Getting dynamic model choices in newforms

This is an excerpt from the form code used on this site; the tricky bit here is making the `choices` for the `language` field get filled in dynamically from `Language.objects.all()` on each form instantiation, so that new languages can be picked up automatically. It also adds a blank choice at the beginning so that users can't accidentally ignore the field and incorrectly end up with whatever Language was first in the list. If you use this, always remember that you have to call the superclass `__init__` _before_ you set your dynamic choices, and that you need to accept `*args` and `**kwargs` so you can pass them to it. In theory, `ModelChoiceField` will solve this, but it still seems to be a bit buggy.

• newforms
• models

Wed 22 Sep 2010 Read More

Filter to resize a ImageField on demand

A filter to resize a ImageField on demand, a use case could be: ` <img src="object.get_image_url" alt="original image"> <img src="object.image|thumbnail" alt="image resized to default 200x200 format"> <img src="object.image|thumbnail:"200x300" alt="image resized to 200x300"> ` The filter is applied to a image field (not the image url get from *get_image_url* method of the model), supposing the image filename is "image.jpg", it checks if there is a file called "image_200x200.jpg" or "image_200x300.jpg" on the second case, if the file isn't there, it resizes the original image, finally it returns the proper url to the resized image. There is a **TODO**: the filter isn't checking if the original filename is newer than the cached resized image, it should check it and resize the image again in this case.

• filter
• models
• thumbnail
• resize
• imagefield

Sun 13 Nov 2011 Read More

Querying on existence of a relationship

When you have two models joined by a foreign key, it's common to want to retrieve a set of objects from the "target" of the foreign key based on whether there are any objects "pointing" to them. This snippet demonstrates how to do so, using the `extra` method of the default model manager. Note that this is probably more efficient than using two ORM methods (e.g., selecting all values from one table, and using an `id__in` lookup on the other) since it does the whole thing in one query and avoids instantiating any intermediate objects.

• models
• orm
• foreign-keys

Sun 11 Jul 2010 Read More

JSONField

This is a great way to pack extra data into a model object, where the structure is dynamic, and not relational. For instance, if you wanted to store a list of dictionaries. The data won't be classically searchable, but you can define pretty much any data construct you'd like, as long as it is JSON-serializable. It's especially useful in a JSON heavy application or one that deals with a lot of javascript. **Example** (models.py): from django.db import models from jsonfield import JSONField class Sequence(models.Model): name = models.CharField(maxlength=25) list = JSONField() **Example** (shell): fib = Sequence(name='Fibonacci') fib.list = [0, 1, 1, 2, 3, 5, 8] fib.save() fib = Sequence.objects.get(name='Fibonacci') fib.list.append(13) print fib.list [0, 1, 1, 2, 3, 5, 8, 13] fib.get_list_json() "[0, 1, 1, 2, 3, 5, 8, 13]" *Note:* You can only save JSON-serializable data. Also, dates will be converted to string-timestamps, because I don't really know what better to do with them. Finally, I'm not sure how to interact with forms yet, so that realm is a bit murky.

