Re: Does imports of related models affect cascade deletion?

I will fetch the actual piece of code and build a small test case out of it, will reply again in this thread when I have some more info.

On Thu, Jul 30, 2015 at 6:39 PM, Marc Aymerich <glicerinu@gmail.com> wrote:

On Thu, Jul 30, 2015 at 3:38 PM, Markus Amalthea Magnuson
<markus.magnuson@gmail.com> wrote:
> Hey,
>
> I stumbled upon a piece of code and a comment that says this:
>
> Deleting a model object that has related objects will only cascade delete
> those objects if their models have been imported.
>
> Is this true? I have not found it in the documentation and would like to add
> a reference to the code comment so others won't be as confused as I am.

Is this models module called models.py and does it live inside a
Django application listed in INSTALLED_APPS?


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Re. My Django Book.

 Say I was wondering if anyone else has ever read this particulkar Django book called "Sams Teach Yourself Django in 24 Hours" I checked out of the library yesterday? [yeah I know it's one of the "24 hours" series] Does it really teach the beginner a lot of things concerning Django?

   

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Custom Field

I wanted your help to make a custom Field Type'm studying it at the time and I'm not getting. I'll put it to you here the code I've done so far. I hope your help.

http://pastebin.com/81Lq86Xn Field

http://pastebin.com/CvATavnd Widget

http://pastebin.com/2tNcKtgf    Form

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Re: Two-to-Many Mapping between Models in Django ORM

Hi Tom,

That was approximately what I was planning to do, I shall do some experimenting to see if I can do any more.  I was just wondering if there was a clever feature for this sort of thing, as ORMs seem pretty magic already!

Thanks,

Rich

On Thursday, 30 July 2015 17:15:40 UTC+1, Tom Evans wrote:

On Thu, Jul 30, 2015 at 4:08 PM, Rich Lewis <rich....@gmx.co.uk> wrote:
> Dear All,
>
> I'm new to the Django ORM, and quite new to ORMs in general.  I have two
> models (lets call them A and B) between which I have an interesting mapping.
> There are precisely 2 B instances associated with each A instance.  Each A
> instance can have many B instances.  The order of Bs are important for As.
>
> I want to do something like this:
>
> class A(models.Model):
>   b_1 = models.ForeignKey(B)
>   b_2 = models.ForeignKey(B)

This would fail here, when you have more than one foreign key to the
same model, you must supply a related_name argument on one of them,
which is used for the inverse relation on the related object. Which
leads us in to...

>
> class B(models.Model):
>   pass
>
> Such that i can do:
>
>>>> b1, b2, b3 = B(), B(), B()
>>>> a1, a2 = A(b_1=b1, b_2=b2), A(b_1=b2, b_2=b3)
>>>> b2.as
> [<A ... >, <A ... >] #(order doesn't matter)

Typically, B instances would have an attribute named 'a_set' (the
lower case model name of the model it is related to, with "_set"
appended). When you have multiple relationships with the same model,
as I mentioned you must supply your own related_name arguments. In
which case, with the models you mentioned, you could then do something
like this:

>>> b2.a1_set.all() | b2.a2_set.all()
> [<A ..>, <A ..>]

>
> I expect I could eventually do something a bit hacky that would work, but
> what would be the best way to handle this?

I would say the above is the hacky way. You could have the same B
assigned to b_1 and b_2 and would have duplicates in the set, and no
way to specify DB constraints to limit it.

Given that you anticipate being able to retrieve all the A's for a
particular B, regardless of how they are related, I would model it as
a M2M with a through table holding any additional information about
the relationship, using the through table to add DB level constraints.

Cheers

Tom

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Re: Two-to-Many Mapping between Models in Django ORM

Oops sorry I meant every B instance can have multiple A instances.  Sorry!

On Thursday, 30 July 2015 16:21:37 UTC+1, monoBOT monoBOT wrote:

2015-07-30 16:08 GMT+01:00 Rich Lewis <rich....@gmx.co.uk>:

There are precisely 2 B instances associated with each A instance.  Each A instance can have many B instances

Isnt that a contradiction?

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Visite mi sitio(Visit my site): monobotsoft.es/blog/

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Re: Does imports of related models affect cascade deletion?

On Thu, Jul 30, 2015 at 3:38 PM, Markus Amalthea Magnuson
<markus.magnuson@gmail.com> wrote:
> Hey,
>
> I stumbled upon a piece of code and a comment that says this:
>
> Deleting a model object that has related objects will only cascade delete
> those objects if their models have been imported.
>
> Is this true? I have not found it in the documentation and would like to add
> a reference to the code comment so others won't be as confused as I am.

Is this models module called models.py and does it live inside a
Django application listed in INSTALLED_APPS?


--
Marc

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Re: Two-to-Many Mapping between Models in Django ORM

On Thu, Jul 30, 2015 at 4:08 PM, Rich Lewis <rich.lewis@gmx.co.uk> wrote:
> Dear All,
>
> I'm new to the Django ORM, and quite new to ORMs in general. I have two
> models (lets call them A and B) between which I have an interesting mapping.
> There are precisely 2 B instances associated with each A instance. Each A
> instance can have many B instances. The order of Bs are important for As.
>
> I want to do something like this:
>
> class A(models.Model):
> b_1 = models.ForeignKey(B)
> b_2 = models.ForeignKey(B)

This would fail here, when you have more than one foreign key to the
same model, you must supply a related_name argument on one of them,
which is used for the inverse relation on the related object. Which
leads us in to...

>
> class B(models.Model):
> pass
>
> Such that i can do:
>
>>>> b1, b2, b3 = B(), B(), B()
>>>> a1, a2 = A(b_1=b1, b_2=b2), A(b_1=b2, b_2=b3)
>>>> b2.as
> [<A ... >, <A ... >] #(order doesn't matter)

Typically, B instances would have an attribute named 'a_set' (the
lower case model name of the model it is related to, with "_set"
appended). When you have multiple relationships with the same model,
as I mentioned you must supply your own related_name arguments. In
which case, with the models you mentioned, you could then do something
like this:

>>> b2.a1_set.all() | b2.a2_set.all()
> [<A ..>, <A ..>]

>
> I expect I could eventually do something a bit hacky that would work, but
> what would be the best way to handle this?

I would say the above is the hacky way. You could have the same B
assigned to b_1 and b_2 and would have duplicates in the set, and no
way to specify DB constraints to limit it.

Given that you anticipate being able to retrieve all the A's for a
particular B, regardless of how they are related, I would model it as
a M2M with a through table holding any additional information about
the relationship, using the through table to add DB level constraints.

Cheers

Tom

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