### Dynamic Permissions

```User < ActiRecord::Base
  has_many :roles
  has_many :permission, through: :roles
  has_many :user_roles
  has_many :custom_permissions
end

UserRole < Base
  belongs_to :user
  belongs_to :role
end

Role < Base
  has_many :permissions
  has_many :users
end

RolePermission < Base
  belongs_to :role
  belongs_to :permission
end

CustomPermission < Base
  belongs_to :user
  belongs_to :permission
end

Permission < Base
  has_many :roles
end

user.roles = [role1, role2]
user.save

user.roles << new_role
```

### Cache class
```
class Cache
  @@storage = {}

  def self.set(key,value)
    @@storage[key] = StoredObject.new(value, 1.day.from_now)
  end
  def self.get(key)
    element = @@storage[key]
    if !element.nil?
      if element.expired?
        @@storage.delete(key)
       return nil
      else
       return element.value
      end
    end
  end
  def self.size
    @@storage.size
  end

  def self.delete(key)
    @@storage.delete(key)
  end
end

class StoredObject
  initialize(value, expiration_date)
    @value = value
    @expiration_date = expiration_date
  end

  private

  def expired?
    expiration_date < Date.today
  end

  attr_reader :value, expiration_date
end

```

### MySQL
```SELECT AVG(e.salary), p.Department as average_salary from Project p left join Members m on m.project_id = p.project_id left join Employee e on e.id = m.employee_id group by p.Department, e.id;```