• models
• model
• json
• db
• field
• json-field

Tue 19 Feb 2013 Read More

PositionField

**This is a model field for managing user-specified positions.** Usage ===== Add a `PositionField` to your model; that's just about it. If you want to work with all instances of the model as a single collection, there's nothing else required. In order to create collections based on another field in the model (a `ForeignKey`, for example), set `unique_for_field` to the name of the field. It's probably also a good idea to wrap the `save` method of your model in a transaction since it will trigger another query to reorder the other members of the collection. Here's a simple example: from django.db import models, transaction from positions.fields import PositionField class List(models.Model): name = models.CharField(max_length=50) class Item(models.Model): list = models.ForeignKey(List, db_index=True) name = models.CharField(max_length=50) position = PositionField(unique_for_field='list') # not required, but probably a good idea save = transaction.commit_on_success(models.Model.save) Indices ------- In general, the value assigned to a `PositionField` will be handled like a list index, to include negative values. Setting the position to `-2` will cause the item to be moved to the second position from the end of the collection -- unless, of course, the collection has fewer than two elements. Behavior varies from standard list indices when values greater than or less than the maximum or minimum positions are used. In those cases, the value is handled as being the same as the maximum or minimum position, respectively. `None` is also a special case that will cause an item to be moved to the last position in its collection. Limitations =========== * Unique constraints can't be applied to `PositionField` because they break the ability to update other items in a collection all at once. This one was a bit painful, because setting the constraint is probably the right thing to do from a database consistency perspective, but the overhead in additional queries was too much to bear. * After a position has been updated, other members of the collection are updated using a single SQL `UPDATE` statement, this means the `save` method of the other instances won't be called. More === More information, including an example app and tests, is available on [Google Code](http://code.google.com/p/django-positions/).

• lists
• models
• fields
• model
• field
• list
• sorting
• ordering
• collection
• collections

Tue 20 Oct 2020 Read More

Updated FileField / ImageField with a delete checkbox

Example model: class MyModel(models.Model): file = RemovableFileField(upload_to='files', \ null=True, blank=True) image = RemovableImageField(upload_to='images', \ null=True, blank=True) A delete checkbox will be automatically rendered when using ModelForm or editing it using form_for_instance. Also, the filename or the image will be displayed below the form field. You can edit the render method of the DeleteCheckboxWidget or add one to RemovableFileFormWidget to customize the rendering. UPDATE: 5. April 2009. Making it work with latest Django (thanks for the comments).

• newforms
• models
• imagefield
• filefield
• remove
• delete

Thu 14 Apr 2011 Read More

Full-Text Searchable Models

A drop-in module to allow for full-text searchable models with very little effort. Tested with PostgreSQL 8.3, but should work on earlier versions with the tsearch2 module installed.

• models
• search
• full-text
• tsearch2

Tue 18 Jun 2013 Read More

ImageWithThumbsField

Easy way to generate image thumbnails for your models. Works with any Storage Backend. From: [http://code.google.com/p/django-thumbs/](http://code.google.com/p/django-thumbs/) **Usage example:** ============== photo = ImageWithThumbsField(upload_to='images', sizes=((125,125),(300,200),) To retrieve image URL, exactly the same way as with ImageField: my_object.photo.url To retrieve thumbnails URL's just add the size to it: my_object.photo.url_125x125 my_object.photo.url_300x200 Note: The 'sizes' attribute is not required. If you don't provide it, ImageWithThumbsField will act as a normal ImageField **How it works:** ============= For each size in the 'sizes' atribute of the field it generates a thumbnail with that size and stores it following this format: available_filename.[width]x[height].extension Where 'available_filename' is the available filename returned by the storage backend for saving the original file. Following the usage example above: For storing a file called "photo.jpg" it saves: photo.jpg (original file) photo.125x125.jpg (first thumbnail) photo.300x200.jpg (second thumbnail) With the default storage backend if photo.jpg already exists it will use these filenames: photo_.jpg photo_.125x125.jpg photo_.300x200.jpg **Note:** It assumes that if filename "any_filename.jpg" is available filenames with this format "any_filename.[widht]x[height].jpg" will be available, too.

• image
• models
• fields
• thumbnail
• field
• thumbnails
• thumbs

Sun 21 Aug 2011 Read More

models with order (+admin editing)

Use this abstract model if you want to add "order" to a given model. Once you do, you will get automatic "up" and "down" links for each model row. One problem is that if the user sorts by another row, the up and down links are still there, but now don't make a lot of sense.

