seeds.rb

ActiveRecord::Base.establish_connection adapter: 'sqlite3',
  database: ':memory:'

ActiveRecord::Migration.verbose = false
ActiveRecord::Schema.define(:version => 1) do
  create_table :employees do |t|
    t.string :first_name
    t.string :last_name
    t.integer :age
  end

  create_table :positions do |t|
    t.integer :employee_id
    t.integer :department_id
    t.string :title
  end

  create_table :departments do |t|
    t.string :name
  end
end

class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true
end

class Employee < ApplicationRecord
  has_many :positions
end

class Position < ApplicationRecord
  belongs_to :employee
  belongs_to :department
end

class Department < ApplicationRecord
  has_many :positions
end

e = Employee.create!(first_name: 'Walter', last_name: 'White', age: 50)
d = Department.create!(name: 'Product')
Position.create!(title: 'Cook', department: d, employee: e)

e = Employee.create!(first_name: 'Jesse', last_name: 'Pinkman', age: 23)
Position.create!(title: 'Junior Cook', department: d, employee: e)

d = Department.create!(name: 'Legal')
e = Employee.create!(first_name: 'Saul', last_name: 'Goodman', age: 47)
Position.create!(title: 'Lawyer', department: d, employee: e)