I think this is best explained with some simple model code (I'm writing this from scratch so possible syntax issues - unimportant here):
class Car(models.Model)
make = models.CharField(...)
model = models.CharField(...)
class StatisticType(models.Model):
name = models.CharField(...)
class Statistic(models.Model)
car = models.ForeignKey('Car')
stype = models.ForeignKey('StatisticType')
data = models.CharField(...)
class Meta:
unique_together = (('car', 'stype'),)
We have a car with some hard-coded stats and we have some database controlled statistics. I might add Colours, Wheel Size, etc. The point is it's editable from the admin so neither I or the client need to climb through the data, but it's limited so users can only pick one of each stat (you can't define "Colours" twice).
So I'm trying to write the data input form for this now and I want a list of optional ModelForms that I can chuck on the page. I've got the simplest ModelForm possible:
class StatisticForm(forms.ModelForm):
class Meta:
model = Statistic
The tricky part (in my head) is generating an instance of this ModelForm for each StatisticType, regardless of it existing yet. That is to say if a Car object doesn't have a Colour assigned to it, the form still shows. Similarly, if it does, that instance of a Statistic is loaded in the ModelForm.
In my view, how do I generate a list of these things, regardless of there being a pre-existing instance of any given Statistic?
This seems like it should be a stupidly simple thing to do but it's late on Friday and everything looks skwonky.
Sounds like you might want to leverage an inline model formset factory.
That would allow you to create as many instances of your Statistic object as you need. If you're needing to create instances of your StatisticType on the fly, that's a bit different.
When Django instantiates forms, for a foreign key, m2m or choice field, it will only accept choices that it deems "valid", and will complain if you add a choice using JavaScript that doesn't exist in a related model or set of choices server-side.
So, if you need to make StatisticTypes on the fly, and then populate formset instances with this new value, I would suggest using Knockout.js. It's very good at keeping lots of DOM elements in sync when data changes.
Related
I have a django model that looks something like this:
class MyModel(models.Model):
a = models.BooleanField(default=False)
b = models.CharField(max_length=33, blank=False)
c = models.CharField(max_length=40, blank=True)
and a corresponding form
class MyForm(ModelForm):
class Meta:
model = MyModel
Asking the user to fill in the form is a two phase process. First I ask whether a is True or False. After a submit (would be better maybe with ajax, but keep it simple at first) I can fill in a dropdown list with choices for b and decide whether or not to show c as an option. No need to show a as a choice any more, it's descriptive text.
So I want to present a form twice, and the same model is behind it, slowly being filled in. First choose a, then be reminded that a is True and be asked about b and c. Or that a is False and be asked about only b. After submitting this second form, I save the object.
I'm not clear how best to do this in django. One way is to make two separate form classes, one of which has hidden fields. But if the same model is behind both and the model has required fields, I'm anticipating this will get me in trouble, since the first form won't have satisified the requirement that b be non-empty. In addition, there's a small amount of fragility introduced, since updating the model requires updating two forms (and a probably at least one view).
Alternatively, I could use non-model forms and have full freedom, but I'd love to believe that django has foreseen this need and I can do it easier.
Any suggestions on what the right idiom is?
You can use Form Wizard from form-tools for that: https://django-formtools.readthedocs.io/en/latest/wizard.html
It works rather simple by defining multiple forms and combining them. Then in the end, you can use the data to your liking with a custom done() form. The docs tell you everything. You can use JS to hide some of your fields for the super quick approach (utilize localStorage for example).
What I want is to retrieve all the fields belonging to a Model of a foreign key.
My models for example:
class BaseProduct(models.Model):
name = models.CharField(max_length=256)
variant = models.CharField(max_length=256, default='N/A')
type = models.ForeignKey(ProductType)
class ProductType(models.Model):
name = models.CharField(max_length=256,blank=False,null=False)
sofa = models.ForeignKey(SofaProduct, blank=True, null=True)
toaster = models.ForeignKey(ToasterProduct, blank=True, null=True)
These are just examples, there can be any number of ProductType models each with any number of fields.
In my template I can display all the fields of the BaseProduct by using the BaseProduct ID. What I want is to display all the fields of the FK.
For example if type = sofa in BaseProduct, I need to retrieve and display all sofa fields as well as BaseProduct fields.
