Re: Adding features to several models

[Eric Walstad]

On Sunday 18 December 2005 06:23 am, Ferry Dave wrote:
> Hi there,
>
> I'm new to django and like it quite much, but have troubles with some
> aspects of it.
>
> I'd like to write a base class with specific features for (almost) every
> model in my app. It should provide several features, like db-fields for
> "hidden", "show after date" with meta.admin configuration and  modified
> X_get_list-functions in views which take care of this fields values, only
> returning e.g. non-hidden objects.
>
> Of course I don't want to cut'n paste this common features into every view
> and model, especially into its meta.admin() part.
>
> Is there a django-way to do something like this?
>
> Regards,
>       Dave

Hi Dave,

Have a look at the patch attached to ticket 419:

<http://code.djangoproject.com/ticket/419>

I use it currently to achieve behaviour that is, I think, very similar to what 
you want to do.  For example, I have a base class that includes created_on, 
created_by, modified_on, modified_by fields.  I inherit from that base class 
in any model class that I want to have those fields.  I also use mix-in 
classes with my model classes.  Some of my mix-ins include get_object and 
get_list like methods that apply to more than one model.

For example, ModelBase uses the no_table patch:

class ModelBase(meta.Model):
    """Base class for all model classes.  This is a table-less base
    class containing accounting info fields including:
     - created_on: Time Stamp, the date and time the record was created
     - created_by: The id of the user that created the record
     - modified_on: The date and time that the record was last modified
     - modified_by: The id of the user that last modified the record.
    """
    modified_by = meta.ForeignKey(auth.User,\
        help_text='The user that last modified the record',
        verbose_name='modified by'
    )
    class META:
        # Explicitly set the no_table flag so that this class doesn't result
        # in a table being created.
        no_table = True
    [...]
    

class StatefulModelBase(ModelBase):
    """A base class for models that will be fed to the Finite State Machine.
    This class has methods and attributes required by the FSM.
    """
    [...]


class CustomerMixIn(object):
    """A mix-in class containing non-model-specific methods.
    The Django model class inherits from this class, thus aquiring all these
    methods.
    """
    def your_custom_get_list(self):
        [...]


class Customer(StatefulModelBase, CustomerMixIn):
    """Customer model.  Gets the accounting info fields from
    StatefulModelBase which gets them from ModelBase.
    """
    # The usual Django model stuff goes here


The patch is really simple and it has worked well for my project which has 
been in heavy development for the last few months.  I update my Django 
install regularly from svn and have never had any conflicts with the patched 
file during svn updates.

It sounds like the magic-removal stuff the core guys are working on will 
render this patch unnecessary.  In the mean time, it works well for me and my 
team and might give you the functionality you are looking for.

Good luck.  I hope this helps.

Eric.