On Tue, Feb 5, 2019 at 9:22 AM Gmoney <grgg...@gmail.com> wrote: > > We have some elaborate logic in a a @hybrid_property and > @<hybrid_property>.expression that I'm keen on setting up some testing to > ensure that both cases return the same value. I'm having some difficulty > sorting out when each version is used at any given time though. > > @hybrid_property > def testProp(self): > ...etc... > > @testProp.expression(cls): > ...etc... > > ... > > # This query generated from this does not include 'test_prop' in the > select. > orm_obj = Task.query.filter(Task.id == '30340').one() > # It then executes through the @hybrid_property (python) code when > you access it. > print (orm_obj.test_prop) > > # This version runs through the @testProp.expression at query > runtime, generating the value from the select > tuples = Task.query.filter(Task.id == > '30340').with_entities(Task.test_prop) > # And of course accessing the tuple here doesn't run the > @hybrid_property code, as it's already there. > print (tuples[0].test_prop) > > > I understand that one is an instance vs a class property, but during the > query processing it's not clear to me when/why one would be used vs the > other. Is the 'with_entities' doing something here?
when you run Task.test_prop, you just ran your hybrid's @expression function. with_entities didn't run it, you did. > What other cases would the expression version be used? any time you use Task.test_prop, e.g., call it from the class. It will never be run otherwise. all occurrences of your @expression function running are in your own source code, not in SQLAlchemy, which knows nothing about this attribute. > I'm just looking for some help understanding the when/where/why of these. I'd > really love to NOT write the expression version at all if possible (we won't > need to filter on it in the query or anything), then you should change it to raise NotImplementedError(). you will then know immediately where it is being called :) and those places can be fixed (since they are all on your end). > because in our case, it's just begging for us to introduce bugs via > inconsistent implementations of the same intended logic. so you should not use @hybrid_property. Use a regular Python @property. > > -- > SQLAlchemy - > The Python SQL Toolkit and Object Relational Mapper > > http://www.sqlalchemy.org/ > > To post example code, please provide an MCVE: Minimal, Complete, and > Verifiable Example. See http://stackoverflow.com/help/mcve for a full > description. > --- > You received this message because you are subscribed to the Google Groups > "sqlalchemy" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to sqlalchemy+unsubscr...@googlegroups.com. > To post to this group, send email to sqlalchemy@googlegroups.com. > Visit this group at https://groups.google.com/group/sqlalchemy. > For more options, visit https://groups.google.com/d/optout. -- SQLAlchemy - The Python SQL Toolkit and Object Relational Mapper http://www.sqlalchemy.org/ To post example code, please provide an MCVE: Minimal, Complete, and Verifiable Example. See http://stackoverflow.com/help/mcve for a full description. --- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To unsubscribe from this group and stop receiving emails from it, send an email to sqlalchemy+unsubscr...@googlegroups.com. To post to this group, send email to sqlalchemy@googlegroups.com. Visit this group at https://groups.google.com/group/sqlalchemy. For more options, visit https://groups.google.com/d/optout.
