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 <niujing...@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 <niujing...@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+unsubscr...@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+unsubscr...@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+unsubscr...@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/CAFHwexcZPm8fWn_5eGgKR28GOv94TgzW6aBSzRAYkpjV%3DJD%3D9A%40mail.gmail.com.
