OK, so I will treat any classes mapped to a join of multiple tables as
being a read-only API, and manually manage the write-API using
relationship().
It doesn't look like I can define a relationship from the composite
mapped class to individual tables though?
>>>
#single table mappers:
mapper(Movies,movies_table)
mapper(Directors,directors_table)
mapper(Genres,genres_table)
j = join(movies_table,md_table).join(directors_table).join(genres_table)
r0 = relationship(Movies,
primaryjoin=(movies_table.c.movie_id==md_table.c.movie_id),
foreign_keys=([md_table.c.movie_id]))
r1 =
relationship(Directors)#,primaryjoin=(and_(movies_table.c.movie_id==md_table.c.movie_id,md_table.c.director_id==directors_table.c.director_id)))#,secondaryjoin=(md_table.c.director_id==directors_table.c.director_id),secondary=md_table)
r2 = relationship(Genres)
mapper(MoviesAndDirectorsAndGenres,
j.select(use_labels=True).alias('mdg'),
properties={'movie':r0,
'director':r1,
'genre':r2
},
passive_updates=False)
Tried lots of permutations of this, but it doesn't look like I can get
sqla to understand a 1-to-1 relationship between the
multiple-table-mapped MoviesAndDirectorsAndGenres objects and the
Movies, Directors, or Genres objects they're built out of...
I think perhaps I'm just trying to do something that SQLA really
wasn't designed for...
hp
On Mon, Jul 19, 2010 at 2:29 AM, Michael Bayer <mike...@zzzcomputing.com> wrote:
>
> On Jul 15, 2010, at 5:11 PM, Harry Percival wrote:
>
>> thanks Michael. I really appreciate your help.
>>
>> How should use .merge()? I've tried both:
>>>>>
>> another_new = MoviesAndDirectorsAndGenres() #init another_new as blank row
>> another_new = session.merge(new) #attempt to merge with my 'new'
>> object that has desired attributes
>>
>> and
>>>>>
>> new = session.merge(another_new) #attempt to merge blank object to my
>> new object with desired attributes
>>
>> the former fails trying to add a genre row that already exists, the
>> latter fails trying to add an object with all null values.
>>
>> the thing is, I'm really not trying to merge anything. I'm trying to
>> create a new object, which happens to be made up partially of new
>> entries in some tables, and existing entries in other tables.
>> genres_genre_id refers to the primary key of the genres table, and a
>> genre with genre_id=6 already exists.
>>
>>
>> am i going to have to map an ORM class to each of the composite
>> tables, and use references to them via relationship() instead of
>> hoping sql can figure it out by itself from the tables that make up
>> the join construct?
>>
>> can supply a sqlite database and some source code if it helps?
>
> It's typically appropriate to map tables individually and connect them via
> relationship(), if you want to be writing rows to them independently.
> That's what the phrase "partially of new entries in some tables, and existing
> entries in other tables." implies.
>
>
>
>>
>> rgds,
>> Harry
>>
>> On Thu, Jul 15, 2010 at 9:48 PM, Michael Bayer <mike...@zzzcomputing.com>
>> wrote:
>>>
>>> On Jul 15, 2010, at 2:52 PM, Harry Percival wrote:
>>>
>>>> A new problem, which seems to occur in both IronPython and normal Python:
>>>>
>>>> I have a database with tables for movies, directors, genres (and a
>>>> bridging table movie_directors)
>>>> I have a class mapped to a join of all three of the above
>>>>
>>>>>>>
>>>> j = join(movies_table,md_table).join(directors_table).join(genres_table)
>>>> mapper(MoviesAndDirectorsAndGenres,j.select(use_labels=True).alias('moviesdirectorsgenres'))
>>>>
>>>> this works fine. i can query the database ok using this mapper.
>>>>
>>>> The tables already have some values in.
>>>> Now, I want to create a new composite object, that references some of
>>>> the existing values, but not all - say it's a new movie by an existing
>>>> director in an existing genre:
>>>>
>>>>>>>
>>>> new = MoviesAndDirectorsAndGenres()
>>>> new.movies_movie_id=8 #does not already exist in the table
>>>> new.directors_director_id=2 #already exists in the table
>>>> new.genres_genre_id=6 #already exists too
>>>> session.add(new)
>>>>
>>>> this fails with a constraint error / integrity error, because SQLA
>>>> tries to re-insert genre #6 even though it already exists. it also
>>>> happens if i use session.merge(new). it also happens if i fully
>>>> specify all of the attributes of each movie/director/genre correctly.
>>>>
>>>> Is this expected behaviour? is there any way to get sqla to
>>>> intelligently only do inserts when necessary? is it something to do
>>>> with cascade configuration, and if so, how do i configure cascades on
>>>> a mapper that's not based on relationship() but based on join()?
>>>
>>> you should be using merge(), and you should ensure that the objects being
>>> merged have the correct primary key values. If "genre #6" refers to some
>>> other column that isn't the primary key of that row, then you'd have to
>>> ensure you put the correct primary key value on your object first before
>>> merging it.
>>>
>>> you can always check what decision merge() made by asking, "obj in
>>> session.new" versus "obj in session.dirty".
>>>
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>>>
>>
>>
>>
>> --
>> ------------------------------
>> Harry J.W. Percival
>> ------------------------------
>> Italy Mobile: +39 389 095 8959
>> UK Mobile: +44 (0) 78877 02511
>> Skype: harry dot percival
>>
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>
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--
------------------------------
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------------------------------
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UK Mobile: +44 (0) 78877 02511
Skype: harry dot percival
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