(disclaimer: I have a tendency to give really long answers. You'll have to forgive me for that)
First rule of schema design - It should reflect your real world business logic (not the actual business action mind you, just the implications of the relationships). For example, if I have a class Person I can create a class Pet with a foreginKey to Person which translates to - every person can have multiple pets.
If we apply that logic to your schema we see that ProductType is a class that has a foreignKey to both Sofas and Toasters, which means each Toaster can have multiple Sofas and vice versa. Last time I checked, I never heard of a Sofa that had a Toaster.
In other words - you need to think what you're actually trying to achieve here. I'm guessing BaseProduct is a basic class that has common fields, and Sofa and Toaster are different types of products. Since they are different, they have their own special fields, and shouldn't be related, so it makes sense to have them as separate models. So why do you even need ProductType? To define the name Toaster? You're already defining an entire model! Why do you need to keep its name on a different table (and not, say, some custom method that always returns "I am a toaster, hear me roar")?
My best guess is that you want to be able to define new types of products on the go. However, if you intend to keep them separated on the model level, then you'll have to create a model for each new product. And if you want to be able to simple define a new model with ProductType, then you either need to have one Product class to manage them all, or you want a complicated dynamic system that can create new models on the fly.
Let's break those options down:
Create a generic product and a type class, like you did there:
class ProductType(models.Model):
name = models.CharField(max_length=256,blank=False,null=False)
class Product(models.Model):
name = models.CharField(max_length=256)
variant = models.CharField(max_length=256, default='N/A')
type = models.ForeignKey(ProductType)
Now each product can only be of one type, and you can always create new types on the go. This of course means all Product objects will share the same fields, and is very limiting. You won't have the same flexibility for each type like you would before (no sofa-only fields), but on the other hand it will be easier to create dynamic types of objects - you just define a new ProductType and bam you have a whole new group of products.
Create a basic abstract Product model, and define a new sub-model for each new type of product. You'll have a lot more flexibility for each one, but defining new types will always require defining a new model and setting up a table for it. With this scheme you don't need the ProductType object at all because the different models define the different types (there's no need for duplicity).
You can create some kind of admin page for the process, but it's not going
to be very easy to setup, and you might find yourself eventually with too many tables
(which can be especially problematic if you need to sometimes query
on all products - you'll have to join a lot of different tables,
which is not very efficient).
Use a non-relational database with some dynamic-models know how and disco*
*ok, it's actually more complicated than that, but the explanation on how to combine them is way too long, even for my answer. If it seems over your head, forget about it. If you have some idea about how non-relation databases work, you can probably figure it out yourself
Your question is somewhat unclear.
I think you want Django modal forms to display all fields of an modal.
def ListForm(Forms.form):
model = MyModel
fields='__all__' #Sets display all
fk_name ="Model_to_use" #Is needed when your model has more then one fk
Django model form
You can use _set for accessing related objects. For example, if you have two models like these:
class MyModel(models.Model):
name = models.CharField(max_length=200)
somedata = models.CharField(max_length=200)
class AnotherModel(models.Model):
name = models.CharField(max_length=256,blank=False,null=False)
referral = models.ForeignKey(MyModel)
type = models.CharField(max_length=256,blank=False,null=False)
you can access the name field of AnotherModel with
>>> m = MyModel.objects.get(id=1)
>>> m.AnotherModel_set.all()[0].name
See: https://docs.djangoproject.com/en/dev/topics/db/queries/#related-objects
On a side note, you should probably rethink your models structure, as yuvi pointed out.
Let us say I have a model which contains related (foreign key) fields. Likewise, those Foreign Key fields may refer to models which may or may not contain related fields. Note that relational fields in Django may be one-to-one, many-to-one, or many-to-many.
Now, given an instance of a model, I want to recursively and dynamically get all instances of the models related to it, either directly or indirectly down the line. Conceptually, i want to perform a traversal of the related objects and return them.
