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/
> >
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