Currently I have the following Django model
class TestUser(models.Model):
name = models.CharField(max_length=250)
email = models.CharField(max_length=250)
...
class Meta:
db_table = 'temp_user'
and I've the following method.
def print_name(self):
return self.name
I want to add this method as a property to TempUser model. I know this can be done by putting the method inside TempUser class and user #property. But I want to do it dynamically.
I tried this from python shell.
In [10]: TempUser.print_name = property(print_name)
In [11]: TempUser.print_name
Out[11]: <property at 0x7efc374e7c58>
In [12]: user = TempUser.objects.get(pk=1)
In [13]: user.print_name
Out[13]: u'Test User'
But once I exit the shell, I loose the property. Is there any way to add the property permanently.
But once I exit the shell, I loose the property. Is there any way to add the property permanently.
No, not through attributes you set at run-time. Data associated with Django model instances is persisted through the database. Your only option for persisting data is to create a field in the database with which to store the information. If you want to persist methods available on a class, edit the source code.
You've expressed a desire to do this dynamically at runtime, too. However, the benefit of doing so is dubious, at best, and likely hazardous to your code base. It's hard to imagine a demonstrable use-case where this solves a real problem that does not already have a better solution. There is almost certainly a better way to get at your goal. If your goal is to keep your code DRY, consider other patterns like inheriting from abstract models.
You can implement regular properties (using the #property method decorator) on your model class that can take existing fields and look at related model fields to compose information on-the-fly, but it is still not persisted in the database...
For example, if you have a model that has a start_time and end_time you could add a total_time property without necessarily needing to create a field for it.
class MyModel(models.Model):
start_time = models.DateTimeField()
end_time = models.DateTimeField()
#property
def total_time(self):
return self.end_time - self.start_time
However, doing this would not allow you to query against this property. MyModel.objects.filter(total_time__lt=delta) for example would not be possible with a property alone.
Some other options you have include annotation and aggregation which can be done dynamically at run-time and gives you the benefit of being able to query against your database.
Using the same example as above, rather than using #property one can annotate a queryset in a similar way, which also lets you query the database on this 'virtual field' and even pass that annotated queryset around. You can even do arithmetic and aggregations like sum, average, and more...
from django.db.models import F, ExpressionWrapper, fields
duration = ExpressionWrapper(F('end_time') - F('start_time'), output_field=fields.DurationField())
qs = MyModel.objects.annotate(duration=duration)
# query for objects with a delta of more than five minutes
results = qs.filter(duration__gt=five_minutes)
See also Query expressions
In short: no, and if you could, it's almost definitely a bad idea. Stick with the established methods of doing things. Django is a very opinionated framework, it is designed for you to do things 'the django way'. You will likely only hurt yourself by going against the grain.
Related
I have an Events table which contains various types of events. I care only about one of those types. As a result, every query I write begins with
Events.objects.filter(event_type="the_type").\
etc(...).etc(...)`.
Obviously this is repetitive and easy to forget. Is there a way to use a custom Manager so that the objects attribute always returns a specific subset of the rows, without explicitly asking for it? Or any other way to restrict the model to specific subset of the rows??
Yes, we can make a manager like:
from django.db import models
class EventManager(models.Manager):
def get_queryset(self):
return super(EventManager, self).get_queryset().filter(event_type="the_type")
and then add the manager to the Event class:
class Event(models.Model):
# ...
objects = EventManager()
Note however that some parts of Django will not use .objects, but ._base_manager, and thus still return the entire set. Furthermore my own experience with overriding the .objects manager is that it turns out to cause a lot of harm, for example if you want to set an attribute of all events, then writing Event.objects.all().update(foo='bar') will only update the events with the_type as type, whereas the code suggests otherwise.
Personally I think it is better to construct a manager with a different name, that at least hints that something is filtered, for example:
class Event(models.Model):
# ...
all_events = models.Manager()
type_events = EventManager()
here Event.objects no longer exist, but you write Event.all_events, or Event.type_events, and thus the code clearly hints what subset you take.