Example:
class Model1{
rfield1 = models.ForeignKey("Model2")
rfield2 = models.ManyToManyField("Model3")
normalfield1 = models.Charfield(max_length=50)
}
class Model2{
sfield = models.ForeignKey("Model3")
normalfield = models.CharField(max_length=50)
}
class Model3{
normalfield = models.CharField(max_length=50)
}
Let's say, I have an instance of model Model1 model1, and I want to get objects directly related to it i.e. all Model2 and Model3 objects, and also those which are indirectly related i.e. all Model3 objects related to the Model2 objects retrieved previously. I also want to consider the case of a One-to-One field where the related field is defined on the OTHER MODEL.
Also, note that it might not be the case that I know the model of an instance I'm currently working on. Let's say in the previous example, I may not now that model1 is an instance of Model1 model. So I want to perform all these dynamically.
In order to this, I think I need a way to get all related fields of an object.
How to get all the related fields of an object?
And how should I use them to get the actual related objects?
Or is there a way to better to do this? Thank you very much!
UPDATE:
I already know how to perform 1, and 2 basically follows directly from 1. :) Update later.
If you have model1 getting all it's many to many field names (etc) is easy since this is well know and these are all stored in the meta's 'local_many_to_many' list:
[field.name for field in model1._meta.local_many_to_many]
The foreign keys are a bit more tricky since they are stored with all other fields in the meta's 'local_fields' list. Hence we need to make sure that it has a relation of sorts. This can be done as follows:
[field.name for field in model1._meta.local_fields if field.rel]
This method has requires no knowledge of your models. Also further interrogation can be done on the field object if the name is not enough.
I've searched around stack overflow for an answer to this (probably simple) question, but most of the solutions I see seem overly complicated and hard to understand.
I have a model "Post" which is an abstract base class. Models "Announcement" and "Event" inherit from Post.
Right now I'm keeping related lists of Events and Announcements in other models. For instance, I have "removed_events" and "removed_announcements" fields in another model.
However, in my project, "removed_events" and "removed_announcements" are treated exactly the same way. There is no need to disambiguate between a "removed event" and a "removed announcement." In other words, a field keeping track of "removed_posts" would be sufficient.
I don't know how to (or perhaps can't) create a field "removed_posts," since Post is abstract. However, right now I feel like I'm repeating myself in the code (and having to do a lot of clutter-some checks to figure out whether the post I'm looking at is an event or an announcement and add it to the appropriate removed field).
What is the best option here? I could make Posts non-abstract, but Post objects themselves should never be created, and I don't think I can enforce this on a non-abstract object.
My understanding of databases is weak, but I'm under the impression that making Post non-abstract would complicate the database due to joins. Is this a big deal?
Finally, there are other fields in other models where I'd like to condense things that amount to an event_list and an announcement_list into a post_list, but those fields do need to be disambiguated. I could filter the post_list based on post type, but the call to filter() would be slower than being able to directly access the event and announcement lists separately, wouldn't it? Any suggestions here?
Thanks a ton for reading through this.
There are two kinds of model subclassing in Django - Abstract Base Classes; and Multi-Table inheritance.
Abstract Base Classes aren't ever used by themselves, and do not have a database table or any form of identification. They are simply a way of shortening code, by grouping sets of common fields in code, not in the database.
For example:
class Address(models.Model):
street = ...
city = ...
class Meta:
abstract = True
class Employee(Address):
name = ...
class Employer(Address):
employees = ...
company_name = ...
This is a contrived example, but as you can see, an Employee isn't an Address, and neither is an Employer. They just both contain fields relating to an address. There are only two tables in this example; Employee, and Employer - and both of them contain all the fields of Address. An employer address can not be compared to an employee address at the database level - an address doesn't have a key of its own.
Now, with multi-table inheritance, (remove the abstract=True from Address), Address does have a table all to itself. This will result in 3 distinct tables; Address, Employer, and Employee. Both Employer and Employee will have a unique foreign key (OneToOneField) back to Address.
You can now refer to an Address without worrying about what type of address it is.
for address in Address.objects.all():
try:
print address.employer
except Employer.DoesNotExist: # must have been an employee
print address.employee
Each address will have its own primary key, which means it can be saved in a fourth table on its own:
class FakeAddresses(models.Model):
address = models.ForeignKey(Address)
note = ...
Multi-table Inheritance is what you're after, if you need to work with objects of type Post without worrying about what type of Post it is. There will be an overhead of a join if accessing any of the Post fields from the subclass; but the overhead will be minimal. It is a unique index join, which should be incredibly quick.
