Transforming data when saving and reading from database

Question

Suppose I have this model (not real code):

class Message(models.Model):
    content = models.CharField(...)
    mentions = models.ManyToManyField(User)

content can for instance be:

Say hi to @Adam and @Bernard

During the Message model save(), "Adam" and "Bernard" are extracted from the text, and their respective user instances added to mentions, so that

 >>> m = Message.objects.create(content="Say hi to @Adam and @Bernard")
 >>> m.mentions.all()
 >>> [User: Adam, User: Bernard]

Now, as to my problem:

How do I cope with Bernard performing a sex-change and renaming himself Bernarda? I would very much like the content then to read Say hi to @Adam and @Bernarda .

Basically, I've tried a strategy of replacing names with ID's when saving ( Say hi to @Adam and @Bernard then becomes Say hi to @1 and @2 , ie their User.pk's). This works fine when saving, and OK-ish when reading.

Doing this replacement in the model's save() method works fine, but read and lookup operations are a major problem. I cannot find an easy way of making sure that what is stored in the database (ie Say hi to @1 and @2 ) 'magically' becomes Say hi to @Adam and @Bernard whenever it is being queried (be that through .filter() and related operations) or its instance being evaluated.

I'm unsure whether I'm on the right track here; any pointers or ideas to doing this differently are very welcome!

Answer 1

1. Write a custom model field class , that extends CharField, you should overread the documentation first so that you have an idea of what's there,

2. that metaclass = models.SubfieldBase is required for the custom Field to call the custom to_python method

3. Override to_python() , it takes a database value and should return a python value, there you can replace @1 by the username of User #1 (say @Adam or @Bernarda),

4. Override get_prep_value() , it takes a python value and should return a value for the SQL, there you can replace @Adam by the pk of User Admin (say @1),

5. Query on mentions , for example Message.objects.filter(mention=someuser) , or make another field: one that contains the replaced version and one that contains the possibly out of date usernames - i don't recommend the latter

6. You could also query on content , for example Message.objects.filter(content__contains='@1')

Voodoo doesn't do any good in general , i would make a model method get_content_display() that does the replacement, and a templatetag that displays the html version of the content - since your probably want to link @Adam to his user page anyway. I would stick to this because that's KISS .

CREDITS to LaundroMat for debugging this answer and fixing it (there was 2 errors and 1 inaccuracy, which he debugged and came back to report !!!). Also DO UPVOTE HIS COMMENT unless you have better way to reward him. Rock'on.