its probably easier to use __missing__ - see attached.
On Mar 13, 2011, at 4:26 AM, Eric Ongerth wrote:
> Thank you, Mike. Hmm... I tried exactly that technique, the last time
> I found myself with a similar need, a year or two ago. I seem to
> remember it only caused the problem to regress one step farther. But
> I must have missed something and will probably give it another shot.
>
> Any suggestions on how to replace the dict in one of these situations
> with a defaultdict? Just override __setitem__ and __getitem__ ?
>
> - Eric
>
>
> On Mar 12, 10:26 pm, Michael Bayer <mike...@zzzcomputing.com> wrote:
>> On Mar 10, 2011, at 6:20 PM, Eric Ongerth wrote:
>>
>>> So, jek, if you're listening, or anyone else -- is there an already
>>> existing, working implementation of a Dict of Lists or Dict of Sets
>>> collection class?
>>
>> the association_proxy is always there to flatten the "object" in the middle
>> of two relationships typically along an association - so just
>> A->relationship->dict of B->relationship->set of C, association proxy from A
>> to B.cs.
>>
>>
>>
>>> On Mar 10, 1:55 pm, Eric Ongerth <ericonge...@gmail.com> wrote:
>>>> Ach, I did it again... proceeded as if column_mapped_collection and
>>>> attribute_mapped_collection provide collection classes that are dicts
>>>> of lists. No, they're just dicts of scalar values!
>>
>>>> Again and again I need dicts of lists. They seem to really fit the
>>>> data that I tend to work with; A's related to C's indexed by B's (the
>>>> B typically indicating the *type* of relationship between the A and
>>>> the C).
>>
>>>> Eventually I need to work my own implementation of dict-of-lists and
>>>> dict-of-dicts collection classes up to full strength so I can share
>>>> them as a recipe or extension or something.
>>
>>>> On Mar 10, 1:02 pm, Eric Ongerth <ericonge...@gmail.com> wrote:
>>
>>>>> I make occasional use of alternate collection classes for my
>>>>> sqlalchemy relationships. For example, column_mapped_collection. The
>>>>> existing collection classes in sqlalchemy.orm.collections work really
>>>>> well for me; I have a lot of relational data that very naturally
>>>>> belongs in dicts rather than lists because it makes sense as one thing
>>>>> indexed or mapped by another.
>>
>>>>> I find that I often need to add an item to one of these dict-mapped
>>>>> collections and would rather not have to check whether the key already
>>>>> exists in the dict. So I'm doing, essentially:
>>>>> collection.setdefault(key, []).append(value).
>>
>>>>> As a matter of syntactic sugar (which alternative collection classes
>>>>> already are, anyway) I would like to have collection classes backed by
>>>>> dicts with the capability of python's defaultdict, so that I could
>>>>> write more simply, and with equivalent results:
>>>>> collection[key].append(value)
>>
>>>>> I understand how to create my own collection class which will behave
>>>>> as desired. My question is, has anyone already got code for this that
>>>>> I could borrow rather than reinvent the wheel? Thanks in advance. --
>>>>> Eric
>>
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from sqlalchemy import String, Integer, Column, create_engine, ForeignKey
from sqlalchemy.orm import relationship, Session
from sqlalchemy.orm.collections import MappedCollection
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.associationproxy import association_proxy
import operator
class Base(object):
id = Column(Integer, primary_key=True)
Base = declarative_base(cls=Base)
class GenDefaultCollection(MappedCollection):
def __missing__(self, key):
self[key] = b = B(key)
return b
class A(Base):
__tablename__ = "a"
associations = relationship("B",
collection_class=lambda: GenDefaultCollection(operator.attrgetter("key"))
)
collections = association_proxy("associations", "values")
class B(Base):
__tablename__ = "b"
a_id = Column(Integer, ForeignKey("a.id"), nullable=False)
elements = relationship("C", collection_class=set)
key = Column(String)
values = association_proxy("elements", "value")
def __init__(self, key, values=None):
self.key = key
if values:
self.values = values
class C(Base):
__tablename__ = "c"
b_id = Column(Integer, ForeignKey("b.id"), nullable=False)
value = Column(Integer)
def __init__(self, value):
self.value = value
engine = create_engine('sqlite://', echo=True)
Base.metadata.create_all(engine)
session = Session(engine)
session.add_all([
A(collections={
"1":set([1, 2, 3]),
})
])
session.commit()
a1 = session.query(A).first()
print a1.collections["1"]
a1.collections["1"].add(4)
session.commit()
a1.collections["2"].update([7, 8, 9])
session.commit()
print a1.collections["2"]
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