Just make sure, that if you need access to the Post, that you use select_related on the queryset.
Events.objects.select_related(depth=1)
That will avoid additional queries to fetch the parent data, but will result in the join occurring. So only use select related if you need the Post.
Two final notes; if a Post can be both an Announcement AND an Event, then you need to do the traditional thing, and link to Post via a ForeignKey. No subclassing will work in this case.
The last thing is that if the joins are performance critical between the parent and the children, you should use abstract inheritance; and use Generic Relations to refer to the abstract Posts from a table that is much less performance critical.
Generic Relations essentially store data like this:
class GenericRelation(models.Model):
model = ...
model_key = ...
DeletedPosts(models.Model):
post = models.ForeignKey(GenericRelation)
That will be a lot more complicated to join in SQL (django helps you with that), but it will also be less performant than a simple OneToOne join. You should only need to go down this route if the OneToOne joins are severely harming performance of your application which is probably unlikely.
Generic relationships and foreign keys are your friend in your path to succeed. Define an intermediate model where one side is generic, then the other side will get a related list of polymorphic models. It's just a little more complicated than a standard m2m join model, in that the generic side has two columns, one to ContentType (actually a FK) and the other to the PK of the actual linked model instance. You can also restrict the models to be linked with using standard FK parameters.
You'll get used with it quickly.
(now that I get an actual keyboard to write with, here there is the example:)
class Post(models.Model):
class Meta: abstract = True
CONCRETE_CLASSES = ('announcement', 'event',)
removed_from = generic.GenericRelation('OwnerRemovedPost',
content_type_field='content_type',
object_id_field='post_id',
)
class Announcement(Post): pass
class Event(Post): pass
class Owner(models.Model):
# non-polymorphic m2m
added_events = models.ManyToManyField(Event, null=True)
# polymorphic m2m-like property
def removed_posts(self):
# can't use ManyToManyField with through.
# can't return a QuerySet b/c it would be a union.
return [i.post for i in self.removed_post_items.all()]
def removed_events(self):
# using Post's GenericRelation
return Event.objects.filter(removed_from__owner=self)
class OwnerRemovedPost(models.Model):
content_type = models.ForeignKey(ContentType,
limit_choices_to={'name__in': Post.CONCRETE_CLASSES},
)
post_id = models.PositiveIntegerField()
post = generic.GenericForeignKey('content_type', 'post_id')
owner = models.ForeignKey(Owner, related_name='removed_post_items')
class Meta:
unique_together = (('content_type', 'post_id'),) # to fake FK constraint
You can't filter into the related collection like a classic many-to-many, but with the proper methods in Owner, and using the concrete classes' managers smartly, you get everywhere you want.
This is a problem concerning django.
I have a model say "Automobiles". This will have some basic fields like "Color","Vehicle Owner Name", "Vehicle Cost".
I want to provide a form where the user can add extra fields depending on the automobile that he is adding. For example, if the user is adding a "Car", he will extra fields in the form, dynamically at run time, like "Car Milage", "Cal Manufacturer".
Suppose if the user wants to add a "Truck", he will add "Load that can be carried", "Permit" etc.
How do I achieve this in django?
There are two questions here:
How to provide a form where the user can add new fields at run time?
How to add the fields to the database so that it can be retrieved/queried later?
There are a few approaches:
key/value model (easy, well supported)
JSON data in a TextField (easy, flexible, can't search/index easily)
Dynamic model definition (not so easy, many hidden problems)
It sounds like you want the last one, but I'm not sure it's the best for you. Django is very easy to change/update, if system admins want extra fields, just add them for them and use south to migrate. I don't like generic key/value database schemas, the whole point of a powerful framework like Django is that you can easily write and rewrite custom schemas without resorting to generic approaches.
If you must allow site users/administrators to directly define their data, I'm sure others will show you how to do the first two approaches above. The third approach is what you were asking for, and a bit more crazy, I'll show you how to do. I don't recommend using it in almost all cases, but sometimes it's appropriate.
