Re: How to get a queryset?

[adelaide_mike]

Thanks Max.  In an ideal world left_connector, right_connector would
be the way to go.  However, I need to account for a cable that, at one
or both ends is split into separate conductors, that go to separate
connectors (thus breaking the rule I had specified.)  Nodes, with a
little embellishment,  allow me to achieve this.  I will investigate
your itertools suggestion.

Mike

On Oct 20, 8:38 am, Max Battcher  wrote:
> adelaide_mike wrote:
> > Hi
> > My models are, essentially :
> > class Cable(models.Model):
> >     cable = models.CharField(max_length=8)
>
> > class Connector(models.Model):
> >     connector = models.CharField(max_length=8)
>
> > class Node(models.Model):
> >     cable = models.ForeignKey(Cable)
> >     connector = models ForeignKey(Connector)
>
> > So, a real world cable can be plugged in to no, one, or two connectors
> > (one at each end).  The Node table describes the junctions between
> > cables and connectors.
>
> > Can I, without resorting to raw SQL, build a queryset that shows
> > connector-cable-connector?
>
> You would have a hard time even accomplishing that in raw SQL with those
> models...  If you have the flexibility to change your models I would try
> something like:
>
> class Connector(models.Model):
>    name = models.CharField(max_length=8)
>
> class Cable(models.Model):
>    name = models.CharField(max_length-8)
>    left_connector = models.ForeignKey(Connector, blank=True, null=True)
>    right_connector = models.ForeignKey(Connector, blank=True, null=True)
>
> This makes your 0, 1, or 2 relationship explicit. I went with the
> left/right distinction as it is an easy "handed" way of describing them,
> but other choices may be more appropriate depending on your usage
> patterns...
>
> Otherwise, your best bet would seem to me to be to use
> Node.objects.all().orderby('cable') and group the
> connector-cable-connector by yourself. Something that I generally find
> useful in situations like that is Python's groupby function from itertools.
>
> --
> --Max Battcher--http://worldmaker.net
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Re: How to get a queryset?

[Max Battcher]

adelaide_mike wrote:
> Hi
> My models are, essentially :
> class Cable(models.Model):
> cable = models.CharField(max_length=8)
> 
> class Connector(models.Model):
> connector = models.CharField(max_length=8)
> 
> class Node(models.Model):
> cable = models.ForeignKey(Cable)
> connector = models ForeignKey(Connector)
> 
> So, a real world cable can be plugged in to no, one, or two connectors
> (one at each end).  The Node table describes the junctions between
> cables and connectors.
> 
> Can I, without resorting to raw SQL, build a queryset that shows
> connector-cable-connector?

You would have a hard time even accomplishing that in raw SQL with those 
models...  If you have the flexibility to change your models I would try 
something like:

class Connector(models.Model):
   name = models.CharField(max_length=8)

class Cable(models.Model):
   name = models.CharField(max_length-8)
   left_connector = models.ForeignKey(Connector, blank=True, null=True)
   right_connector = models.ForeignKey(Connector, blank=True, null=True)

This makes your 0, 1, or 2 relationship explicit. I went with the 
left/right distinction as it is an easy "handed" way of describing them, 
but other choices may be more appropriate depending on your usage 
patterns...

Otherwise, your best bet would seem to me to be to use 
Node.objects.all().orderby('cable') and group the 
connector-cable-connector by yourself. Something that I generally find 
useful in situations like that is Python's groupby function from itertools.

--
--Max Battcher--
http://worldmaker.net


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How to get a queryset?

[adelaide_mike]

Hi
My models are, essentially :
class Cable(models.Model):
cable = models.CharField(max_length=8)

class Connector(models.Model):
connector = models.CharField(max_length=8)

class Node(models.Model):
cable = models.ForeignKey(Cable)
connector = models ForeignKey(Connector)

So, a real world cable can be plugged in to no, one, or two connectors
(one at each end).  The Node table describes the junctions between
cables and connectors.

Can I, without resorting to raw SQL, build a queryset that shows
connector-cable-connector?

Thanks for any input.

Mike
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Re: how to get a queryset by a ManyToMany field

[coldpizza]

This definitely makes sense, I have made a view called
`books_by_author` - this made it easy for me to fit the logic into my
feeble mind, and moreover it looks good in the URLconfs. Thanks a lot
for suggestions!

