Django Multiple inheritance with different tables

Question

I tried to create some models. I actually looked at some examples but most of them don't work. Example:

class Piece(models.Model):
    name =  models.CharField(max_length=100)    
    class Meta:
        abstract = True

class Article(Piece):
    pass        

class Book(Piece):
    pass

class Monday(Book, Article):
    pass    
class Tuesday(Book, Article):
    pass

So my goal is to get the value with something like this -> Monday.Article.name. I want to have different tables for every weekday, containing the article's name. This is the error I get:

ERRORS:
Testing.Monday: (models.E005) The field 'id' from parent model 'Testing.book' clashes with the field 'id' from parent model 'Testing.article'.
Testing.Monday: (models.E005) The field 'name' from parent model 'Testing.book' clashes with the field 'name' from parent model 'Testing.article'.
Testing.Tuesday: (models.E005) The field 'id' from parent model 'Testing.book' clashes with the field 'id' from parent model 'Testing.article'.
Testing.Tuesday: (models.E005) The field 'name' from parent model 'Testing.book' clashes with the field 'name' from parent model 'Testing.article'.

It looks like my Article and Book are using the same name. How does this work?

EDIT: Using this :

class Article(Piece):
   article_id = models.AutoField(primary_key=True)

class Book(Piece):
   book_id = models.AutoField(primary_key=True)

Will result in this error:

Testing.Monday: (models.E005) The field 'piece_ptr' from parent model 
'Testing.book' clashes with the field 'piece_ptr' from parent model 
'Testing.article'.

The only solution is this:article_to_piece = models.OneToOneField(Piece, parent_link=True)But this will not generate a linking between the days and a book/article. I can only add a name to the day.

This is a really bad idea. Just use a single table with a "day" field. — Daniel Roseman
– Daniel Roseman, Commented May 15, 2017 at 21:15
Well I guess i have to do it your way! Can you explain why my approach is bad? Aside from the fact that it doesn't work — ExceptionFinder
– ExceptionFinder, Commented May 15, 2017 at 21:36

Exprator · Accepted Answer · 2017-05-17 03:39:57Z

5

the problem is because the primary field for both the models are id, and a table cannot have 2 columns based on same name, do this and it will solve your error

class Article(Piece):
    id = models.AutoField(primary_key=True)
    name= models.CharField(max_length=100)

class Book(Piece):
    book_id = models.AutoField(primary_key=True)
    book_name= models.CharField(max_length=100)

edited May 17, 2017 at 3:39

answered May 16, 2017 at 4:02

Exprator

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1 Comment

ExceptionFinder Over a year ago

it's not actually what i hoped for but it might work for others!

CataldiR · Accepted Answer · 2019-10-27 21:03:57Z

0

You should try calling init on subclasses, like:

class Monday(Book, Article):
    id = models.AutoField(primary_key=True)
    name= models.CharField(max_length=100)

    def __init__(self, *args, **kwargs):
        return super().__init__(*args, **kwargs)


class OtherDay(Book, Article):
    book_id = models.AutoField(primary_key=True)
    book_name= models.CharField(max_length=100)

    def __init__(self, *args, **kwargs):
        return super().__init__(*args, **kwargs)

This is because, as a Python class, the super() method will call the superclasses definition only one time, while the simple inheritance would call ini() for each superclass.

answered Oct 27, 2019 at 21:03

CataldiR

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1 Comment

Piranna Over a year ago

Can you give a reference for this? Never seen the behaviour you describes...

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