I have been trying to figure out the best way to automate creating multiple SQL tables based on separate but identical models, all based on the same base class. I'm basically creating pseudo message boards or walls with different Groups, and I wanted each Group to have its own db_table of Posts, each Post containing the user id, timestamp, etc.
My first thought was to have one base class of Posts and just include a field for Group name, but I thought this would be bad practice. My rationale was that one table containing every Post for all Groups would get really big (in theory anyway) and slow down filtering, and also that the extra field for group name would in the long run be a waste of memory when I could have separate tables per group and skip this field.
I've also considered using a ForeignKey with a Many-to-One relationship, but as far as I can tell this has the same drawbacks. Am I wrong to think that? Or are these size concerns not really an issue?
So my next idea was to make Posts an abstract class, and then create subclasses based on each Group. This is ultimately what I did. However, I found myself having to copy and paste the code over and over and change the class name each time. This felt very unPythonic to me. It was something like:
class Posts(models.Model):
timestamp = models.DateTimeField(auto_now_add=True, unique=False)
user_id = ...
#etc.
#
class Meta:
abstract = True
class GroupA(Posts):
class Meta(Posts.Meta):
db_table = 'groupa_board'
class GroupB(Posts):
class Meta(Posts.Meta):
db_table = 'groupb_board'
class GroupC...etc.
What I really was looking for was a factory function to do this for me. I tried this sort of thing:
def makeBoard(group):
class Board(Posts):
class Meta(Posts.Meta):
db_table = group
return board #note I tried with and without this line
And then I ran a simple for loop using a list of groups.
for group in groups:
makeBoard(group)
I found myself hitting a RuntimeError: conflicting models in application, and I probably deserved it. So then I figured what I need is something like:
def makeBoard(group):
class group(Posts): #***group here being a variable, not the class name
class Meta(Posts.Meta):
db_table = '%s' % group #maybe issues here too, but the table
return group #name is not that important if the class
#name works
But I couldn't figure out how to make this work! Is there a way to pass a variable from a list to a class name?
Anyway if you're still with me I appreciate it. I've been on stackoverflow all day and while I've found guides for creating abstract base classes and subclasses to solve similar issues, I didn't see a way to create a function to do this for me. I ultimately punted here and just make a subclass for each group by hand. If there is a way to automate this process, I'd love to hear it.
Also, if I'm being stupid for not just going with one db table containing every post, I'd like to know that too, and why! Or if there's a better way to implement this kind of system altogether. I apologize if this has been answered before, I really couldn't find it.
Thank you!
Using a single table would not be bad practice. The extra memory is minimal, on modern systems that shouldn't be a problem. You shouldn't worry about performance either, premature optimization (not including the actual system design) is considered bad practice, but if you run into performance problems you can always specify an index on the group column:
group = models.CharField(max_length=100, db_index=True)
That's not to say that it is the best option, or that your method isn't good. Also, it is entirely possible to dynamically create models, using the type() built-in function. The only difference with dynamically creating models and creating other classes is that you must specifically pass the __module__ attribute. You can create subclasses for Posts in the following way:
def fabric(names, baseclass=Posts):
for name in names:
class Meta:
db_table = '%s_table' % name.lower()
attrs = {'__module__': baseclass.__module__, 'Meta': Meta}
# specify any other class attributes here. E.g. you can specify extra fields:
attrs.update({'my_field': models.CharField(max_length=100)})
newclass = type(str(name), (baseclass,), attrs)
globals()[name] = newclass
fabric(['GroupA', 'GroupB', 'GroupC', etc...])
Put that code in your models.py after your Posts class, and all classes will be created for you. They can be used in any way normal classes can be used: Django doesn't even know you dynamically created this class. Though your Meta class doesn't inherit from Posts.Meta, your meta settings should still be preserved.
Tested with Django 1.4.
