I am finding it hard to understand some of the association defined in the code base.

class Patient < ApplicationRecord   
    belongs_to :g_district, class_name: "District", primary_key: "id", foreign_key: 'district_id', optional: true
    belongs_to :g_perm_district, class_name: "District", primary_key: "id", foreign_key: 'permanent_district_id', optional: true
    belongs_to :g_workplc_district, class_name: "District", primary_key: "id", foreign_key: 'workplace_district_id', optional: true    
end
class District
    belongs_to :province #, optional: true
    belongs_to :division, optional: true
    has_many :hospitals
    has_many :tehsils
    has_many :ucs
    has_many :mobile_users
    has_many :labs
    has_many :insecticides
end

I am not clearly getting these kind of associations defined her.(belongs_to :g_district, class_name: "District", primary_key: "id", foreign_key: 'district_id', optional: true).

In my code, there are no models like g_district, g_perm_district, g_workplc_district.

CodePudding user response：

This is simply creating an association with the same table with different names.

Here g_district, g_perm_district, g_workplc_district are associating with District model with the association names: g_dstrict with foreign_key district_id, g_perm_district with foreign_key permanent_district_id and g_workplc_district with foreign_key workplace_district_id

This basically states that Patients can have 3 districts- District, Permanent District, and Workplace District, as all these 3 are types of District, created an association with the same table with a different names.

Refer this blog for more details.

CodePudding user response：

Your code is associating the same model under different names. When in doubt you can always check this by looking at class_name: "District" which is referencing the actual class name.

In your case a Patient can have an association with three different districts (but all of them point to the District model):

3.times { District.create }
patient = Patient.create(
  g_district: District.find(1), 
  g_perm_district: District.find(2),
  g_workplc_district: District.find(3)
)

patient.g_district #=> #<District:0x00000001083ce048 id: 1,.. >
patient.g_perm_district #=> #<District:0x00000001083ce048 id: 2,.. >
patient.g_workplc_district #=> #<District:0x00000001083ce048 id: 3,.. >

It might also be worth checking the migrations or schema for Patient. The schema table might look like this:

create_table "patients", force: :cascade do |t|
    t.integer "district_id"
    t.integer "permanent_district_id"
    t.integer "workplace_district_id"
    (...other columns here...)
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
end

and the migration:

class CreatePatients < ActiveRecord::Migration[7.0]
  def change
    create_table :patients do |t|
      t.integer :district_id
      t.integer :permanent_district_id
      t.integer :workplace_district_id
      (...other columns here...)

      t.timestamps
    end
  end
end

The foreign_key: 'district_id' part (as well as the other two) suggests that these columns must exist and helps ActiveRecord properly associate the same model multiple times