The mechanism of backrefs is such that when you assign to the
many-to-one side of the relationship, if the one-to-many is not loaded
from the database, the "append" that you want to do does not occur at
that time. When the list is not loaded, the object is placed in a queue
where it will be appended to the list, if and when that list is actually
loaded from the database, otherwise it is just cleared when the session
is next flushed (because the SQL load of that list will now return the
newly appended value as well).
This behavior is because "append to a list" requires that the list be
loaded from the database and long ago this behavior was established to
eliminate the problem of many/huge list loads just by setting
attributes.
So if your list is doing something critical, like setting other database
attributes when something happens to the list in memory, you would need
to ensure the list is appended towards explicitly. Otherwise, setting
up attributes like this is better addressed at flush time in a handler
such as before_flush().
On 09/23/2016 01:03 PM, HP3 wrote:
There seems to be something else ...
The following test passes which indicates that position is set as 0
right after the bullet and the slide are related:
93 deftest_backref_set(self):
94 self._setup(ordering_list('position'))
95
96 #session = create_session()
97 s1 = Slide('Slide #1')
98 #session.add(s1)
99 #session.flush()
100
101 self.assert_(nots1.bullets)
102 self.assert_(len(s1.bullets) == 0)
103
104 b0 = Bullet('s1/b0')
105 assertb0.position == None
106 #session.add(b0)
107 b0.slide = s1
108 assertb0.position == 0
109
110 #session.flush()
111
112 self.assert_(s1.bullets)
113 self.assert_(len(s1.bullets) == 1)
114 self.assert_(s1.bullets[0] == b0)
115 self.assert_(s1.bullets[0].position == 0)
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