Hi Timmy and Michael,
I was wandering here about the same problem. I have a table that defines
keys and value type (ie: key 'username', type 'unicode max 60' - that it is
a constant).
Then, I have the values table, which is polymorphic on value type and also
has a fk to keys table. I think it is a bit lame to duplicate value type in
those two tables, and also not that useful to use the polymorphic as a
"static" value on each record of value table since I already have it on keys
table. By having a fk to keys table, can't I get the polymorphic value from
there?
Thanks in advance,
Richard.
On Wed, May 5, 2010 at 6:47 PM, Timmy Chan <timmy.cha...@gmail.com> wrote:
> thx, that is what i am wondering. they are one-to-one, maybe thats good
> enough for performance. so there's no easy way to do this.
>
>
> On Wed, May 5, 2010 at 5:44 PM, Michael Bayer <mike...@zzzcomputing.com>wrote:
>
>>
>> I meant SQL. if you want to do polymorphic_on based on "a.type" it
>> would at the very least need to issue:
>>
>> SELECT c.*, a.type JOIN b ON c.b_id = b.id JOIN a ON b.a_id=a
>>
>> which is an expensive way to get to where you're going. It can be done
>> of course but not through relation().
>>
>>
>> On May 5, 2010, at 5:36 PM, Timmy Chan wrote:
>>
>> "C"/"D" objects doesn't exist without "A" and "B"? maybe walk up the
>> graph: C.b.a.type
>>
>> im not sure this schema is sensible, if it's really bad, please advise, im
>> not attached to it!
>>
>> thax!
>>
>> On Wed, May 5, 2010 at 5:28 PM, Michael Bayer
>> <mike...@zzzcomputing.com>wrote:
>>
>>> what query do you want to see when you select "C" and "D" objects ?
>>>
>>>
>>>
>>> On May 5, 2010, at 5:26 PM, Timmy Chan wrote:
>>>
>>> thx, is there a way to set that up, or something similar?
>>>
>>> after giving it thought, maybe polymorphic_on isn't what i need. in some
>>> sense, table "A" is similar to a generic container, and i want to constrain
>>> the type of "C" depending on "A"'s type column. is there a way to do this?
>>>
>>> On Wed, May 5, 2010 at 5:15 PM, Michael Bayer
>>> <mike...@zzzcomputing.com>wrote:
>>>
>>>>
>>>> On May 5, 2010, at 5:01 PM, Timmy Chan wrote:
>>>>
>>>> i have 4 tables, a, b, c, d.
>>>>
>>>> a has one-to-many relation with b, b with one-to-one relationship with
>>>> c, c is a polymorphic on a.type, with d being one of the polymorphic types.
>>>>
>>>> is there a way to implement this?
>>>>
>>>> details:
>>>>
>>>> this is what im trying to do in sqlalchemy:
>>>>
>>>> a = Table('a', metadata,
>>>> Column( 'id', Integer(), primary_key=True ),
>>>> Column( 'type', UnicodeText() ) )
>>>>
>>>> b = Table('b', metadata,
>>>> Column( 'id', Integer(), primary_key=True ),
>>>> Column( 'a_id', Integer(), ForeignKey('a.id') ) )
>>>>
>>>> c = Table('c', metadata,
>>>> Column( 'id', Integer(), primary_key=True ),
>>>> Column( 'b_id', Integer(), ForeignKey('b.id') ),
>>>> Column( 'class_id', Integer() ) )
>>>>
>>>> d = Table('d', metadata,
>>>> Column( 'id', Integer(), primary_key=True ),
>>>> Column( 'data', Integer() )
>>>>
>>>> mappers
>>>>
>>>> mapper( A, a )
>>>> mapper( B, b, properties={'a': relationship( A,
>>>> uselist=False,backref='b',
>>>> 'c':relationship( C, uselist=False, backref='b') })
>>>>
>>>> # Does a full join, does not work
>>>> mapper( C, c, polymorphic_on = a.c.type )
>>>> mapper( D, d, inherits=C, polymorphic_identity = "D" )
>>>>
>>>>
>>>> how can i change c to polymorphic on a, through the relationship? is
>>>> there a way to sort by d.data? (b-c/d is one-on-one).
>>>>
>>>>
>>>> this mapping is incorrect. mapper(C) cannot be polymorphic on a
>>>> table which is not part of its mapping, and mapper(C) does not contain an
>>>> "inherits" keyword to that of A. Usually the "polymorphic_on" setting is
>>>> on the base-most mapper in the hierarchy and its not clear here which
>>>> mapper
>>>> you intend for that to be.
>>>>
>>>>
>>>>
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>>>
>>>
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