I'm trying to migrate a desktop application to rails (also dealing with quite old fashioned existing database). The problem is that I don't have a unique ID in one column, but it's three columns of a table that guarantee uniqueness of a record.
Given I have three tables:
authors
author_name,
author_letter,
author_nr1,
author_nr2
...
titles
titel_nr,
titel_name,
...
author_titles
titel_nr,
author_letter,
author_nr1,
author_nr2
The "primary key" of authors consists of author_letter, author_nr1, author_nr2 here.
So do I need sort of a multicolumn primary key here to have rails associations working? Or am I going in the wrong direction here?
No. The Primary Key is (like rails default) the ID of the Record.
In addition you can set unique Keys like
add_index :users, [:merchant_id, :email], unique: true
add_index :users, [:merchant_id, :login], unique: true
This potects your database. To catch the uniqueness in Rails you need to write into your model:
validates_uniqueness_of :email, scope: :merchant_id
validates_uniqueness_of :login, scope: :merchant_id
There exists a gem called composite_primary_keys that will allow to build a primary key using multiple columns.
So, yes, you can use multicolumn primary key.
But, if you are able to change the datamodel (which is not always the case), I would propose to add a column ID to each table, as this will make your life easier (and is also much more performant).
[EDIT]
Your class definition with composite_primary_keys will look like this
class Author
set_primary_keys :author_letter, :author_nr1, :author_nr2
has_many :titles, :through => :author_title
end
class Title
set_primary_key :title_nr
end
class AuthorTitle
belongs_to :title, :foreign_key => :title_nr
belongs_to :authori, :foreign_key => [:author_letter, :author_nr1, :author_nr2]
end
Hope this helps.
As many people said: "if you fight Rails, it'll strike back".
Really try to avoid such situations, It's pain with rails, if you don't have a clean datamodel.