• models
• admin
• order

Tue 23 Nov 2010 Read More

Manager introspecting attached model

[A comment on a recent blog entry of mine](http://www.b-list.org/weblog/2008/feb/25/managers/#c63422) asked about a setup where one model has foreign keys pointing at it from several others, and how to write a manager which could attach to any of those models and query seamlessly on the relation regardless of what it's named. This is a simple example of how to do it: in this case, both `Movie` and `Restaurant` have foreign keys to `Review`, albeit under different names. However, they both use `ReviewedObjectManager` to provide a method for querying objects whose review assigned a certain rating; this works because an instance of `ReviewedObjectManager` "knows" what model it's attached to, and can introspect that model, using [Django's model-introspection API](http://www.b-list.org/weblog/2007/nov/04/working-models/), to find out the correct name to use for the relation, and then use that to perform the query. Using model introspection in this fashion is something of an advanced topic, but is extremely useful for writing flexible, reusable code. **Also**, note that the introspection cannot be done in the manager's `__init__()` method -- at that point, `self.model` is still `None` (it won't be filled in with the correct model until a bit later) -- so it's necessary to come up with some way to defer the introspection. In this case, I'm doing it in a method that's called when the relation name is first needed, and which caches the result in an attribute.

• managers
• models
• introspection

Sun 11 Jul 2010 Read More

DRY with common model fields (another way)

Some time you want to add some common fields to a group of models, for example, in a **Generalization/Specialization** relationship. One could have a base model as the generalization class and specialized models with a foreign key to that base model with an unique attribute but I don't like it that way so, I just do this code to add some commons attributes to a lot of models. If you have many models that all share the same fields, this might be an option. The fields are added directly to each model, e.g. while they will be duplicated on the database level, you only have to define them once in your **python** code. This code is a cleaner way(I think!!!) to do it and will do the same that [this one](http://www.djangosnippets.org/snippets/317/). I hope this piece of code will be useful for you.

• models
• model
• dry
• common

Wed 17 Nov 2010 Read More

Filter on Multiple M2M Objects Simultaneously

This snippet should allow you to do queries not before possible using Django's ORM. It allows you to "Split" up the m2m object you are filtering on. This is best described by example: Suppose you have `Article` and `Tag`, where `Article` has a m2m relation with `Tag` (`related_name = 'tag'`). If I run: from django.db.models.query import Q Article.objects.filter(Q(tag__value = 'A') & Q(tag__value = 'B')) > # no results I would expect to get no results (there are no tags with both a value of 'A' and 'B'). However, I *would* like to somehow get a list of articles with tags meeting that criteria. Using this snippet, you can: from django.db.models.query import Q from path.to.qsplit import QSplit Article.objects.filter(QSplit(Q(tag__value = 'A')) & QSplit(Q(tag__value = 'B'))) > # articles with both tags So now they are split into different `Tag` entries. Notice how the `QSplit()` constructor takes a `Q` object---it's possible to give this any complicated Q-type expression.

• models
• q
• m2m
• db
• orm

Sun 11 Jul 2010 Read More

Quickly create an admin interface for an app.

This creates an admin interface for each model in the "your_appname" app. It also adds each column to the admin list view. Place this in the admin.py of "your_appname".

• models
• admin
• legacy

Sat 31 Aug 2013 Read More

DRY with common model fields

If you have many models that all share the same fields, this might be an option. Please note that order matters: Your model need to inherit from TimestampedModelBase first, and models.Model second. The fields are added directly to each model, e.g. while they will be duplicated on the database level, you only have to define them once in your python code. Not sure if there is a way to automate the call to TimestampedModelInit(). Tested with trunk rev. 5699. There is probably a slight chance that future revisions might break this.

• models
• model
• inheritance

Tue 02 Sep 2014 Read More

Specify a manager for the Admin

This example shows how you can easily limit the objects in the admin by specifying which Manager the admin should use. I haven't seen this documented anywhere (perhaps I've missed it), but it's proven extremely useful to me. The example here will limit objects to those that are attached to the current Site, but you can use any Manager you want (for example, a Manager that shows only published Articles). Finally -- not that I'm suggesting this -- but you *could* combine this with the ThreadLocals trick to show only objects that have been created by that user.

• managers
• models
• admin
• sites

Sun 11 Jul 2010 Read More