Session and Transaction Management
It is desirable to wrap individual HTTP requests in database transactions and ORM sessions. This ensures, for example, that if a web request fails part way through, no changes are made to the database. More advanced session management can retry a request if it fails due to a transitive error. Some also avoid the overhead of a database transaction for read-only requests.
Many web frameworks include session and transaction management. For example, TurboGears 2 uses Repoze.tm and zope.sqlalchemy to support this.
Loading and Saving Data
For loading, data needs to be converted from the format loaded from the database into a format that can be displayed by the forms library. This conversion needs to be performed in reverse for saving. ToscaWidgets itself makes this relatively straightforward, but some conversion is still necessary. A database layer should aim to do all such conversion in a transparent manner.
Applications often contain quite repetitive code to initiate the act of loading and saving data. A database layer should aim to do this automatically.
Populating Selection Fields
Selection fields, such as dropdown lists, often have their options sourced from a database table. A database layer should load these automatically, and ideally support cacheing for efficiency.
Generating Widget Definitions
Many applications contain long widget definitions that closely match the underlying database models. The idea is to reduce application code by automatically generating these definitions. Some tools exist that automatically generate source code at design time, but tw2.sqla avoids that approach and generates the definitions at run time.
For flexibility it is very important to be able to override the automatic definitions. This needs to be possible on a per-field basis. It should also be possible to provide a customised policy, specifying the rules for generating widgets from model definitions. For example, an application may decide that all fields named “comment” should have a TextArea, instead of a TextField.
Django has long had the Django admin site, which is a key feature and receives much development attention. There have been several projects in the Python WSGI space to provide automatic form creation, or administrative interfaces. For example, TurboGears 1.0 had both FastData and Catwalk. Such projects have tended to be relatively fragmented and unmaintained. A particular challenge was that FastData and Catwalk originally only worked with SQLObject and could not easily be changed to support SQLAlchemy.
As of 2010, the leading efforts in the Python WSGI space are Sprox and Rum. Sprox helps automatically define forms and views from database models; it is a relatively thin layer that can be readily customised. Rum is a somewhat thicker layer, almost a web framework in itself, and is primarily aimed at producing automatic admin interfaces. Both work with SQLAlchemy and ToscaWidgets, while making efforts to abstract the dependencies.
Sprox and Rum are the primary influences for tw2.sqla. One major difference is that tw2.sqla is only intended to work with SQLAlchemy and ToscaWidgets 2, and makes no attempt to abstract the dependencies. Here is a high-level comparison of their functionality:
| Feature | Sprox | Rum | tw2.sqla |
|---|---|---|---|
| Session and transaction management | None; relies on the containing framework | Supported, same technique as TG2 | Supported, same technique as TG2 |
| Loading and saving data | None; responsibility of the application | Supported, with both conversion and initiation | Supported, with both conversion and initiation |
| Generating widget definitions | Supported, with customisation of both fields and policy | Supported; Sprox can be used if desired | Supported, with customisation of both fields and policy |
| Populating selection fields | Supported; no cacheing | Supported; no cacheing | Supported, with cacheing |
RelativeValidator, efficiency consideration: Say we have a ManyToOne relation, “status” using the column “status_id”. We could have a SelectionField on “status” using RelatedValidator, or one on “status_id” using IntValidator. The former would do stronger validation, while the latter would be more efficient.