CODEWITHSUNDEEP
ruby-on-railsrubyruby-on-rails-3activerecordhas-and-belongs-to-many
datalost
datalost

Reputation: 3773

Rails3 has_and_belongs_to_many - error on getting an attribute from join table

Given that I have following structure:

class A < ActiveRecord::Base
  has_and_belongs_to_many :bs, :class_name => "B", :join_table => "ab"
end

class AB < ActiveRecord::Base
  #ab has a date column "creation_date"
  #ab also has a text column "creatior"
end

class B < ActiveRecord::Base
end

I successfully retrieve "creation_date" attribute in rails console 

console> A.find(1).bs.first.creation_date
        => "14/08/1874"

In controller 

@a = A.find(1)
@bs = a.bs

But using it in a view (partial), I got following error 

bs.each do |b|
  b.b_attribute #this is O.K.
  b.creation_date # cause error =>    undefined method `creation_date` for #<B:...>
end

 # also try to debug in partial
 A.find(1).bs.first.creation_date #=> this return data correctly

The problem as shown above, what can possibly cause undefined method whilst the direct attributes are still accessible.

Anyone have any idea what is wrong with my code?

Upvotes: 1

Views: 535

Answers (1)

anon
anon

Reputation:

When you're dealing with an intermediate model, you shouldn't use has_and_belongs_to_many. That method works only when the join table is not a model. You need to make a clear distinction between the concepts of a model and a table. In rails, you rarely have access to the underlying table, most often you deal with the wrapping models.

If your join table has anything more than the foreign keys to A and B, and you need to access that additional data, then it needs to be a model. In your case, it is, but you're not using the correct relation. It should look like this:

class A < ActiveRecord::Base
  has_many :abs
  has_many :bs, :through => :abs
end

class AB < ActiveRecord::Base
end

class B < ActiveRecord::Base
end

Afterwards, accessing creation_date and creator should be done through the AB model, since it really is an attribute that belongs to it.

Take a look here for a quick explanation with examples: http://guides.rubyonrails.org/association_basics.html#choosing-between-has_many-through-and-has_and_belongs_to_many

Upvotes: 2

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