20180501090936_create_doctors.rb

class CreateDoctors < ActiveRecord::Migration[5.1]
  def change
    create_table :doctors do |t|
      t.string :first_name
      t.string :last_name
      t.timestamps
    end
  end
end

20180501090940_create_interns.rb

class CreateInterns < ActiveRecord::Migration[5.1]
  def change
    create_table :interns do |t|
      t.string :first_name
      t.string :last_name
      t.references :doctor, index: true
      t.timestamps
    end
  end
end

20180501093211_create_patients.rb

class CreatePatients < ActiveRecord::Migration[5.1]
  def change
    create_table :patients do |t|
      t.string :first_name
      t.string :last_name
      t.timestamps
    end
  end
end

20180501093215_create_consultations.rb

class CreateConsultations < ActiveRecord::Migration[5.1]
  def change
    create_table :consultations do |t|
      t.references :doctor, index: true
      t.references :patient, index: true
      t.timestamps
    end
  end
end

consultation.rb

class Consultation < ActiveRecord::Base
  belongs_to :patient
  belongs_to :doctor
end

doctor.rb

class Doctor < ActiveRecord::Base
  has_many :interns
  has_many :consultations
  has_many :patients, through: :consultations

  validates :last_name, presence: true
end

intern.rb

class Intern < ActiveRecord::Base
  belongs_to :doctor
end

patient.rb

class Patient < ActiveRecord::Base
  has_many :consultations
  has_many :doctors, through: :consultations
end