Forum: Ruby on Rails HABTM - primary key problem

Hi everyone,

I ran into a serious problem when I was trying to do something similar
to twitter follow mechanism. The thing is like this:

First, I have a self-referential table, named "users"
  create_table :users do |t|
      t.column :name, :string
      t.column :user_name, :string
      t.column :password, :string
      t.column :email, :string
  end

I also have a HABTM join table, named "follow_relations"
  create_table :follow_realtions do |t|
      t.column :a_id, :integer  # a_id follows b_id
      t.column :b_id, :integer
  end

My HABTM join is defined as follows
   has_and_belongs_to_many :followers,
                                                 :class_name =>
"User",
                                                 :join_table =>
"follow_relations",
                                                 :foreign_key =>
"b_id",
                                                 :association_foreign_key
=> "a_id"

  has_and_belongs_to_many :followings,
                                                :class_name => "User",
                                                :join_table =>
"follow_relations",
                                                :foreign_key =>
"a_id",
                                                :association_foreign_key
=> "b_id"

When I executed:
    gaoxh = User.create(:name => "gaoxh")
    micai = User.create(:name => "micai")
    xiexin = User.create(:name => "xiexin")
    gaoxh.followers << micai
    xiexin.followers << micai

The first << operation executes the following SQL :
INSERT INTO `follow_relations (`id`, `b_id`, `a_id`) VALUES (3, 1, 3)
The second << operation executes the following SQL:
INSERT INTO `follow_relations (`id`, `b_id`, `a_id`) VALUES (3, 2, 3)
Thus, key id is duplicated. It looks like that every time, the value
of column id is copied from column a_id.
What's going on here? How to fix this problem?

Thanks in advance,
xiahong

Xiahong Gao wrote:
> Hi everyone,
> 
> I ran into a serious problem when I was trying to do something similar
> to twitter follow mechanism. The thing is like this:
> 
> First, I have a self-referential table, named "users"
>   create_table :users do |t|
>       t.column :name, :string
>       t.column :user_name, :string
>       t.column :password, :string
>       t.column :email, :string
>   end
> 
> I also have a HABTM join table, named "follow_relations"
>   create_table :follow_realtions do |t|
>       t.column :a_id, :integer  # a_id follows b_id
>       t.column :b_id, :integer
>   end
> 
> My HABTM join is defined as follows
>    has_and_belongs_to_many :followers,
>                                                  :class_name =>
> "User",
>                                                  :join_table =>
> "follow_relations",
>                                                  :foreign_key =>
> "b_id",
>                                                  :association_foreign_key
> => "a_id"
> 
>   has_and_belongs_to_many :followings,
>                                                 :class_name => "User",
>                                                 :join_table =>
> "follow_relations",
>                                                 :foreign_key =>
> "a_id",
>                                                 :association_foreign_key
> => "b_id"
[...]
> The first << operation executes the following SQL :
> INSERT INTO `follow_relations (`id`, `b_id`, `a_id`) VALUES (3, 1, 3)
> The second << operation executes the following SQL:
> INSERT INTO `follow_relations (`id`, `b_id`, `a_id`) VALUES (3, 2, 3)
> Thus, key id is duplicated. It looks like that every time, the value
> of column id is copied from column a_id.
> What's going on here? How to fix this problem?

If you're using HABTM, the join table (follow_relations) should not have 
an id column.  Remove it and see if that does the trick.

> 
> Thanks in advance,
> xiahong

Best,
--
Marnen Laibow-Koser
http://www.marnen.org
marnen@marnen.org

If I remove id column, what is the primary key?

2009/7/3 Marnen Laibow-Koser <rails-mailing-list@andreas-s.net>

2009/7/3 gaoxh gaoxh04 <gaoxh04@gmail.com>:
> If I remove id column, what is the primary key?

Have a look at the rails guides, particularly the one on ActiveRecord
associations at http://guides.rubyonrails.org/index.html

Colin

thanks all... problem solved.

2009/7/3 Colin Law <clanlaw@googlemail.com>