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. I hope this piece of code will be useful for you.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
import types from copy import deepcopy from django.db import models class ExpanderField(object): """ This type of field can be used to include some common fields and methods in models. It take a class with those common things as parameter in the constructor and will add all fields in that class to models classes in which a field of this class type is declared. If the class containing common fields have fields of type: ForeignKey and ManyToManyField, then the related_name attribute of those fields will be renamed to a value of the form: model_name_original_value . Ej: if related_name = child_set and model name is MenuItem, it will be renamed to MenuItem_child_set. To add methods of the common class you must inherit from it. An importan thing to note here is that you must inherit first from the common class, order matters. Use case: class ContentManager(models.Manager): pass class CommonFields: pub_date = models.DateTimeField() created_by = models.ForeignKey(User, related_name = 'created_by_set') last_modified_by = models.ForeignKey(User, related_name = 'last_modified_by_set') objects = ContentManager() def save(self): #do something pass class NewsItem(CommonFields, models.Model): title = models.CharField(maxlength = 100) body = models.CharField(maxlength = 200) common = ExpanderField(CommonFields) this will create a class of this form: class NewsItem(models.Model): title = models.CharField(maxlength = 100) body = models.CharField(maxlength = 200) pub_date = models.DateTimeField() created_by = models.ForeignKey(User, related_name = 'created_by_set') last_modified_by = models.ForeignKey(User, related_name = 'last_modified_by_set') objects = ContentManager() """ def __init__(self, field_container_class): self.field_container_class = field_container_class def contribute_to_class(self, cls, name): attr_list = [attr for attr in dir(self.field_container_class) if attr not in ('__doc__', '__module__', '__class__', '__dict__')] container = self.field_container_class for attr in attr_list: clone = None attr_value = getattr(container, attr) if type(attr_value) != types.MethodType: clone = deepcopy(attr_value) if (isinstance(clone, models.ForeignKey) or isinstance(clone, models.ManyToManyField)) and \ clone.rel.related_name is not None: clone.rel.related_name = cls.__name__ + '_' + clone.rel.related_name if clone is not None: cls.add_to_class(attr, clone)
More like this
- Image compression before saving the new model / work with JPG, PNG by Schleidens 2 weeks ago
- Help text hyperlinks by sa2812 1 month, 1 week ago
- Stuff by NixonDash 3 months, 2 weeks ago
- Add custom fields to the built-in Group model by jmoppel 5 months, 2 weeks ago
- Month / Year SelectDateWidget based on django SelectDateWidget by pierreben 9 months ago
Just a few improvements: - you can directly define objects = models.Manager() even if a custom manager could be useful ; - you can use a tuple instead of a list for performances issues.
Anyway, thanks for sharing this. Useful.
It's only to save you from typing, I hate to repeat the same code over and over again!!!
Great snippet, much better than my attempt.
I suggest using:
attr_list = [attr for attr in dir(self.field_container_class) if not attr.startswith('__')]
Which makes it work better with new-style classes.
I have a problem when validating. I get this:
File ".../utils.py", line 93, in contribute_to_class cls.add_to_class(attr, clone)
File "/var/lib/python-support/python2.5/django/db/models/base.py", line 89, in add_to_class value.contribute_to_class(cls, name)
File "/var/lib/python-support/python2.5/django/db/models/options.py", line 49, in contribute_to_class if self.meta:
AttributeError: 'Options' object has no attribute 'meta'
I found the same problem as ogw and the solution for me was that my "expander field" models needed to inherit from object rather than models.Models that is;
Please login first before commenting.