Other posters have previously said in this forum that when your Django app starts getting big and unmanageable, you should split it up into several apps. I'm at that point now. What are the best practices for allowing communication between these apps?
One of my apps (let's call it Processor) processes very large data sets. Once an hour it produces a small amount of new data for the other app. This other app (let's call it Presenter) displays the data to users.
How should Processor hand new data to Presenter? Should it simply import parts of Presenter's model, so it can create and save records in Presenter's database? That seems like tight coupling to me. Or should it pass the data by calling a function in Presenter? Or put the data in some sort of data store that both Processor and Presenter know about?
How do you all usually solve this problem?
/Martin
I would definitely go for the importing Processor's models in the Presenter app. That's how, for instance, you can add extra user info: you have a UserPreferences model with a ForeignKeyField to django.contrib.auth.models.User. Your might have less of a bad feeling doing that that between your two apps because django.contrib is the "standard library", but nevertheless, it is direct coupling.
If your applications are coupled, then your code should be coupled to reflect this. This follows the idea that explicit is better than implicit, right?
If, however, your're designing something a tad more generic (i.e. you're going to use multiple Presenter app instances for Processors of different kinds), you can store the specific models as a setting:
import processor_x.models
PRESENTER_PROCESSOR_MODELS = presenter_x.models
Then, in your Presenter models:
from django.conf import settings
class Presenter:
processor = models.ForeignKey(settings.PRESENTER_PROCESSOR_MODELS)
Caveat: I have never attempted this, but I don't recall a limitation on settings to be only strings, tuples or lists!
Related
I have a bunch of REST views and a bunch of data transfers posted from templates that needs to be cleaned. I am wondering where to put all the sanitizing code and processing code should go: views.py or models.py ? Am I missing some secret tricks that I don't know about? (like usage of model.manager) ?
Currently I kept the models clean with object creation and update. Views is nicely organized with strictly REST views. But as the project grows, more and more code, especially handling of data that has accumulated. (views.py file had grown in size to 150lines) Should I put them in another file. If so, is that ok to pass the whole "request" to that file? (along with having to import models and possibly sessions and messages)
I have implemented my django apps like this:
In models.py: just add the definition of the models and some code usefull for the Signals because in my application will work with other models
In view.py: here i add the code usefull for manage the form for the frontend and create the json necessary for the API call to external app
of course you can split the code in other files and call the functions when needed like fuctions.py or options.py
from .functions import calc
External app with Rest Framework: here is the CORE of the application, where i'll call the models, where all the logic is available
This is how i use it, by the way is always better use the view.py for write the code.
Some Tips about The Model View Controller pattern in web applications
I have a general algorithm design question. I am creating a Django app that will connect to an API, but I won't be storing these results (at least not at first). After I retrieve the data from the API I manipulate it accordingly and have already created a class with numerous methods to do this.
Should the programming logic for this be performed in the model or the view for the Django framework? Is one more sustainable than the other (e.g. in a few months I decide to store the information). Also, is it best to encapsulate my class in its own file, and them import into the model/view?
Thanks!
Rob
If you don't need to store data, don't use Djnago's built in models.
Write views and import your own modules/classes.
Bonus: If your views share a lot of logic (probably related to request/response handling), use class based views and write a common MyBaseView.
I'm working on an e-commerce framework for Django. The chief design goal is to provide the bare minimum functionality in terms of models and view, instead allowing the users of the library to extend or replace the components with their own.
The reasoning for this is that trying to develop a one-size-fits-all solution to e-commerce leads to overcomplicated code which is often far from optimal.
One approach to tackling this seems to be using inversion-of-control, either through Django's settings file or import hacks, but I've come up against a bit of a problem due to how Django registers its models.
The e-commerce framework provides a bunch of abstract models, as well as concrete versions in {app_label}/models.py. Views make use of Django's get_model(app_label,model) function to return the model class without having to hard-code the reference.
This approach has some problems:
Users have to mimic the structure of the framework's apps, ie the app_label and effectively replace our version of the app with their own
Because of the way the admin site works by looking for admin.py in each installed app, they have to mimic or explicitly import the framework's admin classes in order to use them. But by importing them, the register method gets called so they have to be unregistered if a user wants to customise them.
The user has to be extremely careful about how they import concrete models from the core framework. This is because Django's base model metaclass automatically registers a model with the app cache as soon as the class definition is read (ie upon __new__), and the first model registered with a specific label is the one you're stuck with. So you have to define all your override models BEFORE you import any of the core models. This means you end up with messy situations of having a bunch of imports at the bottom of your modules rather than the top.
My thinking is to go further down the inversion-of-control rabbit hole:
All references to core components (models, views, admin, etc) replaced with calls to an IoC container
For all the core (e-commerce framework) models, replace the use of Django's base model metaclass with one that doesn't automatically register the models, then have the container explicitly register them on startup.
My question:
Is there a better way to solve this problem? The goal is to make it easy to customise the framework and override functionality without having to learn lots of annoying tricks. The key seems to be with models and the admin site.
I appreciate that using an IoC container isn't a common pattern in the Django world, so I want to avoid it if possible, but it is seeming like the right solution.
Did you look at the code from other projects with a similar approach?
Not sure if this way covers your needs, but imo the code of django-shop is worth to look at.
