Hi Mike, Thanks for the pointers. I take your point about directionality, but it feels like this is a special case that intuitively should work the same way for a single model that it does for two. However for now, it does what it does.
I took at look at using a union @property, and while it does work to return both sides of the relationship, it does not, as you say support querying. I took a look at http://docs.sqlalchemy.org/en/latest/orm/join_conditions.html#relationship-to-non-primary-mapper, and I"ll be honest, I didn't understand most of it. Also http://docs.sqlalchemy.org/en/latest/orm/join_conditions.html#building-query-enabled-properties seems to suggest that only predefined queries would be possible? I'm wondering if instead I should use one-to-many relationships, and use an event to create the opposing relationship when a relationship is added? Would appreciate your thoughts. Thanks! Matt. On Tuesday, 21 February 2017 23:25:27 UTC, Mike Bayer wrote: > > you want "Node.connected" to be the set of all nodes connected in either > direction. The problem is that relationally, all graphs are > "directed", so we have the "left", "right" aspect of things. > > The easiest way to get ".connected" as the union of both sets (hint!) is > to just union them in Python. Give Node a "left_nodes" and > "right_nodes" relationship (example: see > > http://docs.sqlalchemy.org/en/latest/_modules/examples/graphs/directed_graph.html) > > > then make a regular Python @property that returns > "self.left_nodes.union(self.right_nodes)", easy enough. > > If you want to get into querying this relationally, then you need to do > the UNION on the SQL side and you'd need to get into creating a mapping > against a UNION and then building a relationship to it. This is > significantly more involved and would be using some of the techniques at > > http://docs.sqlalchemy.org/en/latest/orm/join_conditions.html#relationship-to-non-primary-mapper. > > > > But, a @property that does a Python union of two relationships, quick > and easy. > > > > On 02/21/2017 04:43 PM, Matthew Brookes wrote: > > > > I apologize if this is well trodden ground, but having googled, and > > stack-overflowed, read the docs, and searched this list, where lots of > > people have asked the same (or similar) questions, I couldn't find a > > concrete answer... > > > > I'm trying to set up a Model such that an entry can be connected to one > > or more other entries, and that the reverse relationship can be found > > from the same attribute. > > > > Effectively I'm trying to do this: > > > > ``` > > > > from sqlalchemy import Integer, ForeignKey, String, Column, Table > > from sqlalchemy.ext.declarative import declarative_base > > from sqlalchemy.orm import relationship > > > > Base = declarative_base() > > > > node_to_node = Table("node_to_node", Base.metadata, > > Column("left_node_id", Integer, ForeignKey("node.id"), > primary_key=True), > > Column("right_node_id", Integer, ForeignKey("node.id"), > primary_key=True) > > ) > > > > class Node(Base): > > __tablename__ = 'node' > > id = Column(Integer, primary_key=True) > > label = Column(String) > > connected = relationship("Node", > > secondary=node_to_node, > > primaryjoin=id==node_to_node.c.left_node_id, > > secondaryjoin=id==node_to_node.c.right_node_id, > > backref="connected" > > ) > > > > ``` > > > > However, that naturally fails (`Error creating backref 'translations' on > > relationship 'Sentence.translations': property of that name exists`) as > > there's no magic to figure out that `translations` should be > > bi-directional. > > > > Is there another way to achieve this? > > > > Thanks! > > > > Matt. > > > > -- > > 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 <javascript:> > > <mailto:sqlalchemy+unsubscr...@googlegroups.com <javascript:>>. > > To post to this group, send email to sqlal...@googlegroups.com > <javascript:> > > <mailto:sqlal...@googlegroups.com <javascript:>>. > > 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.
