rails polymorphic association (legacy database)

Question

I am using a legacy database, so i do not have any control over the datamodel. They use a lot of polymorphic link/join-tables, like this

create table person(per_ident, name, ...)

create table person_links(per_ident, obj_name, obj_r_ident)

create table report(rep_ident, name, ...)

where obj_name is the table-name, and obj_r_ident is the identifier. So linked reports would be inserted as follows:

insert into person(1, ...)
insert into report(1, ...)
insert into report(2, ...)

insert into person_links(1, 'REPORT', 1)
insert into person_links(1, 'REPORT', 2)

And then person 1 would have 2 linked reports, 1 and 2.

I can understand possible benefits having a datamodel like this, but i mostly see one big shortcoming: using constraints is not possible to ensure data integrity. But alas, i cannot change this anymore.

But to use this in Rails, i was looking at polymorphic associations but did not find a nice way to solve this (since i cannot change the columns-names, and did not readily find a way to do that).

I did come up with a solution though. Please provide suggestions.

class Person < ActiveRecord::Base

  set_primary_key "per_ident"
  set_table_name "person"
  has_and_belongs_to_many :reports,
                         :join_table => "person_links",
                         :foreign_key => "per_ident",
                         :association_foreign_key => "obj_r_ident",
                         :conditions => "OBJ_NAME='REPORT'"
end

class Report < ActiveRecord::Base

  set_primary_key "rep_ident"
  set_table_name "report"
  has_and_belongs_to_many :persons,
                     :join_table => "person_links",
                     :foreign_key => "obj_r_ident",
                     :association_foreign_key => "per_ident",
                     :conditions => "OBJ_NAME='REPORT'"
end

This works, but i wonder if there would be a better solution, using polymorphic associations.

Answer 1

At least as of Rails 4.2.1, you can pass foreign_type to a belongs_to declaration to specify the name of the column to be used for the 'type' of the polymorphic association

http://apidock.com/rails/v4.2.1/ActiveRecord/Associations/ClassMethods/belongs_to

Answer 2

You can override the column names, sure, but a quick scan of the Rails API didn't show me anywhere to override the polymorphic 'type' column. So, you wouldn't be able to set that to 'obj_name'.

It's ugly, but I think you'll need a HABTM for each type of object in your table.

You might be able to do something like this:

{:report => 'REPORT'}.each do |sym, text|
  has_and_belongs_to_many sym,
    :join_table => "person_links",
    :foreign_key => "obj_r_ident",
    :association_foreign_key => "per_ident",
    :conditions => "OBJ_NAME='#{text}'"
end

At least that way all the common stuff stays DRY and you can easily add more relationships.