Jonathan LaCour wrote:
>
> Michael Hipp wrote:
>
>> I don't understand how to use the "two" fields created in a
>> ManyToOne relationship?
>>
>> From the tutorial, as below, Elixir will create (in Python) two
>> fields: 'director' and 'director_id'. How are they used? And how
>> are they kept in sync?
>
> So, you aren't actually creating two "fields". You are creating a
> single column in the database, representing the foreign key to the
> referenced table. In this case, that column is "director_id." The
> `director_id` attribute will be populated with the foreign key
> to the referenced row in your `director` table. The `director`
> attribute on your `Movie` entity will use that foreign key to load
> the referenced row from the `director` table into a `Director`
> object.
>
> SQLAlchemy will keep these fields in sync, but not until after the
> object is flushed back to the database. In your example, if you add
> a "flush" of the objects to the database, then the synchronization
> will happen:
Thanks. I took your more complete example and added to it. It illustrates the
part that is (was?) throwing me:
from elixir import *
class Movie(Entity):
director = ManyToOne('Director')
class Director(Entity):
name = Field(Unicode(60))
movies = OneToMany('Movie')
metadata.bind = 'sqlite:///'
setup_all()
metadata.create_all()
d = Director(name=u"George")
m = Movie(director=d)
print '---------------------------------'
print m.director, m.director_id # Inconsistent
d.flush()
m.flush()
print m.director, m.director_id # Consistent
print '---------------------------------'
# Down with the director!
m.director_id = None
print m.director, m.director_id # Inconsistent
d.flush()
m.flush()
print m.director, m.director_id # Inconsistent!!!
print '---------------------------------'
# The correct way...
m = Movie(director=d)
d.flush()
m.flush()
print m.director, m.director_id # Consistent
# Down with the director!
m.director = None # Don't touch the '_id' thing
print m.director, m.director_id # Inconsistent
d.flush()
m.flush()
print m.director, m.director_id # Consistent
Output:====================================================
---------------------------------
<__main__.Director object at 0x00F3BA70> None
<__main__.Director object at 0x00F3BA70> 1
---------------------------------
<__main__.Director object at 0x00F3BA70> None
<__main__.Director object at 0x00F3BA70> None
---------------------------------
<__main__.Director object at 0x00F3BA70> 1
None 1
None None
I'm still struggling with the idea of the director and director_id being in an
inconsistent state until flush(). I'll *definitely* have to remember that.
Seems like it could lead to many hard to find bugs.
In that middle block if I try to get rid of the Director by setting director_id
to None, the object never comes back into a consistent state even after
flush(). I guess the dictum is to never-ever mess with the "director_id" thing
and consider it read-only?
Sorry for being so verbose.
Thanks,
Michael
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