On Fri, Jul 5, 2019, at 6:16 AM, Simon King wrote: > In Python, modules are executed when they are imported. Class > definitions are executable statements, where the metaclass is called > to construct the *class*. When you import a module containing > declarative classes, the SQLAlchemy metaclass is called for each of > those classes, building the mappings, registering the polymorphic > identities and so on.
The pattern from a software architecture point of view was identified by Martin Fowler as the "registry" pattern, https://martinfowler.com/eaaCatalog/registry.html, however this pattern doesn't say much about the mechanics used. The registry pattern refers to linking the creation of a class or object to its automatic inclusion in some semi-global or in some cases global registry, which is one SQLAlchemy uses a lot, in some cases explicitly and in other cases only behind the scenes. The declarative metaclass contains a function which scans your class for attributes, builds a Table from these attributes which is associated with the MetaData collection that the metaclass has access towards (this is registry pattern #1). Then, the mapper() function is invoked against the class you created along with this Table, which creates a new Mapper object that is added to a global, weak-referencing collection inside the sqlalchemy.orm.mapper module (this is registry pattern #2). When the mapper "inherits" from another one as seems to be the case here, that inheriting mapper is also amended to include this new mapper in its collection of subclasses. The overall "mapper configure" step which you may have seen scans through this registry of mapper objects and makes sure all the mappers that refer to each other, usually through the relationship() linkage, are fully linked and have been found. > > Simon > > On Fri, Jul 5, 2019 at 10:58 AM natsjoo sodillepa <snats...@gmail.com> wrote: > > > > For the curious. One thing that lingers around is the question: how does > > SQLalchemy make the connection between the parent and the child classes? > > > > In the separate modules, where the child classes are defined there is no > > other code. The only thing that gets executed is the class construction, > > but as > > far as I know that doesn't trigger execution of code in the parent class. I > > know that SQLalchemy uses a metaclass but that's used only for > > instantiation of instances, not of classes, or is it? > > > > -- > > 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. > > To view this discussion on the web visit > > https://groups.google.com/d/msgid/sqlalchemy/c90cafee-13a4-4759-99a7-b2fd656697b2%40googlegroups.com. > > 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. > To view this discussion on the web visit > https://groups.google.com/d/msgid/sqlalchemy/CAFHwexebd5F9HXSX_kcBCY3Xey9aiPLThLX3h6GD1v7bomXd%3Dg%40mail.gmail.com. > 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. To view this discussion on the web visit https://groups.google.com/d/msgid/sqlalchemy/199d9628-bb4a-48bb-9c5a-cb88df2124cc%40www.fastmail.com. For more options, visit https://groups.google.com/d/optout.
