如何使用peewee的Use作为装饰器来动态指定数据库？

Question

Despite numerous recipes and examples in peewee's documentation; I have not been able to find how to accomplish the following:

For finer-grained control, check out the Using context manager / decorator . This allows you to specify the database to use with a given list of models for the duration of the wrapped block.

I assume it would go something like...

db = MySQLDatabase(None)


class BaseModelThing(Model):
    class Meta:
        database = db


class SubModelThing(BaseModelThing):
    '''imagine all the fields'''  
    class Meta:
        db_table = 'table_name'  

runtime_db = MySQLDatabase('database_name.db', fields={'''imagine field mappings here''', **extra_stuff)

@Using(runtime_db, [SubModelThing])
@runtime_db.execution_context()
def some_kind_of_query():
    '''imagine the queries here'''

but I have not found examples, so an example would be the answer to this question.

Answer 1

Yeah, there's not a great example of using Using or the execution_context decorators, so the first thing is: don't use the two together. It doesn't appear to break anything, just seems to be redundant. Logically that makes sense as both of the decorators cause the specified model calls in the block to run in a single connection/transaction.

The only(/biggest) difference between the two is that Using allows you to specify the particular database that the connection will be using - useful for master/slave (though the Read slaves extension is probably a cleaner solution).

If you run with two databases and try using execution_context on the 'second' database (in your example, runtime_db ) nothing will happen with the data. A connection will be opened at the start of the block and closed and the end, but no queries will be executed on it because the models are still using their original database.

The code below is an example. Every run should result in only 1 row being added to each database.

from peewee import *

db = SqliteDatabase('other_db')
db.connect()

runtime_db = SqliteDatabase('cmp_v0.db')
runtime_db.connect()


class BaseModelThing(Model):
    class Meta:
        database = db


class SubModelThing(Model):
    first_name = CharField()
    class Meta:
        db_table = 'table_name'

db.create_tables([SubModelThing], safe=True)
SubModelThing.delete().where(True).execute() # Cleaning out previous runs

with Using(runtime_db, [SubModelThing]):
    runtime_db.create_tables([SubModelThing], safe=True)
    SubModelThing.delete().where(True).execute()

@Using(runtime_db, [SubModelThing], with_transaction=True)
def execute_in_runtime(throw):
    SubModelThing(first_name='asdfasdfasdf').save()
    if throw: # to demo transaction handling in Using
        raise Exception()

# Create an instance in the 'normal' database
SubModelThing.create(first_name='name')

try: # Try to create but throw during the transaction
    execute_in_runtime(throw=True)
except:
    pass  # Failure is expected, no row should be added

execute_in_runtime(throw=False) # Create a row in the runtime_db

print 'db row count: {}'.format(len(SubModelThing.select()))

with Using(runtime_db, [SubModelThing]):
    print 'Runtime DB count: {}'.format(len(SubModelThing.select()))