Tim Chase wrote:
>> I'm in the process of turning telemeta [1] into a django
>> application, and need dynamic model fields: we want to allow
>> site administrators to easily add/remove "fields" to our main
>> data structure, the MediaItem, which represents an audio/video
>> resource.
>
> Our current app does something like this...our basic solution was
> something like one of the two following ideas (depending on
> whether particular categories should be consistent across labs,
> such as "media" meaning the same thing in Lab1 and Lab2)
Actually the labs where just an example: one laboratory will have one
installation of the application, and the dynamic fields will only apply within
the scope of this installation. There's no such thing as consistency accross
labs.
> or the more simple "every lab has its own bag of categories that
> don't relate to anybody else" (this is how we settled on it
> because our clients rarely have much in common for these
> category/value pairs):
>
>
> class Lab(Model):
> pass
> class Category(Model):
> lab = ForeignKey(Lab)
> name = CharField(maxlength=40)
> class Value(Model):
> category = ForeignKey(Category)
> value = CharField(maxlength=40)
> class MediaItem(Model):
> lab = ForeignKey(Lab)
> values = ManyToMany(Values)
Well, that's what I almost did... with support for enumerations (some fields
must only accept predetermined values):
class MediaItem(models.Model):
"Describe an audio/video item with metadata"
title = models.CharField(maxlength=250)
author = models.CharField(maxlength=250)
def get_dynamic_properties(self):
"Retrieve dynamic properties associated with a given media item"
[...]
class MediaItemPropertyDefinition(models.Model):
"Define a media item dynamic property"
TYPE_CHOICES = (('text', 'Text'), ('enumeration', 'Enumeration'))
name = models.CharField(maxlength=64)
type = models.CharField(maxlength=64, choices = TYPE_CHOICES)
class MediaItemPropertyEnumerationItem(models.Model):
"Define a possible value for media item dynamic enumeration properties"
definition = models.ForeignKey(MediaItemPropertyDefinition, core=True)
name = models.CharField(maxlength=250)
class MediaItemProperty(models.Model):
"Associate a value to a media item dynamic property"
definition = models.ForeignKey(MediaItemPropertyDefinition, core=True)
media_item = models.ForeignKey(MediaItem)
value = models.CharField(maxlength=250)
enum_item = models.ForeignKey(MediaItemPropertyEnumerationItem, null=True)
However, watching the MySQL verbose log, I realized that retrieving properties
as well as their definitions (and enumeration values) to properly display an
detail view or edition form execute a _lot_ of SQL queries, when this could be
achieved with one or two joins... I'm afraid it won't scale.
Or does your "ManyToMany(Values)" field helps for optimization?
May I expect any help from manipulators, managers, query sets (all of these are
still a bit obscure to me) ?
Regards,
--
Olivier - http://www.samalyse.com
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