I have a foreign key field called books (in the model Book) in an intermediate model called Link_Book_Courses.
I'd like to add multiple Book objects to this. How do I do that in django?
Related Objects have an add() method:
https://docs.djangoproject.com/en/1.3/ref/models/relations/#django.db.models.fields.related.RelatedManager.add
You can add several like this
mymodel.related.add(*OtherModel.objects.filter(...))
For one ForeignKey you can only have one book. So to add multiple entries, you’ll have to add multiple Link_Book_Courses-objects. However what you might really want is a ManyToManyField.
Related
My question is related to this one, where the author wanted to store a reference to a model in the attribute of another model using Django's models.ForeignKey field type. My question is, would the exact same method work, if an article had multiple authors, or is something additional needed to establish this?
If an article can have multiple authors and each author can have multiple articles, then you want to use a ManyToManyField.
I have a model that has a Charfield (let's name it advantages) with a choices attribute. After a while, I've decided that this field should be "upgraded" to some kind of ListField, since more than one choice can be selected.
From what I have searched, I have two options:
1 - Create a new model, and use a ManyToManyField in the first model referencing this new model. This way, the "multiple select" default field used in admin will be rendered. Life is good.
2- Create a custom field that saves my field as a string with some kind of separator.
These two approaches are summarized in What is the most efficent way to store a list in the Django models? and the 2nd approach in more examples: How to create list field in django, http://cramer.io/2008/08/08/custom-fields-in-django/, https://djangosnippets.org/snippets/1200/, https://djangosnippets.org/snippets/1491/
Fact is: I don't want to create another model just to have the ManyToManyField. This is a controlled list of choices that I have (and don't want people adding new items) and think that creating a table for this is overkill (although I can create a fixture to this table and not register the model in admin.py, so people wouldn't be adding new items. But I don't know how would migrations work when changing these values in fixtures, when in the past I would just chance the choices tuple in my model definition).
...and creating a new custom field, I don't know. This seems like problems in the long run since I don't know the implications, problems when upgrading Django, etc.
Why there isn't a built in ListField? Which problems do you see in the long run for the two approaches I'm thinking of? I'm planning to do the first but I'm a little lost about migrations.
django.contrib.postgres has an ArrayField.
You seem to not be willing to create a new table with only the List inside (because it would be "overkill"), but what you are suggesting is to copy/paste the same exact values in all the entries of your table, which isn't a good model solution in my opinion.
I'd advise to go for the ManyToMany (or any other implementation doing the trick with another table).
What is the defacto way of creating model relationships in Django via frontend forms.
For example a user signs up for service using a form, they start a quote.
In getting a quote they can select and add products to their quote specifying variable such as sizes in this process.
This is modelled with relevant User, Quote, Product models and relevant relationships.
I am trying to work out the best way that these are linked together by frontend forms and views.
Would I load into the quote form a hidden field for the related user_id for example, which I can then process manually to form the one-to-many relationship.
I am just wondering if this is something accounted for within forms or if I have to manually create the forms to achieve my goal.
This is one of the more complicated things to try and achieve but there are several things in Django which will help you.
You're going to need a ManyToMany field on the Quote model to link the Products to it.
This can be displayed in forms simply via a ModelMultipleChoiceField:
https://docs.djangoproject.com/en/dev/ref/forms/fields/#modelmultiplechoicefield
... which is just renders a basic multiple select list of existing products.
The interface you want probably looks more like an inline formset however. The complication here is that they are designed for ForeignKey relations rather than ManyToMany.
Under the covers, a ManyToMany relation is actually just two ForeignKey relations, via an intermediate 'through' model. We can exploit this to build an inline formset on the through model, see this answer:
https://stackoverflow.com/a/10999074/202168
You'll note the caveat in that answer, the inline rows won't know which Quote they belong to unless you override some code.
You may like to look at some helper apps which provide custom widgets for ManyToMany fields:
https://code.google.com/p/django-ajax-filtered-fields/
http://django-autocomplete-light.readthedocs.org/en/latest/
By default, Django automatically gives each model an id field.
Are there any additional fields django's ORM adds automatically? Perhaps in specific cases?
There are only 2 other situations I can think of where fields are automatically created. One is when sub-classing another model. The sub-class will inherit the parent's fields, see here. The other is a Many-to-Many relationship. For a M2M relationship not only will a field get created but an entire intermediate table. Again, the relevant docs
Also, you can avoid having Django create the id field if you specify primary=True for the field you want to use as the primary key. See here
There are some other model/DB naming conventions as well. For example, the actual database table names will be prefixed with the Django app name that contains them plus an underscore. For example, a model named Author in an app named library will get called library_author. I'm sure there are other examples as well, so this is not an exhaustive list.
I was wondering if there is already a way to create a separate autoincrement-ID-per-user field in Django?
Basically, I'm storing many related models and I need the IDs generated to be autoincrement per user.
I don't want to change how id works, just need a new field that I can add which is unique=True per user.
Any suggestions (other than overriding save and implementing it myself)?
No, there's no such field, but I wonder why you think you need it. The ID is really just for the model's internal use, you shouldn't ever care what it is.
For example, if you want to know how many related items there are for a user then you would just use the count() method on the related queryset. If you want something to be unique per user, you can use the unique_together meta property.
Can you given an example of a use case for a per-user unique id?
Edited in response to comments: to get the object from a URL as you mention, you just need to do:
myuser.myobject_set.all()[7]