Dynamic models
Once you know what to do, this is relatively straightforward. You'll need:
1 or 2 models to store the names and types of the fields
(optional) An abstract model to define common functionality for your (subclassed) dynamic models
A function to build (or rebuild) the dynamic model when needed
Code to build or update the database tables when fields are added/removed/renamed
1. Storing the model definition
This is up to you. I imagine you'll have a model CustomCarModel and CustomField to let the user/admin define and store the names and types of the fields you want. You don't have to mirror Django fields directly, you can make your own types that the user may understand better.
Use a forms.ModelForm with inline formsets to let the user build their custom class.
2. Abstract model
Again, this is straightforward, just create a base model with the common fields/methods for all your dynamic models. Make this model abstract.
3. Build a dynamic model
Define a function that takes the required information (maybe an instance of your class from #1) and produces a model class. This is a basic example:
from django.db.models.loading import cache
from django.db import models
def get_custom_car_model(car_model_definition):
""" Create a custom (dynamic) model class based on the given definition.
"""
# What's the name of your app?
_app_label = 'myapp'
# you need to come up with a unique table name
_db_table = 'dynamic_car_%d' % car_model_definition.pk
# you need to come up with a unique model name (used in model caching)
_model_name = "DynamicCar%d" % car_model_definition.pk
# Remove any exist model definition from Django's cache
try:
del cache.app_models[_app_label][_model_name.lower()]
except KeyError:
pass
# We'll build the class attributes here
attrs = {}
# Store a link to the definition for convenience
attrs['car_model_definition'] = car_model_definition
# Create the relevant meta information
class Meta:
app_label = _app_label
db_table = _db_table
managed = False
verbose_name = 'Dynamic Car %s' % car_model_definition
verbose_name_plural = 'Dynamic Cars for %s' % car_model_definition
ordering = ('my_field',)
attrs['__module__'] = 'path.to.your.apps.module'
attrs['Meta'] = Meta
# All of that was just getting the class ready, here is the magic
# Build your model by adding django database Field subclasses to the attrs dict
# What this looks like depends on how you store the users's definitions
# For now, I'll just make them all CharFields
for field in car_model_definition.fields.all():
attrs[field.name] = models.CharField(max_length=50, db_index=True)
# Create the new model class
model_class = type(_model_name, (CustomCarModelBase,), attrs)
return model_class
4. Code to update the database tables
The code above will generate a dynamic model for you, but won't create the database tables. I recommend using South for table manipulation. Here are a couple of functions, which you can connect to pre/post-save signals:
import logging
from south.db import db
from django.db import connection
def create_db_table(model_class):
""" Takes a Django model class and create a database table, if necessary.
"""
table_name = model_class._meta.db_table
if (connection.introspection.table_name_converter(table_name)
not in connection.introspection.table_names()):
fields = [(f.name, f) for f in model_class._meta.fields]
db.create_table(table_name, fields)
logging.debug("Creating table '%s'" % table_name)
def add_necessary_db_columns(model_class):
""" Creates new table or relevant columns as necessary based on the model_class.
No columns or data are renamed or removed.
XXX: May need tweaking if db_column != field.name
"""
# Create table if missing
create_db_table(model_class)
# Add field columns if missing
table_name = model_class._meta.db_table
fields = [(f.column, f) for f in model_class._meta.fields]
db_column_names = [row[0] for row in connection.introspection.get_table_description(connection.cursor(), table_name)]
for column_name, field in fields:
if column_name not in db_column_names:
logging.debug("Adding field '%s' to table '%s'" % (column_name, table_name))
db.add_column(table_name, column_name, field)
And there you have it! You can call get_custom_car_model() to deliver a django model, which you can use to do normal django queries:
CarModel = get_custom_car_model(my_definition)
CarModel.objects.all()
Problems
Your models are hidden from Django until the code creating them is run. You can however run get_custom_car_model for every instance of your definitions in the class_prepared signal for your definition model.
ForeignKeys/ManyToManyFields may not work (I haven't tried)
You will want to use Django's model cache so you don't have to run queries and create the model every time you want to use this. I've left this out above for simplicity
You can get your dynamic models into the admin, but you'll need to dynamically create the admin class as well, and register/reregister/unregister appropriately using signals.
