, modelTable.Name, modelCol.Name),
+					ObjectType: "alter column nullability",
+					Schema:     schema.Name,
+					Priority:   145,
+					Sequence:   len(scripts),
+					Body:       sql,
+				}
+				scripts = append(scripts, script)
+			}
 		}
 	}
 

pkg/writers/pgsql/migration_writer_test.go

@@ -136,6 +136,44 @@ func TestWriteMigration_AltersColumnTypeWhenActualTypeDiffers(t *testing.T) {
 	}
 }
 
+func TestWriteMigration_AltersColumnNullabilityWhenNotNullDiffers(t *testing.T) {
+	current := models.InitDatabase("testdb")
+	currentSchema := models.InitSchema("public")
+	currentTable := models.InitTable("service_instance", "public")
+	currentType := models.InitColumn("rid_service_instance_type", "service_instance", "public")
+	currentType.Type = "text"
+	currentType.NotNull = true
+	currentTable.Columns["rid_service_instance_type"] = currentType
+	currentSchema.Tables = append(currentSchema.Tables, currentTable)
+	current.Schemas = append(current.Schemas, currentSchema)
+
+	model := models.InitDatabase("testdb")
+	modelSchema := models.InitSchema("public")
+	modelTable := models.InitTable("service_instance", "public")
+	modelType := models.InitColumn("rid_service_instance_type", "service_instance", "public")
+	modelType.Type = "text"
+	modelType.NotNull = false
+	modelTable.Columns["rid_service_instance_type"] = modelType
+	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, "ALTER COLUMN rid_service_instance_type DROP NOT NULL") {
+		t.Fatalf("expected migration to drop NOT NULL on existing column, got:\n%s", output)
+	}
+}
+
 func TestWriteMigration_UsesStorageTypeForSerialAlterStatements(t *testing.T) {
 	current := models.InitDatabase("testdb")
 	currentSchema := models.InitSchema("public")

pkg/writers/pgsql/templates.go

@@ -116,6 +116,14 @@ type AlterColumnDefaultData struct {
 	DefaultValue string
 }
 
+// AlterColumnNullabilityData contains data for alter column nullability template
+type AlterColumnNullabilityData struct {
+	SchemaName string
+	TableName  string
+	ColumnName string
+	NotNull    bool
+}
+
 // CreatePrimaryKeyData contains data for create primary key template
 type CreatePrimaryKeyData struct {
 	SchemaName     string
@@ -331,6 +339,16 @@ func (te *TemplateExecutor) ExecuteAlterColumnDefault(data AlterColumnDefaultDat
 	return buf.String(), nil
 }
 
+// ExecuteAlterColumnNullability executes the alter column nullability template
+func (te *TemplateExecutor) ExecuteAlterColumnNullability(data AlterColumnNullabilityData) (string, error) {
+	var buf bytes.Buffer
+	err := te.templates.ExecuteTemplate(&buf, "alter_column_nullability.tmpl", data)
+	if err != nil {
+		return "", fmt.Errorf("failed to execute alter_column_nullability template: %w", err)
+	}
+	return buf.String(), nil
+}
+
 // ExecuteCreatePrimaryKey executes the create primary key template
 func (te *TemplateExecutor) ExecuteCreatePrimaryKey(data CreatePrimaryKeyData) (string, error) {
 	var buf bytes.Buffer

pkg/writers/pgsql/templates/alter_column_nullability.tmpl

@@ -0,0 +1,7 @@
+{{- if .NotNull -}}
+ALTER TABLE {{qual_table .SchemaName .TableName}}
+  ALTER COLUMN {{quote_ident .ColumnName}} SET NOT NULL;
+{{- else -}}
+ALTER TABLE {{qual_table .SchemaName .TableName}}
+  ALTER COLUMN {{quote_ident .ColumnName}} DROP NOT NULL;
+{{- end -}}