I have Django application with 30+ models. I want to write an application that can take a snapshot of the data in some of the models. I want to write the models once and reuse them in each application so that if I maintain it in one place, the only difference being that when I call python manage.py syncdb the same table are created with different table prefixes.
Is there any way to do this?
This is exactly where the reusable app principle comes into play.
(as explained at the django website)
Related
If I am creating my first python web app using Django, am I able to at a later date, edit/change up models/views and have the program do so accordingly? Do I have to run a specific command to update my web app? Or Do I need to create the web application the way I want too right away?
I have been trying to google this, but am unable to come to a concrete answer. I am just using tutorials, but will want to add my own features at a later date, and want to make sure I do not need to create another app for example. Please remember, I am still fairly new.
You can alter models. Then you should use manage.py makemigrations to construct migrations which reflect the changes you made. Use manage.py migrate to then apply the changes to the database.
It's important to notice that "this is the way it works." You build your models, then make migrations both to initially create the corresponding tables and to make any subsequent changes to them. (It is also possible to scrape from an existing database.)
I'm a few weeks into Python/Django and encountering an annoying problem. I have some existing databases set up in settings.py, everything looks good, I've even accessed the databases using connections[].cursor()
But the databases (and data) are not making their way into models that I want to use, despite doing the makemigrations and migrate commands. I was able to use py manage.py inspectdb --database-dbname and copied that class information manually into my models.py, but that didn't work either (typing py manage.py inspectdb on its own does not pull up these databases, I was only able to view by that --database extension). So I'm stumped, as it seems I'm doing all the right steps but not able to use these existing databases in Django.
Any other hints and steps I can take are welcome!
(Almost) all the tutorials, examples, and third-party app you'll find on the internet, and most of the Django documentation assume you use one database for your app. That's because it's fairly tricky and unusual to use multiple databases in one app.
But it's not impossible to use multiple databases and the documentation contains instructions on how to do this and what changes you'll need to make to make it work.
IMO, these are the pre-conditions to use multiple databases in one project:
The databases contain explicitly unrelated information, i.e. you won't have SQL relationships between tables in different databases. One database may contain a table with a column that maps to a column in a table in another database, but they aren't explicit (no ForeignKey or ManyToManyField in your models).
You don't need to mix databases in one query: This basically derives from the previous condition. It just means that if you need to get objects from one database that depend on the rows coming from another database, you establish the relationship in python. E.g. fetching as list of names from one database and using that list to filter a queryset on the other database.
For example, if you have an existing database that contains Strava routes (which are regularly updated via some external mechanism) and your app is a broader app that helps users getting to know their neighbourhood where they can recommend locations and things to do, being able to offer a list of routes with a starting point nearby might be something you'd want to show.
Now that you know this, the way to go is described in the doc linked above:
Create a database router so that queries for certain models are automatically routed to the correct database. E.g. Route.objects.filter(start_city=city) would automatically fetch routes from your Strava routes database.
If you need to save information about a route in your app, save it in a model in the default database and use a unique identifier of the route that will map to the strava database. Use separate queries (no relationships) to fetch information about a specific route.
That being said, if the Strava database is not regularly updated via 3rd channels and its purpose is just to pre-populate your default database, then export the data from the Strava database as json and import it into your django db using manage.py loaddata or a migration file, the latter being more flexible as to the structure of the json file.
I am setting a web server based on Django Framework with PostgreSQL. I need to update the models in my Django app every hour and I am wondering if is it better way to update the database from another script file using python and psycopg2 or using script that directly connects to Django models and update them using Django API for models? I use this Database only in Django web server.
Summarizing... What is better logic?
update_db.py that contains:
connect to database via psycopg2, update the tables
update_db.py that contains:
connect to Django app models, update them using Django API for models
I will run script every hour using Cron, and the Django Project is already connected to that specific database and it works fine. I am just asking for better performance and better way in logic sense. Data won't be updated very often but it would have big jsons.
Unless you have very compelling reasons to do otherwise, the obvious solution here is to use a custom management command and the ORM - your code will be at the obvious place (for someone else having to work on the project), can be tested as part of your whole project's test suite, and won't require having to mentally "translate" from raw SQL to Django ORM code (not to mention that you'll have more chance to catch a mismatch between your Django models and your script's code when your schema changes - from experience, raw sql scripts that aren't under test are usually forgotten when doing a schema migration).
FWIW, optimizing "for performances" doesn't really makes sense in your case, unless your updates are long and complex and do block access to the site - but even then the overhead of the ORM is certainly not going to be the main bottleneck (just make sure you use properly).
I have a few databases in MongoDB that I want to create models for dynamically, since there are many databases and I cannot do it manually. Questions:
What should my models.py look like? (Does inspectdb work with mongodb databases or only SQL based dbs?)
Since the database models are created dynamically, how do I code the serializer class to return the dynamic fields?
Thanks in advance.
Django supports an object-relational mapper, that is aimed at traditional relational databases. While there are a number of mongodb packages for Django, none of them support inspectdb to construct your models. Either way, inspectdb is a kludge designed as a one of process to help a one-of migratation away from a legacy system, i.e. you'd build your models.py file once and never run inspectdb again. This is not what you want to do, as you seem to want dynamic models that can be added or altered at runtime.
On the bright side, Django MongoDB Engine has some support for arbitrary embedded models within pre-defined models. But even then they don't seem too supportive of it:
As you can see, generic embedded models add a lot of overhead that bloats up your data records. If you want to use them anyway, here’s how you’d do it...
In summary, try to build your models as best you can to actually match your requirements. If you know nothing about your models ahead of production, then perhaps Django isn't the right solution for you.
I need to add a couple fields to Group model in django contrib.auth app using:
field_name = models.CharField(...)
field_name.contribute_to_class(Group, 'field_name')
My issue is while creating the migrations with South, since they are created in the "migrations" directory inside the auth app and, since the system is already in production, I'm not allowed to alter the current django installation in the server in order to migrate auth.
Does anyone know how to create and load such migrations?
Thanks in advance for your help.
Django doesn't make it particularly easy to modify the standard models. I wouldn't recommend that you sublass Group, because it's quite annoying to get built-in functionality to reference the new model instead.
The usual thing to do here is to create a GroupProfile model that has a Group as a unique foreign key. It might not be elegant, but it won't have the huge maintenance overhead that comes with forking Django's source code.
Also: if you can't modify the Django code on the server, you aren't going to be able to do this with raw SQL hackery or a clever migration. South isn't going to be the problem -- the problem is that the Django ORM is going to notice that there are fields present in the SQL table that aren't specified in the code, which will cause it to throw an exception.
Since you use a hack to patch up the model, I think you should write a migration manually. Try copying another migration and changing add_column and models first, if it fails - there is always an option named "raw sql" :)