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Beginner's question on has_and_belongs_to_many

Answer 1

+1 A:

You want something like:

Section
has_and_belongs_to_many :blogs
has_many :posts, :through => :blogs

I'm not certain the syntax is exactly correct, but the through attribute is what you are looking for.

macabail 2010-02-03 18:23:22

Sadly this raises the exception:Mysql::Error: Unknown column 'blogs.section_id' in 'where clause': SELECT count(*) AS count_all FROM `posts` INNER JOIN `blogs` ON `posts`.blog_id = `blogs`.id WHERE ((`blogs`.section_id = 1))

Erik 2010-02-03 18:37:29

@Erik - You have to set up your models/tables slightly differently when using has many through as opposed to has and belongs to many. Check out the docs: http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html

Jimmy Cuadra 2010-02-03 18:42:09

@Jimmy Cuadra: Do you mean I should abandon the has_and_belongs_to_many in favor for has_many :through?

Erik 2010-02-03 18:47:23

@jimmy, thanks for posting the doc link, should help clear that up.

macabail 2010-02-03 18:47:39

@erik, it depends; are blogs and sections really co-dependent, or is it that blogs belong to a section, and a section has_many blogs ?

macabail 2010-02-03 18:49:14

@Erik - Yes, you use one or the other. The basic rule of thumb is to always use has many through unless you're sure you will never need to work with the join model directly.

Jimmy Cuadra 2010-02-03 18:51:03

@macabail: A section can have many blogs and a blog can also have many sections, so as I see it there must be some kind of join table.

Erik 2010-02-03 18:51:11

@Jimmy Cuadra: Ok, thank you, I'll give it a go!

Erik 2010-02-03 18:51:38

@Erik: did that work out for you?

macabail 2010-02-04 23:15:55

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