I have a couple models as follows:
class Student(models.Model):
name = models.CharField()
classes = models.ManyToManyField(Class, related_name="students")
class Class(models.Model):
nombre = models.CharField
What I need is a way such that, when querying students in certain class, instead of just returnig the students enrolled in that class, it returns a QuerySet of all the students, each one with an aggregated field indicating if such student is enrolled in that class, e.g.:
[{'name':'student A', 'enrolled_in_physics':True}, {'name':'student B', 'enrolled_in_physics':False}]
I think it can be achieved through F() expressions along with ExpressionWrapper, but have no idea of how implement them; additionaly, the documentation and the examples are not very noob-friendly. Any help is appreciated, thanks!.
EDIT: Ok, I think using the word "list" is not the correct one, I need a normal QuerySet, such that let me do something like this:
student_a = query[0]
student_a.name
>>>'A'
student_a.enrolled_in_physics
>>>True
UPDATED
You could try defining a method within the student class:
class Student(models.Model):
name = models.CharField()
classes = models.ManyToManyField(Class, related_name="students")
def enrolled_in_class(self,class_name):
if len(Class.objects.filter(nombre=class_name,students__in=self)) > 0:
return True
else:
return False
students = Student.objects.all()
student_a = students[0]
student_a.name
student_a.enrolled_in_class('physics')
Original Answer:
Edit: Sorry misread question, I'll try and get an answer together for what you actually wanted: A list of all students, and binary true false if they are enrolled in a specific class.
Alright, I'm reading this as a:
I need a query that returns a list of dictionaries, indicating the student, and enrollment in a class.
#Assume: class_name = 'physics'
def get_class_list(class_name):
filtered_class = Class.object.get(nombre=class_name)
student_names = Students.objects.filter(classes__contains=filtered_class).values_list('name',flat=True)
class_list = []
for name in student_names:
class_list = {}
class_list['name'] = name
enrolled_class_string = 'enrolled_in_' + class_name
class_list[enrolled_class_string] = True
return class_list
This will give you a list of dictionaries with the keys named 'name', and 'class__name'
Hi I have a model that has a return statement like this.
def __unicode__(self):
return u'Car from: %s' % self.car_from
If I call the model and print it, it shows.
mymodel = CarFromData.objects.filter(user = self.user )
<QuerySet [<CarFromData: Car from: CarRoom 1>]>
I would like to access the self.car_from in my view. So I can store the
CarRoom1 inside a variable.
I tried this way
x = mymodel.model.__name__
but that does not give me that string information.
car_from is nothing but an attribute to a CarFromData Model. So, its value can get in following way:
mymodel = CarFromData.objects.filter(user=self.user)
myobject = mymodel.first()
x = None
if myobject:
x = myobject.car_from
I have two Django models and connected via Foreignkey element and in the second model I need to use the firs model's attribute - example (pseudocode):
Class Category(models.Model):
c_attribute = "Blue"
Class Object(models.Model):
o_category = models.ForeignKey(Category)
o_color = o_category.c_attribute
The key here is the last line - I got error saying that ForeignKey object has no attribute c_attribute.
Thanks
Because o_category is a Key to a Category, not a Category itself!
you can check by type(o_category) to check is not Category!
so you have access to related Cateogry of a Object in other parts of application when connected to database.for example in shell you can write:
c = Category()
c.save()
o = Object(o_category = c, ...) #create Object with desired params
... #some changes to o
o.save()
o.o_category.c_attribute #this will work! :)
You can use to_field='', but that might give you an error as well.
Class Object(models.Model):
o_category = models.ForeignKey(Category)
o_color = models.ForeignKey(Category, to_field="c_attribute")
The best thing is do create a function in your Object model, that would get you the categories c_attribute like so:
def get_c_attribute(self):
return self.o_category.c_attribute
def index(request):
expiring_list = probe.objects.filter("isExpired=True")
output = ', '.join([p.serial for p in expiring_list])
return HttpResponse(output)
isExpired is a Boolean function. How should I modify the filter so that the filter does not raise a ValueError?
You are making the query in a wrong format.
Your query should be of the form:
expiring_list = probe.objects.filter(isExpired = True)
This was the query you needed to make in case isExpired was your model field. But since you say its a function, assuming that function is inside the class you need to get all the objects in the following way:
expiring_list = []
objects = probe.objects.all()
for obj in objects:
if obj.isExpired() == True:
expiring_list.append(obj)
The expiring_list will now contain all the objects of the model probe where isExpired function returns True
I think isExpired is not a field in your models, as reference to your previous question Refresh a field from another table [Django]
I think exp_date is the field which you are looking for.
Try this:
import datetime
def index(request):
expiring_list = probe.objects.filter(exp_date__lt=datetime.date.today())
output = ', '.join([p.serial for p in expiring_list])
return HttpResponse(output)
I want to create a new type of field for django models that is basically a ListOfStrings. So in your model code you would have the following:
models.py:
from django.db import models
class ListOfStringsField(???):
???
class myDjangoModelClass():
myName = models.CharField(max_length=64)
myFriends = ListOfStringsField() #
other.py:
myclass = myDjangoModelClass()
myclass.myName = "bob"
myclass.myFriends = ["me", "myself", "and I"]
myclass.save()
id = myclass.id
loadedmyclass = myDjangoModelClass.objects.filter(id__exact=id)
myFriendsList = loadedclass.myFriends
# myFriendsList is a list and should equal ["me", "myself", "and I"]
How would you go about writing this field type, with the following stipulations?
We don't want to do create a field which just crams all the strings together and separates them with a token in one field like this. It is a good solution in some cases, but we want to keep the string data normalized so tools other than django can query the data.
The field should automatically create any secondary tables needed to store the string data.