Overview
If you're fine with the added complication and problems, enjoy! One it's running, it works exactly as expected thanks to Django and Python's flexibility. You can feed your model into Django's ModelForm to let the user edit their instances, and perform queries using the database's fields directly. If there is anything you don't understand in the above, you're probably best off not taking this approach (I've intentionally not explained what some of the concepts are for beginners). Keep it Simple!
I really don't think many people need this, but I have used it myself, where we had lots of data in the tables and really, really needed to let the users customise the columns, which changed rarely.
Database
Consider your database design once more.
You should think in terms of how those objects that you want to represent relate to each other in the real world and then try to generalize those relations as much as you can, (so instead of saying each truck has a permit, you say each vehicle has an attribute which can be either a permit, load amount or whatever).
So lets try it:
If you say you have a vehicle and each vehicle can have many user specified attributes consider the following models:
class Attribute(models.Model):
type = models.CharField()
value = models.CharField()
class Vehicle(models.Model):
attribute = models.ManyToMany(Attribute)
As noted before, this is a general idea which enables you to add as much attributes to each vehicle as you want.
If you want specific set of attributes to be available to the user you can use choices in the Attribute.type field.
ATTRIBUTE_CHOICES = (
(1, 'Permit'),
(2, 'Manufacturer'),
)
class Attribute(models.Model):
type = models.CharField(max_length=1, choices=ATTRIBUTE_CHOICES)
value = models.CharField()
Now, perhaps you would want each vehicle sort to have it's own set of available attributes. This can be done by adding yet another model and set foreign key relations from both Vehicle and Attribute models to it.
class VehicleType(models.Model):
name = models.CharField()
class Attribute(models.Model):
vehicle_type = models.ForeigngKey(VehicleType)
type = models.CharField()
value = models.CharField()
class Vehicle(models.Model):
vehicle_type = models.ForeigngKey(VehicleType)
attribute = models.ManyToMany(Attribute)
This way you have a clear picture of how each attribute relates to some vehicle.
Forms
Basically, with this database design, you would require two forms for adding objects into the database. Specifically a model form for a vehicle and a model formset for attributes. You could use jQuery to dynamically add more items on the Attribute formset.
Note
You could also separate Attribute class to AttributeType and AttributeValue so you don't have redundant attribute types stored in your database or if you want to limit the attribute choices for the user but keep the ability to add more types with Django admin site.
To be totally cool, you could use autocomplete on your form to suggest existing attribute types to the user.
Hint: learn more about database normalization.
Other solutions
As suggested in the previous answer by Stuart Marsh
On the other hand you could hard code your models for each vehicle type so that each vehicle type is represented by the subclass of the base vehicle and each subclass can have its own specific attributes but that solutions is not very flexible (if you require flexibility).
You could also keep JSON representation of additional object attributes in one database field but I am not sure this would be helpfull when querying attributes.
Here is my simple test in django shell- I just typed in and it seems work fine-
In [25]: attributes = {
"__module__": "lekhoni.models",
"name": models.CharField(max_length=100),
"address": models.CharField(max_length=100),
}
In [26]: Person = type('Person', (models.Model,), attributes)
In [27]: Person
Out[27]: class 'lekhoni.models.Person'
In [28]: p1= Person()
In [29]: p1.name= 'manir'
In [30]: p1.save()
In [31]: Person.objects.a
Person.objects.aggregate Person.objects.all Person.objects.annotate
In [32]: Person.objects.all()
Out[33]: [Person: Person object]
It seems very simple- not sure why it should not be a considered an option- Reflection is very common is other languages like C# or Java- Anyway I am very new to django things-
Are you talking about in a front end interface, or in the Django admin?
You can't create real fields on the fly like that without a lot of work under the hood. Each model and field in Django has an associated table and column in the database. To add new fields usually requires either raw sql, or migrations using South.
From a front end interface, you could create pseudo fields, and store them in a json format in a single model field.
For example, create an other_data text field in the model. Then allow users to create fields, and store them like {'userfield':'userdata','mileage':54}
But I think if you're using a finite class like vehicles, you would create a base model with the basic vehicle characteristics, and then create models that inherits from the base model for each of the vehicle types.
class base_vehicle(models.Model):
color = models.CharField()
owner_name = models.CharField()
cost = models.DecimalField()
class car(base_vehicle):
mileage = models.IntegerField(default=0)
etc