This document discusses database modeling for a system with companies, departments, and employees. It describes setting up composite primary keys to link the three tables together and using associations and joins to efficiently query across the tables. Methods are defined on the Company model to count employees. A view is created to select large companies with over 1000 employees.

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11. Po w ere d by vs

17. class Employee < ActiveRecord::Base
validates_presence_of :name
CREATE TABLE employees (
name VARCHAR(40) NOT NULL,

18. class Employee < ActiveRecord::Base
validates_length_of :login,
:minimum => 5
CREATE TABLE employees (
login VARCHAR(10)
check(length(login) > 4),

19. begin
emp = Employee.new(:login => '')
emp.save_with_validation(false)
fail
rescue ActveRecord::StatementInvalid
end

20. Departments Employees

21. class Employee < ActiveRecord::Base
belongs_to :department
validates_presence_of :department
CREATE TABLE employees (
department_id INTEGER NOT NULL
REFERENCES departments(id),

24. Companies Departments Employees

25. has_many :though
company.employees.size
SELECT count(*) AS count_all
FROM employees
INNER JOIN departments
ON employees.department_id =
departments.id
WHERE departments.company_id = 1

26. SELECT count(*) AS count_all
FROM employees
WHERE company_id = 1
Companies
?
Departments Employees

27. company_id department_id
Companies Departments Employees
id id id
company_id company_id
department_id

28. Companies Departments Employees
id company_id id
department_id company_id
department_id

29. departments
CREATE TABLE departments(
company_id INTEGER NOT NULL
REFERENCES companies(id),
department_id SERIAL NOT NULL,
PRIMARY KEY (company_id,
department_id)

30. employees
CREATE TABLE employees(
company_id INTEGER NOT NULL
REFERENCES companies(id),
department_id INTEGER NOT NULL,
FOREIGN KEY
(company_id, department_id)
REFERENCES department
(company_id, department_id)

31. departments company_id
department_id employees
Companies Departments Employees
id company_id id
department_id company_id
department_id

32. # gem install composite_primary_keys

33. Department
class Department < ActiveRecord::Base
set_primary_keys :company_id,
:department_id
has_many :employees,
:foreign_keys =>
[:company_id,
:department_id]

34. Company
class Company < ActiveRecord::Base
def employees_size
Employee.count(
:conditions =>
['company_id = ?', id])
end

38. CREATE VIEW big_campanies AS
SELECT * FROM companies WHERE id IN
(SELECT company_id FROM
(SELECT compay_id, count(id) AS c
FROM employees
GROUP BY company_id) AS t
WEHRE c > 1000)

39. BigCampany
class BigCampany < ActiveRecord::Base
has_many :department
end

40. SELECT * FROM (SELCT * FROM ...)