The secondary table should ideally have only one copy of each unique string. This is optional, but would be nice to have.
Looking in the Django code it looks like I would want to do something similar to what ForeignKey is doing, but the documentation is sparse.
This leads to the following questions:
Can this be done?
Has it been done (and if so where)?
Is there any documentation on Django about how to extend and override their model classes, specifically their relationship classes? I have not seen a lot of documentation on that aspect of their code, but there is this.
This is comes from this question.
There's some very good documentation on creating custom fields here.
However, I think you're overthinking this. It sounds like you actually just want a standard foreign key, but with the additional ability to retrieve all the elements as a single list. So the easiest thing would be to just use a ForeignKey, and define a get_myfield_as_list method on the model:
class Friends(model.Model):
name = models.CharField(max_length=100)
my_items = models.ForeignKey(MyModel)
class MyModel(models.Model):
...
def get_my_friends_as_list(self):
return ', '.join(self.friends_set.values_list('name', flat=True))
Now calling get_my_friends_as_list() on an instance of MyModel will return you a list of strings, as required.
What you have described sounds to me really similar to the tags.
So, why not using django tagging?
It works like a charm, you can install it independently from your application and its API is quite easy to use.
I also think you're going about this the wrong way. Trying to make a Django field create an ancillary database table is almost certainly the wrong approach. It would be very difficult to do, and would likely confuse third party developers if you are trying to make your solution generally useful.
If you're trying to store a denormalized blob of data in a single column, I'd take an approach similar to the one you linked to, serializing the Python data structure and storing it in a TextField. If you want tools other than Django to be able to operate on the data then you can serialize to JSON (or some other format that has wide language support):
from django.db import models
from django.utils import simplejson
class JSONDataField(models.TextField):
__metaclass__ = models.SubfieldBase
def to_python(self, value):
if value is None:
return None
if not isinstance(value, basestring):
return value
return simplejson.loads(value)
def get_db_prep_save(self, value):
if value is None:
return None
return simplejson.dumps(value)
If you just want a django Manager-like descriptor that lets you operate on a list of strings associated with a model then you can manually create a join table and use a descriptor to manage the relationship. It's not exactly what you need, but this code should get you started.
Thanks for all those that answered. Even if I didn't use your answer directly the examples and links got me going in the right direction.
I am not sure if this is production ready, but it appears to be working in all my tests so far.
class ListValueDescriptor(object):
def __init__(self, lvd_parent, lvd_model_name, lvd_value_type, lvd_unique, **kwargs):
"""
This descriptor object acts like a django field, but it will accept
a list of values, instead a single value.
For example:
# define our model
class Person(models.Model):
name = models.CharField(max_length=120)
friends = ListValueDescriptor("Person", "Friend", "CharField", True, max_length=120)
# Later in the code we can do this
p = Person("John")
p.save() # we have to have an id
p.friends = ["Jerry", "Jimmy", "Jamail"]
...
p = Person.objects.get(name="John")
friends = p.friends
# and now friends is a list.
lvd_parent - The name of our parent class
lvd_model_name - The name of our new model
lvd_value_type - The value type of the value in our new model
This has to be the name of one of the valid django
model field types such as 'CharField', 'FloatField',
or a valid custom field name.
lvd_unique - Set this to true if you want the values in the list to
be unique in the table they are stored in. For
example if you are storing a list of strings and
the strings are always "foo", "bar", and "baz", your
data table would only have those three strings listed in
it in the database.
kwargs - These are passed to the value field.
"""
self.related_set_name = lvd_model_name.lower() + "_set"
self.model_name = lvd_model_name
self.parent = lvd_parent
self.unique = lvd_unique
# only set this to true if they have not already set it.
# this helps speed up the searchs when unique is true.
kwargs['db_index'] = kwargs.get('db_index', True)
filter = ["lvd_parent", "lvd_model_name", "lvd_value_type", "lvd_unique"]
evalStr = """class %s (models.Model):\n""" % (self.model_name)
evalStr += """ value = models.%s(""" % (lvd_value_type)
evalStr += self._params_from_kwargs(filter, **kwargs)
evalStr += ")\n"
if self.unique:
evalStr += """ parent = models.ManyToManyField('%s')\n""" % (self.parent)
else:
evalStr += """ parent = models.ForeignKey('%s')\n""" % (self.parent)
evalStr += "\n"
evalStr += """self.innerClass = %s\n""" % (self.model_name)
print evalStr
exec (evalStr) # build the inner class
def __get__(self, instance, owner):
value_set = instance.__getattribute__(self.related_set_name)
l = []
for x in value_set.all():
l.append(x.value)
return l
def __set__(self, instance, values):
value_set = instance.__getattribute__(self.related_set_name)
for x in values:
value_set.add(self._get_or_create_value(x))
def __delete__(self, instance):
pass # I should probably try and do something here.
def _get_or_create_value(self, x):
if self.unique:
# Try and find an existing value
try:
return self.innerClass.objects.get(value=x)
except django.core.exceptions.ObjectDoesNotExist:
pass
v = self.innerClass(value=x)
v.save() # we have to save to create the id.
return v
def _params_from_kwargs(self, filter, **kwargs):
"""Given a dictionary of arguments, build a string which
represents it as a parameter list, and filter out any
keywords in filter."""
params = ""
for key in kwargs:
if key not in filter:
value = kwargs[key]
params += "%s=%s, " % (key, value.__repr__())
return params[:-2] # chop off the last ', '
class Person(models.Model):
name = models.CharField(max_length=120)
friends = ListValueDescriptor("Person", "Friend", "CharField", True, max_length=120)
Ultimately I think this would still be better if it were pushed deeper into the django code and worked more like the ManyToManyField or the ForeignKey.
I think what you want is a custom model field.