Try smth like this
import app.models as group_models
from django.db.models.base import ModelBase
def fabric(group):
for item in dir(group_models):
c = getattr(group_models, item)
if type(c) is ModelBase:
if c._meta.db_table == '%s_table' % group:
return c
return None
I have a model that has a pickled set of strings. (It has to be pickled, because Django has no built in set field, right?)
class Foo(models.Model):
__bar = models.TextField(default=lambda: cPickle.dumps(set()), primary_key=True)
def get_bar(self):
return cPickle.loads(str(self.__bar))
def set_bar(self, values):
self.__bar = cPickle.dumps(values)
bar = property(get_bar, set_bar)
I would like the set to be editable in the admin interface. Obviously the user won't be working with the pickled string directly. Also, the interface would need a widget for adding/removing strings from a set.
What is the best way to go about doing this? I'm not super familiar with Django's admin system. Do I need to build a custom admin widget or something?
Update: If I do need a custom widget, this looks helpful: http://www.fictitiousnonsense.com/archives/22
Update 2: Now I'm looking through different relational models to see if that will work. One idea I'm toying with:
class FooMember(models.Model):
name = models.CharField(max_length=120)
foo = models.ForeignKey('Foo')
class Foo(models.Model):
def get_names(self):
return FooMember.objects.filter(foo__exact=self)
Disadvantages of this include:
It feels excessive to make an entire model for one data field (name).
I would like the admin interface for Foo to allow the user to enter a list of strings. I'm not sure how to do that with this setup; making a custom form widget seems like less work.
Uhm. Django usually stores it's data in an SQL database. Storing a set as a pickled string is definietly not the best way to use an SQL database. It's not immediately obvious which is the right solution in your case, that depends what is in that set, but this is the wrong solution in any case.
You might want a new table for that set, or at least save it as comma separated values or something.
I'm setting up a data model in django using multiple-table inheritance (MTI) like this:
class Metric(models.Model):
account = models.ForeignKey(Account)
date = models.DateField()
value = models.FloatField()
calculation_in_progress = models.BooleanField()
type = models.CharField( max_length=20, choices= METRIC_TYPES ) # Appropriate?
def calculate(self):
# default calculation...
class WebMetric(Metric):
url = models.URLField()
def calculate(self):
# web-specific calculation...
class TextMetric(Metric):
text = models.TextField()
def calculate(self):
# text-specific calculation...
My instinct is to put a 'type' field in the base class as shown here, so I can tell which sub-class any Metric object belongs to. It would be a bit of a hassle to keep this up to date all the time, but possible. But do I need to do this? Is there some way that django handles this automatically?
When I call Metric.objects.all() every objects returned is an instance of Metric never the subclasses. So if I call .calculate() I never get the sub-class's behavior.
I could write a function on the base class that tests to see if I can cast it to any of the sub-types like:
def determine_subtype(self):
try:
self.webmetric
return WebMetric
except WebMetric.DoesNotExist:
pass
# Repeat for every sub-class
but this seems like a bunch of repetitious code. And it's also not something that can be included in a SELECT filter -- only works in python-space.
What's the best way to handle this?
While it might offend some people's sensibilities, the only practical way to solve this problem is to put either a field or a method in the base class which says what kind of object each record really is. The problem with the method you describe is that it requires a separate database query for every type of subclass, for each object you're dealing with. This could get extremely slow when working with large querysets. A better way is to use a ForeignKey to the django Content Type class.
#Carl Meyer wrote a good solution here: How do I access the child classes of an object in django without knowing the name of the child class?
Single Table Inheritance could help alleviate this issue, depending on how it gets implemented. But for now Django does not support it: Single Table Inheritance in Django so it's not a helpful suggestion.
But do I need to do this?
Never. Never. Never.
Is there some way that django handles this automatically?
Yes. It's called "polymorphism".
You never need to know the subclass. Never.
"What about my WebMetric.url and my TextMetric.text attributes?"
What will you do with these attributes? Define a method function that does something. Implement different versions in WebMetric (that uses url) and TextMetric (that uses text).
That's proper polymorphism.
Please read this: http://docs.djangoproject.com/en/1.2/topics/db/models/#abstract-base-classes
Please make your superclass abstract.
Do NOT do this: http://docs.djangoproject.com/en/1.2/topics/db/models/#multi-table-inheritance
You want "single-table inheritance".
