I am attempting to migrate a desktop application to rails (also coping with quite traditional existing database). However , I do not possess a unique ID in a single column, but it is three posts of the table that guarantee originality of the record.

Given I've 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 includes author_letter, author_nr1, author_nr2 here.

So I want kind of a multicolumn primary key here to possess rails associations working? Or shall we be held pointed in the wrong direction here?

unsure basically got your question right:

no. The Main Secret is (like rails default) the ID from the Record.

additionally you are able to set unique Secrets like

    add_index :users, [:merchant_id, :email], :unique => true
    add_index :users, [:merchant_id, :login], :unique => true

this potects your database. to trap this in rails you have to write inside your model:

  validates_uniqueness_of :email,    :scope => :merchant_id
  validates_uniqueness_of :login,    :scope => :merchant_id

There is available a jewel known as composite_primary_keys that will permit to construct a principal key using multiple posts.

So, yes, you should use multicolumn primary key.

But, if you can to alter the datamodel (which isn't always the situation), I'd offer give a column ID to every table, as this makes your existence simpler (and it is a lot more performant).

[EDIT]

Your class definition with composite_primary_secrets may be 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 can help.