rails 4 how to use foreign key and primary key - ruby-on-rails-4

Question

How to set the following association:
class Midatum < ActiveRecord::Base
# ..., diagn1, diagn2, diagn3
# sample data:
# ..., "0123", nil ,"0124"
# ..., "0123", nil ,"0124"
# ..., "0123", "1123", nil
belongs_to :icd9, :foreing_key => :diagn1
belongs_to :icd9, :foreing_key => :diagn2
belongs_to :icd9, :foreing_key => :diagn3
end
class icd9 < ActiveRecord::Base
# icd9, description
# sample data:(unique)
#"0123", "some text"
#"0124", "some other text"
#"1123", "description text"
#"1133", "description text"
has_many :midata, :foreing_key => :icd9, :primary_key => :icd9
end
This does not work. It may be obvious for someone but not for me. The database
is a legacy DB and readonly. I need to establish this assoc to able to work with the data.

Answer 1

This answer comes from a Rails expert and it does solve my problem. I am posting it in case someone else have the same problem.
belongs_to :icd9_a, :foreign_key => :diagn1, :class_name => "Icd9"
belongs_to :icd9_b, :foreign_key => :diagn2, :class_name => "Icd9"
belongs_to :icd9_c, :foreign_key => :diagn3, :class_name => "Icd9"
But that means you'll need to query the association using all three methods:
m = Midatum.first
m.icd9_a
m.icd9_b
m.icd9_c
In the same way, over in the Icd9 class you'll need three separate associations with unique names:
class Icd9 < ActiveRecord::Base
self.primary_key = :icd9
has_many :midata_a, :foreign_key => :diagn1, :class_name => "Midatum"
has_many :midata_b, :foreign_key => :diagn2, :class_name => "Midatum"
has_many :midata_c, :foreign_key => :diagn3, :class_name => "Midatum"
end
Note also that since the icd9 table doesn't have an 'id' column but uses the 'icd9' column as the primary key, you'll need to set it as I've done:
self.primary_key = :icd9