sqlobject-discuss

[SQLObject] How do I update a many-to-many field?

[Mike K. <mrm...@co...>]

Given the following simple schema:

class Author(SQLObject):
    lastName = StringCol(length=40, notNone=True)
    books = RelatedJoin('Book')  # An author can write many books

class Book(SQLObject):
    title = StringCol(length=100, notNone=True)
    authors = RelatedJoin('Author')  # A book can have many authors

I then make an Author and a Book:

author = Author("Seuss")
book = Book("The Cat In The Hat")

Now I want to give the book an author:

book.addAuthor(author)

Hmm, let's make a new author and change the book's author:

author = Author("Geissel")
book.addAuthor(author)

No, that's not right.  I just gave that book two authors.

What I want is some kind of update that will properly handle the deletion of the
previous author and addition of the new author to the magic book-to-author table
SQLObject created for me.  Or at least a way for me to delete the previous
author entries for this book, so I can add the new author entries.  So what's
the correct way to handle this type of CRUD?

Re: [SQLObject] How do I update a many-to-many field?

[Oleg B. <ph...@ma...>]

On Fri, Dec 23, 2005 at 10:23:52PM +0000, Mike Kent wrote:
> author = Author("Geissel")
> book.addAuthor(author)
> 
> No, that's not right.  I just gave that book two authors.

   Yes, you did, and that's right. This is exactly what you did (not what
you meant).

> What I want is some kind of update that will properly handle the deletion of the
> previous author and addition of the new author to the magic book-to-author table

   Do you want to replace the author? Than you don't want many-to-many
relationship.

> Or at least a way for me to delete the previous
> author entries for this book

   book.removeAuthor(author)
   author.removeBook(book)

Oleg.
-- 
     Oleg Broytmann            http://phd.pp.ru/            ph...@ph...
           Programmers don't die, they just GOSUB without RETURN.

[SQLObject] Re: How do I update a many-to-many field?

[Mike K. <mrm...@co...>]

Oleg Broytmann <phd <at> mail2.phd.pp.ru> writes:

>    Do you want to replace the author? Than you don't want many-to-many
> relationship.

Oleg, can you expand on that please?  My thinking is this: as I said in the
schema, a book can have many authors, and an author can write many books, so it
sounds like a many-to-many relationship.  Yet you still need to be able to
update a book entry, to say, correct the name of an author.  Now, SQLObject
gives you the ability to add an author to a book with the automatic handling of
the many-to-many relationship table.  Apparently it does not give you automatic
handling of the many-to-many relationship table for an update operation.  So I
was looking for the correct way to handle this myself.

Re: [SQLObject] Re: How do I update a many-to-many field?

[Oleg B. <ph...@ma...>]

On Sat, Dec 24, 2005 at 02:38:40PM +0000, Mike Kent wrote:
> My thinking is this: as I said in the
> schema, a book can have many authors, and an author can write many books, so it
> sounds like a many-to-many relationship.

   Yes, this is many-to-many. In many-to-many, you cannot replace an author
automatically. You have to remove one manually...

Oleg.
-- 
     Oleg Broytmann            http://phd.pp.ru/            ph...@ph...
           Programmers don't die, they just GOSUB without RETURN.

Re: [SQLObject] How do I update a many-to-many field?

[Steve Z. <sl...@gm...>]

> a way for me to delete the previous author entries for this book

How about "book.removeAuthor(author)"?

[SQLObject] Re: How do I update a many-to-many field?

[Mike K. <mrm...@co...>]

Thanks, Oleg and Steve.  In many readings of the SQLObject docs, I never
encountered any mention of an automatically-created 'remove' function 
that's a companion to the automatically-created 'add' function of a 
'to-many' relationship.