On Feb 8, 7:57 pm, "Jacob Kaplan-Moss" <[EMAIL PROTECTED]>
wrote:
> On 2/8/08, coldpizza <[EMAIL PROTECTED]> wrote:
>
> > And other related question: where it would be more logically correct
> > to put this code? In URLConf as an parameter for the view, in a view
> > of its own, or in a method bound to the Book model?
>
> I wouldn't put it in a URLconf. URLconfs are supposed to be
> "configuration", so putting logic there seems dirty (to me). From
> there, though, it's a bit of a matter of taste: do you consider "books
> by an author" to be model logic, or view logic?
>
> For me, I'm usually guided by how I'll be using the query in question:
> if it's designed to be accessed from non-view areas (template tags,
> CLI scripts, etc.) I'll put it on the model, but if that particular
> query is only used in some specific view, I'll leave it in the view.
> Again, though, it's a matter of taste.
>
> Jacob
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Re: how to get a queryset by a ManyToMany field

[Jacob Kaplan-Moss]

On 2/8/08, coldpizza <[EMAIL PROTECTED]> wrote:
> And other related question: where it would be more logically correct
> to put this code? In URLConf as an parameter for the view, in a view
> of its own, or in a method bound to the Book model?

I wouldn't put it in a URLconf. URLconfs are supposed to be
"configuration", so putting logic there seems dirty (to me). From
there, though, it's a bit of a matter of taste: do you consider "books
by an author" to be model logic, or view logic?

For me, I'm usually guided by how I'll be using the query in question:
if it's designed to be accessed from non-view areas (template tags,
CLI scripts, etc.) I'll put it on the model, but if that particular
query is only used in some specific view, I'll leave it in the view.
Again, though, it's a matter of taste.

Jacob

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Re: how to get a queryset by a ManyToMany field

[coldpizza]

Thanks a lot! Before posting I have spent over an hour reading and
rereading that particular page in the doc section and in the
Djangobook but still somehow the concept eluded me. I do admit though
that I do not have previous experience solving this sort of issues in
other languages. Maybe adding a real use case to the doc could make it
easier to grasp.

And other related question: where it would be more logically correct
to put this code? In URLConf as an parameter for the view, in a view
of its own, or in a method bound to the Book model?

On Feb 8, 6:24 pm, "Jacob Kaplan-Moss" <[EMAIL PROTECTED]>
wrote:
> On 2/8/08, coldpizza <[EMAIL PROTECTED]> wrote:
>
> > Given that I have a Book model with a ManyToManyField called authors
> > how do I get all the books for a particular author? Each book may have
> > more than one author, so I cannot filter the books using the standard
> > methods.
>
> You'll want to more closely read the documentation on related object
> lookups:http://www.djangoproject.com/documentation/db-api/#related-objects.
>
> The short version, though, is this:
>
> Given a author ID, to find all the books by that author you'd use::
>
> Book.objects.filter(authors__pk=author_id)
>
> Or, given a particular author object::
>
> author.book_set.all()
>
> (Where did "book_set" come from? It's automatically created on the
> "other side" of the many-to-many relation you created from Book to
> Author. You can change the name of that relation attribute using the
> "related_name" option to ManyToManyField).
>
> Jacob
>
> PS: Also see Chapter 5 of the Django Book
> (http://djangobook.com/en/1.0/chapter05/);it's got examples pretty
> similar to what you're doing.
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Re: how to get a queryset by a ManyToMany field

[Jacob Kaplan-Moss]

On 2/8/08, coldpizza <[EMAIL PROTECTED]> wrote:
> Given that I have a Book model with a ManyToManyField called authors
> how do I get all the books for a particular author? Each book may have
> more than one author, so I cannot filter the books using the standard
> methods.

You'll want to more closely read the documentation on related object
lookups: http://www.djangoproject.com/documentation/db-api/#related-objects.

The short version, though, is this:

Given a author ID, to find all the books by that author you'd use::

Book.objects.filter(authors__pk=author_id)

Or, given a particular author object::

author.book_set.all()

(Where did "book_set" come from? It's automatically created on the
"other side" of the many-to-many relation you created from Book to
Author. You can change the name of that relation attribute using the
"related_name" option to ManyToManyField).

Jacob

PS: Also see Chapter 5 of the Django Book
(http://djangobook.com/en/1.0/chapter05/); it's got examples pretty
similar to what you're doing.

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how to get a queryset by a ManyToMany field

[coldpizza]

Sorry if this is too trivial but I have looked through the docs and
could not find a clear answer.

Given that I have a Book model with a ManyToManyField called authors
how do I get all the books for a particular author? Each book may have
more than one author, so I cannot filter the books using the standard
methods.

My models are:
class Author(models.Model):
name = models.CharField(max_length=124)

class Book(models.Model):
title = models.CharField(max_length=124)
authors = models.ManyToManyField(Author)

If I got it right, according to the documentation to get to a 'child'
queryset I have to iterate with 'for each b in Book.objects.all()',
and then call the b.authors.all() on each instance in order to check
if a particular author.id is found in the authors queryset. But how do
I get the results as a queryset? I could not get this to work.

To get all the books for a given author.id, I have tried:

def books_by_author(request, object_id):
'''does not work'''
filtered_by_author = []
for b in Book.objects.all():
if object_id in [a.id for a in b.authors.all()]:
filtered_by_author.append(b)
return render_to_response('polls/book_list.html', {'object_list':
filtered_by_author,} )

But this, of course, does not work, because it returns a list and not
a query set. What would be a proper, nice solution?
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