> -----Original Message----- > From: sqlalchemy@googlegroups.com > [mailto:sqlalch...@googlegroups.com] On Behalf Of Chris Withers > Sent: 08 July 2010 09:28 > To: sqlalchemy@googlegroups.com > Subject: Re: [sqlalchemy] Comparable properties > > Oliver Beattie wrote: > > @property > > def is_visible(self): > > return (self.enabled and not self.is_deleted) > > > > This can clearly be mapped quite easily to SQL expression > > `Klass.enabled == True & Klass.is_deleted == False` > > You could always add a class-level attribute that stored this... > > @property > def is_visible(self): > return (self.enabled and not self.is_deleted) > > visible = enabled==True & is_deleted==False > > You may need to wrap that into a method with the > classproperty decorator... > > But, it'd be nice to have one attribute of the object fulfil > both roles, > and I don't know how to do that :-S > > Chris >
I think the 'Derived Attributes' example does what you want: http://www.sqlalchemy.org/docs/examples.html#module-derived_attributes http://www.sqlalchemy.org/trac/browser/examples/derived_attributes/attri butes.py As far as I can tell, it uses some Python descriptor magic to allow your property to work both at the instance and at the class level (so 'self' will either be the instance or the class). Accessing Klass.is_visible returns the SQL expression construct, but instance.is_visible works as normal. You'd be more restricted in what you can write inside your property definition though. For example, you can't use plain Python 'and', or assume that 'self.enabled' evaluates to True or False. I think something like this would work though: @hybrid def is_visible(self): return (self.enabled == True) & (self.is_deleted == False) Hope that helps, Simon -- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To post to this group, send email to sqlalch...@googlegroups.com. To unsubscribe from this group, send email to sqlalchemy+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/sqlalchemy?hl=en.
