On 02/23/2016 06:11 PM, Piotr Dobrogost wrote:
On Tuesday, February 23, 2016 at 5:44:45 PM UTC+1, Mike Bayer wrote:
That the old TextValue objects need to be deleted I understand. What I
don't see
is what makes SA specifically blank-out primary key column
'text_value.text_id'?
the "blanking out" is what occurs if you do *not* delete the old
TextValue. It means to be de-associated with the parent Text object
which necessarily means setting TextValue.text_id to None, which in this
mapping happens to also be part of the primary key. If the TextValue
is instead to be deleted, then this step does not occur.
Asking differently; I would understand if I got some integrity errors
while SA
would be trying to insert new rows with the same keys without first deleting
old rows but I have no idea how SA "came up with the idea" to blank-out
primary key?
In this specific example, marking the TextValue as deleted ends up
gving
you an UPDATE of TextValue, because the ORM converts an
INSERT/DELETE of
the same primary key into a single UPDATE. But if your dictionary
Is this an implementation detail or official semantics?
this is how the ORM deals with the case of a DELETE of a row, then an
INSERT of a row with that same identity. The unit of work currently has
the limitation that within a single flush, all INSERT statements happen
before all DELETE statements, so in this case it takes the presence of
both commands and turns them into an UPDATE. however, it's still
considered to be a delete + insert - if you're using the versioning
extensions, the operation will still show up as these two at the session
level.
It's very
important for me
to keep identity of existing TextValue objects and only update values in
their 'value' column.
so this is what your example currently does, minus association proxy and
the dict mapping:
# values = {'pl': u'org', 'en': u'org'}
session.add(
Text(_values=[
TextValue(lang_id='pl', value='org'),
TextValue(lang_id='en', value='org')])
text = session.query(Text).one()
# text.values = {'pl': u'modified', 'en': u'modified'}
text._values.clear()
text._values.append(TextValue(lang_id='pl', value='modified'))
text._values.append(TextValue(lang_id='en', value='modified'))
now if it instead was written like this, we wouldn't get redundant
TextValue objects:
values = {'pl': u'org', 'en': u'org'}
session.add(Text(values=values))
text = session.query(Text).one()
text.values['pl'] = 'modified'
text.values['en'] = 'modified'
it would even keep the same TextValue if you did it like this:
text.values.update({'pl': u'modified', 'en': u'modified'})
so currently, the assignment you're doing treats this as "remove the
old, add the new". This is called a "bulk assignment" inside the
collection mechanics. The associationproxy has a few hooks to modify
this but none of them currently provide a direct link to not first do
the "clear" of the current value, if the incoming value is not the same
object. However, we can just override __set__ and make assignment do
anything we want:
from sqlalchemy.ext.associationproxy import AssociationProxy
class DictUpdateProxy(AssociationProxy):
def __set__(self, obj, values):
proxy = self.__get__(obj, None)
keys_to_remove = set(proxy).difference(values)
for key in keys_to_remove:
del proxy[key]
proxy.update(values)
class Text(Base):
__tablename__ = 'text'
id = Column(Integer, primary_key=True)
_values = relationship('TextValue',
collection_class=attribute_mapped_collection('lang_id'))
values = DictUpdateProxy(
'_values', 'value',
creator=lambda k, v: TextValue(lang_id=k, value=v))
in that way you can control exactly what __set__() does and it will
never remove a TextValue object where the same identity is coming in on
assignment.
(The reason it's important is that I'm using sqlalchemy-continuum
extension for automatic
versioning). Is there some preferred way to make sure identity is intact
and modifications
are done through UPDATE and not DELETE/INSERT?
Also, you say that in this specific example there is no DELETE needed
yet there's an error.
Does it mean SA tries to make sure DELETEs would be possible if they
were needed even if
in this specific example they are not needed?
Regards,
Piotr
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