In Django, when you have a parent class and multiple child classes that inherit from it you would normally access a child through parentclass.childclass1_set or parentclass.childclass2_set, but what if I don't know the name of the specific child class I want?
Is there a way to get the related objects in the parent->child direction without knowing the child class name?
(Update: For Django 1.2 and newer, which can follow select_related queries across reverse OneToOneField relations (and thus down inheritance hierarchies), there's a better technique available which doesn't require the added real_type field on the parent model. It's available as InheritanceManager in the django-model-utils project.)
The usual way to do this is to add a ForeignKey to ContentType on the Parent model which stores the content type of the proper "leaf" class. Without this, you may have to do quite a number of queries on child tables to find the instance, depending how large your inheritance tree is. Here's how I did it in one project:
from django.contrib.contenttypes.models import ContentType
from django.db import models
class InheritanceCastModel(models.Model):
"""
An abstract base class that provides a ``real_type`` FK to ContentType.
For use in trees of inherited models, to be able to downcast
parent instances to their child types.
"""
real_type = models.ForeignKey(ContentType, editable=False)
def save(self, *args, **kwargs):
if self._state.adding:
self.real_type = self._get_real_type()
super(InheritanceCastModel, self).save(*args, **kwargs)
def _get_real_type(self):
return ContentType.objects.get_for_model(type(self))
def cast(self):
return self.real_type.get_object_for_this_type(pk=self.pk)
class Meta:
abstract = True
This is implemented as an abstract base class to make it reusable; you could also put these methods and the FK directly onto the parent class in your particular inheritance hierarchy.
This solution won't work if you aren't able to modify the parent model. In that case you're pretty much stuck checking all the subclasses manually.
In Python, given a ("new-style") class X, you can get its (direct) subclasses with X.__subclasses__(), which returns a list of class objects. (If you want "further descendants", you'll also have to call __subclasses__ on each of the direct subclasses, etc etc -- if you need help on how to do that effectively in Python, just ask!).
Once you have somehow identified a child class of interest (maybe all of them, if you want instances of all child subclasses, etc), getattr(parentclass,'%s_set' % childclass.__name__) should help (if the child class's name is 'foo', this is just like accessing parentclass.foo_set -- no more, no less). Again, if you need clarification or examples, please ask!
Carl's solution is a good one, here's one way to do it manually if there are multiple related child classes:
def get_children(self):
rel_objs = self._meta.get_all_related_objects()
return [getattr(self, x.get_accessor_name()) for x in rel_objs if x.model != type(self)]
It uses a function out of _meta, which is not guaranteed to be stable as django evolves, but it does the trick and can be used on-the-fly if need be.
It turns out that what I really needed was this:
Model inheritance with content type and inheritance-aware manager
That has worked perfectly for me. Thanks to everyone else, though. I learned a lot just reading your answers!
You can use django-polymorphic for that.
It allows to automatically cast derived classes back to their actual type. It also provides Django admin support, more efficient SQL query handling, and proxy model, inlines and formset support.
The basic principle seems to be reinvented many times (including Wagtail's .specific, or the examples outlined in this post). It takes more effort however, to make sure it doesn't result in an N-query issue, or integrate nicely with the admin, formsets/inlines or third party apps.
Here's my solution, again it uses _meta so isn't guaranteed to be stable.
class Animal(models.model):
name = models.CharField()
number_legs = models.IntegerField()
...
def get_child_animal(self):
child_animal = None
for r in self._meta.get_all_related_objects():
if r.field.name == 'animal_ptr':
child_animal = getattr(self, r.get_accessor_name())
if not child_animal:
raise Exception("No subclass, you shouldn't create Animals directly")
return child_animal
class Dog(Animal):
...
for a in Animal.objects.all():
a.get_child_animal() # returns the dog (or whatever) instance
You can achieve this looking for all the fields in the parent that are an instance of django.db.models.fields.related.RelatedManager. From your example it seems that the child classes you are talking about are not subclasses. Right?
An alternative approach using proxies can be found in this blog post. Like the other solutions, it has its benefits and liabilities, which are very well put in the end of the post.