extremely helpful, like a beacon for a lost ship:) thanks On Friday, December 11, 2020 at 5:59:50 AM UTC-8 Simon King wrote:
> I think this page might explain it: > > > https://docs.sqlalchemy.org/en/13/core/sqlelement.html#column-elements-and-expressions > > The "case" function, like many other SQLAlchemy functions, returns an > instance of a ClauseElement subclass. This instance is used later when > building SQL queries. Columns themselves are also ClauseElement > subclasses. You can construct them at any time and pass them around > like any other python object. When it comes time to query the > database, the SQLAlchemy compiler will build the query by traversing > all the ClauseElements and converting them to the appropriate SQL. > > The column_property mechanism accepts a ClauseElement and causes it to > be added to the SQL when you later query the table. > > Simon > > On Fri, Dec 11, 2020 at 11:11 AM Jinghui Niu <niuji...@gmail.com> wrote: > > > > Thanks. One thing to clarify, I noticed that here you used `case` > without using in a context of `select`. Is this considered a shorthand > within sqlalchemy? > > > > On Fri, Dec 11, 2020 at 2:16 AM Simon King <si...@simonking.org.uk> > wrote: > >> > >> You can do it, but you need to use an SQL conditional rather than a > >> python one. In this case that would probably be a CASE expression: > >> > >> > https://docs.sqlalchemy.org/en/13/core/sqlelement.html#sqlalchemy.sql.expression.case > >> > >> I think it would look something like this: > >> > >> from sqlalchemy.sql import case > >> > >> aggr = column_property( > >> case([(attr_a == 'go', attr_b + attr_c)], > >> else_=attr_b - attr_c) > >> ) > >> > >> Hope that helps, > >> > >> Simon > >> > >> On Fri, Dec 11, 2020 at 9:13 AM niuji...@gmail.com <niuji...@gmail.com> > wrote: > >> > > >> > I have a mapped class: > >> > > >> > class Model(sqlalchemy.declarative_base()): > >> > attr_a = Column(String) > >> > attr_b = Column(Integer) > >> > attr_c = Column(Integer) > >> > > >> > aggr = column_property(attr_b + attr_c IF attr_a=='go' ELSE attr_b - > attr_c) > >> > > >> > Last line is pseoudo code that requires some conditional logic. Is > such logic allowed inside column_property? Thanks a lot! > >> > > >> > -- > >> > 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+...@googlegroups.com. > >> > To view this discussion on the web visit > https://groups.google.com/d/msgid/sqlalchemy/736f0ded-e39a-47fd-a0db-8ed33057d2a3n%40googlegroups.com > . > >> > >> -- > >> 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 a topic in the > Google Groups "sqlalchemy" group. > >> To unsubscribe from this topic, visit > https://groups.google.com/d/topic/sqlalchemy/a_-Bqhh5wY0/unsubscribe. > >> To unsubscribe from this group and all its topics, send an email to > sqlalchemy+...@googlegroups.com. > >> To view this discussion on the web visit > https://groups.google.com/d/msgid/sqlalchemy/CAFHwexeO2qgWeKeWPN-WH9LD8_Zch4jQRYeiB-WNbZrcMBrZFQ%40mail.gmail.com > . > > > > -- > > 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+...@googlegroups.com. > > To view this discussion on the web visit > https://groups.google.com/d/msgid/sqlalchemy/CAOQAhNeTVkH5m-XXotNyHV_wBrb3Ev9SJnJpTYF2sRuJvxmOFA%40mail.gmail.com > . > -- 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 view this discussion on the web visit https://groups.google.com/d/msgid/sqlalchemy/93992e51-4b60-4093-b0fe-defd43c951e9n%40googlegroups.com.
