Forum: Ruby on Rails how to design database: has_and_belongs_to_many

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1a9637acaccba514bf6c2cecf80c3f16?d=identicon&s=25 LeonB (Guest)
on 2007-03-05 22:36
(Received via mailing list)
I'm not sure how to design my database. This is the casus:

There are users who can make declarations. And there are users who are
coupled to the declaration. They are the ones who have to pay for the
declaration. So the first users did pay for something in a store.
Enters it in the system and add other users to the declaration. The
costs are splitted among the coupled users.

For example: user A bought a crate of Beer for €9. His roommates (B
and C) are going to drink too and the all decide to share the costs. A
enters the crate in the system and everyone has to pay €3.

This is an existing application but I'm going to remake it in Ruby. (A
part of) the current structure is like this:

Table: declarations

Table: declarations_users

Is it possible to have the user_id in the declarations table and in
the many_to_many table? And does get this? Do I have to make an
has_and_belongs_to_many declaration AND an belongs_to?Or just one?

Thanks in advance!
2017657725dd1bce83dc8a1e2e991d04?d=identicon&s=25 Luke Ivers (Guest)
on 2007-03-05 22:55
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You can just... do what it is you're trying to do.

class User
  has_and_belongs_to_many :declarations
  has_many :my_declarations, :class_name => 'Declarations'

class Declaration
  has_and_belongs_to_many :users
  belongs_to :owning_user, :class_name => 'User'

Now you refer to declaration.users to find all the users that belong to
You refer to declaration.owning_user to find out the original owner

You refer to user.declarations to find all the declarations that belong
to a
You refer to user.my_declarations to find all the declarations that are
owned by that user

This should work given the table descriptions in your email.
1a9637acaccba514bf6c2cecf80c3f16?d=identicon&s=25 LeonB (Guest)
on 2007-03-05 23:04
(Received via mailing list)
Ah thanks! I think this is exactly what I want.

I'm starting with Ruby on Rails, so maybe a dumb question:
my_declarations and owning_user is an alias? So the db table field
(for example) is still called "user_id"?
2017657725dd1bce83dc8a1e2e991d04?d=identicon&s=25 Luke Ivers (Guest)
on 2007-03-05 23:25
(Received via mailing list)
Right... basically, the first part of declaring an association is
telling it
what name to give it in the class...
If they didn't set up default values for you, then EVERY declaration
look like
has_many :users, :class_name => 'User', foreign_key => 'user_id'
But that's the whole "convention over configuration" thing they talk
in the Rails community... since the chances are that 99% of the time,
are going to be right, it just has them set as defaults.

But, you can ALWAYS specify them, if you feel the need to, and
especially if
you're needing to associations to the same object... you just use
first arguments, and then specify class_name and foreign_key if you have

You're welcome... and there are no dumb questions: just relative levels
1a9637acaccba514bf6c2cecf80c3f16?d=identicon&s=25 LeonB (Guest)
on 2007-03-05 23:37
(Received via mailing list)
Thanks again for the nice reply. Is everyone that nice at the email

With this line:
has_many :users, :class_name => 'User', foreign_key => 'user_id'
you can access the users with the instance variable @users in the

I have created this table (for deposits from one user to another to
pay for depts).

class CreateDeposits < ActiveRecord::Migration
  def self.up
    transaction do
      create_table :deposits do |table|
        table.column :from_user_id, :integer
        table.column :to_user_id, :integer
        table.column :amount, :integer
        table.column :confirmed, :boolean
        table.column :created_at, :datetime
        table.column :updated_at, :datetime

  def self.down
    drop_table :deposits

with this line the logic would be complete if I understand it all
belongs_to :payer, :class_name => 'User', foreign_key =>
belongs_to :casher, :class_name => 'User', foreign_key =>

Where did you get this knowledge? By reading the API?
2017657725dd1bce83dc8a1e2e991d04?d=identicon&s=25 Luke Ivers (Guest)
on 2007-03-06 01:52
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On 3/5/07, LeonB <> wrote:
> Thanks again for the nice reply. Is everyone that nice at the email
> lists?

The Rails mailing list is surprisingly polite and helpful.

With this line:
>       create_table :deposits do |table|
>     drop_table :deposits
>   end
> end
> with this line the logic would be complete if I understand it all
> corectly?
> belongs_to :payer, :class_name => 'User', foreign_key =>
> 'from_user_id'
> belongs_to :casher, :class_name => 'User', foreign_key =>
> 'from_user_id'

I believe you intended one of those to be 'to_user_id' as opposed to
'from_user_id', but other than that, yes it's correct.

Where did you get this knowledge? By reading the API?

I got it just like this: trying to build something, not understanding,
either asking questions, or looking up the answer to specific questions
the API... just generally trying to read the API or really any other
of information without a specific example that you're working towards
usually only get you 1/4 of the way to understanding what you're
actually using the information is what makes you really understand. :)
1a9637acaccba514bf6c2cecf80c3f16?d=identicon&s=25 LeonB (Guest)
on 2007-03-07 00:15
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Ah, thanks.
I'm gonna try to make this work tomorrow. With a bit of luck I've got
a working application some weeks from now.
1a9637acaccba514bf6c2cecf80c3f16?d=identicon&s=25 LeonB (Guest)
on 2007-03-08 00:12
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It just don't get it. I thought the extra relationships would auto-
generate some extra variables.
So let's say I do "declarations/show/1" and I would like to see all
the coupled users (from declarations_users). How do I access them in
the view? Do I have to edit some additional controller code?

1a9637acaccba514bf6c2cecf80c3f16?d=identicon&s=25 LeonB (Guest)
on 2007-03-08 00:14
(Received via mailing list)
Ahhhhh.... never mind. I got it! I read this sourcecode and found out
@declaration.users would work. And it dit!
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