This framework provides the basic logic, allowing you to provide custom logic where needed.
customize via models
eg see the productmodel.py
#==============================================================================
# Extensibility
#==============================================================================
PRODUCT_MODEL = getattr(settings, 'SHOP_PRODUCT_MODEL',
'shop.models.defaults.product.Product')
Product = load_class(PRODUCT_MODEL, 'SHOP_PRODUCT_MODEL')
customize via logic/urls
eg see the shop's simplevariation-plugin
It extends the cart-logic, so it hooks in via urlpattern:
(r'^shop/cart/', include(simplevariations_urls)),
(r'^shop/', include(shop_urls)),
and extends the views:
...
from shop.views.cart import CartDetails
class SimplevariationCartDetails(CartDetails):
"""Cart view that answers GET and POSTS request."""
...
The framework provides several points to hook-in, the simplevariation-plugin mentionned above additionally provides a cart-modifier:
SHOP_CART_MODIFIERS = [
...
'shop_simplevariations.cart_modifier.ProductOptionsModifier',
...
]
I worry that this explanation is not very understandable, it is difficult to briefly summarize this concept. But take a look at the django-shop project and some of its extensions: ecosystem
For example, Account 1--> *User --> 1 Authentication
1 account has multiple users and each user will have 1 authentication
I come from java background so what I usually do is
define these classes as java beans (ie, just getter and setter, no logic attached)
create AccountManager ejb class, define create_account method (with takes 1 account, list of users)
preparing data in the web layer, then pass data into AccountManager ejb, for instance:
accountManager.createAccount(account, userList)
But in django, the framework advocates that you put domain logic into model classes (row level) or associated manager classes (table level), which makes things bit awkward. Yes, it is fine that if your logic is only involves one table, but in the real application, usually each step will involve multiple different tables or even databases, so what should I do in this case?
Put the logic into View? I don't think this is good practice at all. or even overwrite the save method in model class, passing in extra data by using **kwargs? then the backend will break.
I hope this illustrates my confusion with where business logic should be placed in a django application.
Not sure if you've read the section on Managers in Django, it seems to solve your current situation. Assuming you have the following Account model defined, User is built-in.
# accounts/models.py
class AccountManager(models.Manager):
def create_account(self, account, user_list):
...
class Account(models.Model):
objects = AccountManager()
Feel free to separate your manager code in a separate file if it gets too big. In your views:
# views.py
from accounts.models import Account
Account.objects.create_account(account, user_list)
The business logic is still in the models.
EDIT
The keyword here is override, not overwrite. If you override the model's save method, you have to keep in mind that any create, update operations from within your web app and admin will use this new functionality. If you only want those business logic to happen once in a specific view, it might be best to keep it out of save.
I guess you can put your business logic in its own regular class. You will have to instantiate that class each time you need to run your business logic. Alternatively, you can put your business logic as a static function in this new class if you want to skip the OOP approach.
What I'm doing is that most of my apps have service.py file with business logic. This is because if I need a function that works with models from multiple apps I simply cannot do this:
# shop/models.py
from auth.models import User, Team
This code will be stuck in circular reference loop.
But I'm gonna move most likely from service.py to app with structure like this:
service/
auth.py
customers.py
another_set_of_functions.py
...
Well the example that you illustrated above with the accountManager.createAccount(account, userList) seems like something that could be easily done my adding a createAccount method to the account model. If you feel the need to take business logic outside of the model though you could always just create a new module that host your business logic, and then import and used in in your views.
While working on my Django-based projects I'm always trying to follow Django's approach to reusable apps - I'm trying to decouple my applications from each other and especially trying to avoid cross references but sometimes it does not seem to be possible.
Let's consider a simple example with 2 applications: articles and users. Articles application defines article model, articles list view and single article view, users application defines user model and user profile view. Article is referencing user from the author field, so articles application is obviously dependent on users application which is fine.
But when it comes to user profile, I want to display latest articles authored by the user (and may be latest articles viewed by the user) on that page but that makes users application aware of articles application which is what I'm trying to avoid.
I can obviously try to push all such references to the template level but it still does not solve the issue completely and at the same time may be very inefficient in terms of database queries sometimes.
What do you guys do in such cases?
If you are really set on not having any conversation between the 'user' app and the 'article' app, then you need a third app to act as interface. That would know about users and articles and define all the coupling between them. Your article view would be in there, since it has to get the user data, and your user profile view would be in there, because it needs to say "Fred wrote 5 articles".
Whether this level of decoupling is worth it, I don't know. Sometimes programming for re-usability gets in the way of making the thing usable in the first place.
The standard (or preferred) way of keeping coupled apps decoupled is to add a conditional coupling - like in some apps that try to import django-notification and only if they find it, they report events to it.
Still, if you have two apps that talks to each other by design, then I don't see any point in decoupling them - there are plenty of examples in Django world of apps that just require other apps. Note that I'm talking here about writing real-world software, not about some academic delibrations :-)
It seems that in this case, the dependency of users on articles is in a method, not a field. (Whether it's a model method, a model class method, or a manager method is immaterial). If that's so, you can do a lazy import of articles inside the method. By the time this import is performed, users.models will be fully loaded, so even if this is a circular import, it will not cause problems. The "import users" statement in articles will not have to re-load users and will have the full users namespace available.