[PonyORM-list] Where is a good place to create the database object? (was Re: no subject)

[Alexander Kozlovsky]

Hi Stefan! I answered you on StackOverflow:
https://stackoverflow.com/questions/53748730/when-using-pony-orm-or-sqlalchemy-where-to-create-database-object/53765112#53765112

Best regards,
Alexander

On Thu, Dec 13, 2018 at 11:38 AM Stefan Dröge <stefan at sdroege.de> wrote:

> sorry, forgot to add subject.
>
> Am Do., 13. Dez. 2018 um 09:35 Uhr schrieb Stefan Dröge <stefan at sdroege.de
> >:
>
>> I already asked the question on stackoverflow, however I just found this
>> mailing list, so I thought I'd cross post my question here:
>>
>> (Original question from Stackoverflow:
>> https://stackoverflow.com/questions/53748730/when-using-pony-orm-or-sqlalchemy-where-to-create-database-object
>> )
>>
>> I began playing a bit with Object Relational Mappers (*Pony ORM*
>> specifically).
>>
>> In *Pony*, all entity definitions inherit from the [`db.Entity`][1]
>> class. However in order to do that of course the `db` object needs to be
>> created first somewhere. (`db.Entity` is somewhat similar to the
>> declarative base in *sqlalchemy*, so I believe my question below is also
>> similar valid for *sqlalchemy*)
>>
>> All the examples I have seen in the Pony ORM docs present the examples
>> inline, where the Database object `db` is simply declared in the
>> interpreter prompt before the entities are declared.
>>
>> This leaves for me the question: **Where should I create my db object in
>> a "real" project?.**
>>
>> Especially consider the case where I want to keep my entity definitions
>> separate from where I actually use these entities (say I only want to build
>> a nice ORM wrapper package to access a database, which then should be used
>> in multiple different other projects). Then I probably want the users to
>> provide the own `db` object that is configured to their needs in order to
>> access the DB.
>>
>> ## Some example code:
>>
>> Say I have a Database that stores *persons* and *addresses* and my
>> package `my_orm` should provide an ORM for the database, which then will be
>> used in `app.py`:
>>
>>
>> *my_orm/__init__.py*
>>
>>     from my_orm.person import Person
>>     from my_orm.address import Address
>>
>> *my_orm/person.py:*
>>
>>     from pony.orm import Required
>>
>>     class Person(db.Entity): # Where should `db` be defined?
>>         name = Required(str)
>>         age = Required(int)
>>
>> *my_orm/address.py:*
>>
>>     from pony.orm import Required
>>
>>     class Address(db.Entity): # Where should `db` be defined?. Must be
>> the same `db` object as in person.py
>>         street_name = Required(str)
>>         zip_code = Required(int)
>>
>> *app.py*
>>
>>     from pony.orm import Database
>>     db = Database()
>>     import my_orm
>>
>> Besides already looking ugly because it mixes the imports with the
>> creation of the db, this will also throw the error `NameError: name 'db' is
>> not defined`. So what should I do instead?
>>
>>   [1]: https://docs.ponyorm.com/api_reference.html#Database.Entity
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>>
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