Re: [sqlalchemy] relationsip(): In Parent or Child Class?

[Mike Bayer]

On 5/21/15 4:32 PM, Rich Shepard wrote:

  There are a number of many-to-one table/class relationships in the
application. In the many class I use ForeignKey() to relate that 
column to

the appropriate 'one' class and column.

  Reading the ORM tutorial tells me that the relationship() function 
can be

in either table. I can specify the backref from the many class to the one
class or define it as a child in the one class with the backref to the 
many

class.

  Is one placement preferred over the other? If not, what criteria are 
used

to determine where the relationship() function should be placed?
I think the best use case is to put it on both, using back_populates.  
This is the focus of current documentation: 
http://docs.sqlalchemy.org/en/rel_1_0/orm/backref.html


It's more verbose but I think it's clearer and functionally this is what 
backref does behind the scenes in any case.









Rich


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Re: [sqlalchemy] relationsip(): In Parent or Child Class?

[Rich Shepard]

On Thu, 21 May 2015, Mike Bayer wrote:


I think the best use case is to put it on both, using back_populates.
This is the focus of current documentation:
http://docs.sqlalchemy.org/en/rel_1_0/orm/backref.html

It's more verbose but I think it's clearer and functionally this is what 
backref does behind the scenes in any case.


Mike,

  OK. I'm carefully working through the ORM tutorial and cleaning code as I
go.

Thanks,

Rich

[sqlalchemy] relationsip(): In Parent or Child Class?

[Rich Shepard]

  There are a number of many-to-one table/class relationships in the
application. In the many class I use ForeignKey() to relate that column to
the appropriate 'one' class and column.

  Reading the ORM tutorial tells me that the relationship() function can be
in either table. I can specify the backref from the many class to the one
class or define it as a child in the one class with the backref to the many
class.

  Is one placement preferred over the other? If not, what criteria are used
to determine where the relationship() function should be placed?

Rich

Re: [sqlalchemy] relationsip(): In Parent or Child Class?

[Rich Shepard]

On Thu, 21 May 2015, Mike Bayer wrote:


I think the best use case is to put it on both, using back_populates.


  Validation check: am I correctly using relationship() in the following set
of three tables? (N.B. Other columns removed for clarity and space saving.)

class Agencies(Base):
__tablename__ = 'agencies'
name = Column(Unicode(48), Sequence('name_seq'), primary_key = True)
...
child1 = relationship('AgencyUnits', back_populates = 'agency', cascade = 
'all, delete, delete-orphan')
child2 = relationship('Permits', back_populates = 'agency', cascade = 'all, 
delete, delete-orphan')
child3 = relationship('Inspect', back_populates = 'agency', cascade = 'all, 
delete, delete-orphan')

class AgencyUnits(Base):
__tablename__ = 'agencyUnits'
name = Column(Unicode(48), Sequence('name_seq'), primary_key = True)
parent_name = Column(Unicode(48), nullable = False, ForeignKey(\
'agencies.name'))
parent_id = relationship(Agency, back_populates = 'units')
...
child = relationship(AgencyContacts, back_populates = 'units', cascade = 
'all, delete, delete-orphan')

class AgencyContacts(Base):
__tablename__ = 'agencyContacts'
contact_id = Column(Integer, Sequence('contact_id_seq'), primary_key=True)
...
unit_id = Column(Unicode(48), ForeignKey('agencyUnits.name'))
parent = relationship(AgencyUnits, back_populates = 'contacts')

Rich