On 05/19/2016 08:58 AM, Михаил Доронин wrote:
Suppose I have models (with corresponding tables) in pseudocode
table1
| some_unique_name | some_property |
|------------------+---------------|
| foo | 1 |
|------------------+---------------|
| bar | 2 |
|------------------+---------------|
table2
| another_unique_name | another_property |
|---------------------+------------------|
| jack | married |
|---------------------+------------------|
| marry | widow |
|---------------------+------------------|
Another database
foo_jack_some_huge_data
| kids_count | dog_coung | point_in_time |
|------------+-----------+---------------|
| 1 | 0 | 0:01 |
|------------+-----------+---------------|
| 2 | 1 | 3:00 |
|------------+-----------+---------------|
bar_jack_some_huge_data
| kids_count | dog_coung | point_in_time |
|------------+-----------+---------------|
| 2 | 0 | 0:01 |
|------------+-----------+---------------|
| 3 | 1 | 3:00 |
|------------+-----------+---------------|
foo_marry_some_huge_data
| kids_count | dog_coung | point_in_time |
|------------+-----------+---------------|
| 4 | 0 | 0:01 |
|------------+-----------+---------------|
| 5 | 1 | 3:00 |
|------------+-----------+---------------|
So what I'm trying to say that we are making a shard on
table1.some_unique_name and table2.another_unique_name.
So what I've already done is that
|
|
|
@lru_cache(maxsize=None)
def candles_table(some_unique_name, another_unique_name):
return Table('_'.join((some_unquie_name, another_unique_name,
'some_huge_data')), Base.metadata,
Column('kids_count', Integer),
Column('dog_count', Integer),
Column('point_in_time', DateTime),
)
|
|
|
this works fine but I'd like to use class and a mapper instead of raw
table object (for obvious reasons)
I saw this question
<http://stackoverflow.com/questions/19163911/dynamically-setting-tablename-for-sharding-in-sqlalchemy>,
but I don't like the solutions there.
Can I create a custom mapper class that would have api like that
|
mapper.map(klass, *tables)
|
and then somehow when quering this class via session.query I would be
able to say what tables exactly would I like to query. Like some
.shard(...) or something like that.
Well if you want to see some examples of on-the-fly mappers for
different databases that nonetheless map to the same class take a look
at https://bitbucket.org/zzzeek/sqlalchemy/wiki/UsageRecipes/EntityName.
The kind of thing you're doing has been done before but it's not
totally easy.
Along with the entity name thing, you can customize Session as well as
how it generates a query. If you look at the source to the horizontal
sharding extension
http://docs.sqlalchemy.org/en/latest/orm/extensions/horizontal_shard.html?highlight=horizontal#module-sqlalchemy.ext.horizontal_shard
and maybe also my post at
http://techspot.zzzeek.org/2012/01/11/django-style-database-routers-in-sqlalchemy/
you can get some ideas for how to game things on the Session side.
Those are the two tools you'd be working with here so hopefully you can
find a way to combine them in a way that works for you.
I haven't created the issue because I didn't know if solution (not a
workaround like mentioned in stackoverflow) already possible with
sqlalchemy. If not it would be beneficial to add I think.
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