RoR - More Active Record

Active Record Scope:

default_scope:   默认的检索方式

#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"]]

Scope 总是返回ActiveRecord::Relation

Validations:

数据总是要经过Validations才能存在数据库之中

#: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 

Writing your own validation:

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 

N+1 Queries Issue and DB Transactions:

#solve for n+ queries 

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