class Child < ApplicationRecord
  attr_readonly :mother_id
  belongs_to :mother
end

class Mother < ApplicationRecord
  has_many :children
end

2.4.1 :001 > c = Child.new
 => #<Child id: nil, mother_id: nil, created_at: nil, updated_at: nil, name: nil> 
2.4.1 :002 > m = Mother.new
 => #<Mother id: nil, created_at: nil, updated_at: nil> 
2.4.1 :003 > c.mother = m
 => #<Mother id: nil, created_at: nil, updated_at: nil> 
2.4.1 :004 > c.save
   (0.1ms)  begin transaction
  SQL (0.4ms)  INSERT INTO "mothers" ("created_at", "updated_at") VALUES (?, ?)  [["created_at", "2017-06-30 05:47:37.580014"], ["updated_at", "2017-06-30 05:47:37.580014"]]
  SQL (0.2ms)  INSERT INTO "children" ("mother_id", "created_at", "updated_at") VALUES (?, ?, ?)  [["mother_id", 9], ["created_at", "2017-06-30 05:47:37.589372"], ["updated_at", "2017-06-30 05:47:37.589372"]]
   (1.4ms)  commit transaction
 => true 
2.4.1 :005 > c.id
 => 6 
2.4.1 :006 > m.id
 => 9 
2.4.1 :007 > m2 = Mother.new
 => #<Mother id: nil, created_at: nil, updated_at: nil> 
2.4.1 :008 > m2.save
   (0.1ms)  begin transaction
  SQL (2.4ms)  INSERT INTO "mothers" ("created_at", "updated_at") VALUES (?, ?)  [["created_at", "2017-06-30 05:47:46.046904"], ["updated_at", "2017-06-30 05:47:46.046904"]]
   (1.4ms)  commit transaction
 => true 
2.4.1 :009 > m2.id
 => 10 
2.4.1 :010 > c.mother = m2
 => #<Mother id: 10, created_at: "2017-06-30 05:47:46", updated_at: "2017-06-30 05:47:46"> 
2.4.1 :012 > c.save
   (0.1ms)  begin transaction
  SQL (0.5ms)  UPDATE "children" SET "mother_id" = ?, "updated_at" = ? WHERE "children"."id" = ?  [["mother_id", 10], ["updated_at", "2017-06-30 05:47:54.513353"], ["id", 6]]
   (1.5ms)  commit transaction
 => true 
2.4.1 :013 > c.reload
  Child Load (0.3ms)  SELECT  "children".* FROM "children" WHERE "children"."id" = ? LIMIT ?  [["id", 6], ["LIMIT", 1]]
 => #<Child id: 6, mother_id: 10, created_at: "2017-06-30 05:47:37", updated_at: "2017-06-30 05:47:54", name: nil> 
2.4.1 :014 > c.mother.id
  Mother Load (0.3ms)  SELECT  "mothers".* FROM "mothers" WHERE "mothers"."id" = ? LIMIT ?  [["id", 10], ["LIMIT", 1]]
 => 10