Add basic functionality

app/models/admission.rb

@@ -1,4 +1,24 @@
 class Admission < ApplicationRecord
-  belongs_to :country
-  belongs_to :person
+  belongs_to :country, inverse_of: :admissions
+  belongs_to :person, inverse_of: :admissions
+  belongs_to :previous, class_name: name, inverse_of: :next, optional: true
+  has_one :next, class_name: name, foreign_key: :previous_id, inverse_of: :previous
+
+  scope :ordered, -> { order(:arrived_on, :previous_id) }
+  scope :before, -> (date) { where("#{table_name}.arrived_on <= ?", date) }
+  scope :after, -> (date) { where("#{table_name}.arrived_on >= ?", date) }
+  
+  around_create :set_previous
+  before_destroy :update_nexts_previous
+
+  def set_previous
+    self.previous ||= person.admissions.ordered.before(arrived_on).last
+    future_next = previous&.reload_next
+    yield
+    future_next&.update(previous: self)
+  end
+
+  def update_nexts_previous
+    self.next&.update(previous: previous)
+  end
 end

app/models/country.rb

@@ -1,2 +1,4 @@
 class Country < ApplicationRecord
+  has_many :admissions, inverse_of: :country
+  has_many :people, foreign_key: :birth_country_id, inverse_of: :birth_country
 end

app/models/person.rb

@@ -1,2 +1,18 @@
 class Person < ApplicationRecord
+  belongs_to :birth_country, class_name: Country.name
+  has_many :admissions
+
+  def stays
+    stays = []
+    admissions.ordered.includes(:country).all.each_cons(2) do |a, b|
+      stays.push(
+        country: a.country,
+        arrival: a.arrived_on,
+        departure: b.arrived_on,
+        duration: (b.arrived_on - a.arrived_on).to_i + 1
+      )
+    end
+
+    stays
+  end
 end