#Use unscoped to break out of the default 

class Hobby < ActiveRecord::Base

    has_and_belongs_to_many :people 

    default_scope { order :name }

end 

Hobby.pluck :name

# => ["Music","Programming"]

Hobby.unscoped.pluck :name

# => ["Programming", "Music"]

#named scopes

#scope :name, lambda

scope :ordered_by_age, -> {order age: :desc}

scope :starts_with, -> (starting_string) {where("first_name LIKE ?", "#{starting_string}%")}

Person.ordered_by_age.pluck :age

# => [,,,,] 

Person.ordered_by_age.starts_with("Jo").pluck :age, :first_name

# => [[,"Josh"],[,"John"],[,"John"]]

Person.ordered_by_age.limit().starts_with("Jo").pluck :age, :first_name

# => [[,"Josh"],[,"John"]]

#:presence and :uniqueness

presence: true

# Make sure the field contains some data

uniqueness: true

# 确保只有一个data

class Job < ActiveRecord::Base

    belongs_to :person

    has_one :salary_range 

    validates :title, :company, presence: true

end 

#Other Common Validators 

:umericality

# validates numeric input 

:length

# validates value is a certain length 

:format

# validates value complies with  some regular expression format 

:inclusion

# validates value is inside specified range 

:exclusion

# validates value is out of the specified range 

class SalaryRange < ActiveRecord::Base

    belongs_to :job 

    validate :min_is_less_than_max 

    def min_is_less_than_max

        if min_salary > max_salary

            errors.add(:min_salary, "cannot be greater than maximum salary!")

        end

    end

end 

#solve for n+ queries 

Person.includes(:personal_info).all.each { |p| puts p.personal_info.weight }