rails g model personal_info height:float weight:float person:references 

build_personal_info(hash)
#=> 在数据库中创建record

create_personal_info(hash)
#=> 在memory中创建record

#=> Both remove the previous reference in the DB

rails g model job title company position_id person:references

#in model 

class Person < ActiveRecord::Base

    has_one :personal_info

    has_many :jobs

end 

class Job < ActiveRecord::Base

    belongs_to :person

end 

person.jobs = jobs

# 用新的array替换现有的jobs

person.jobs << job(s)

# 添加新的jobs

person.jobs.clear

# 将foreign key 设置为NULL， 解除这个person和jobs之间的关系

create!   where!

#加了！才会提醒是否成功

# :dependent

class Person < ActiveRecord::Base

    has_one :personal_info, dependent: :destroy

 end 

# 这样删除person的时候同时会删除跟person相关的personal_info

#join table 

rails g model hobby name 

rails g migration create_hobbies_people person:references hobby:references

#生成Migration

class CreateHobbiesPeople < ActiveRecord::Migration

    def change

        create_table :hobbies_people, id: false do |t|

            t.references :person, index: true, foreign_key: true

            t.references :hobby, index:true, foreign_key: true

        end

    end

end 

#model

class Person < ActiveRecord::Base

    has_and_belongs_to_many :hobbies

end 

class Hobby < ActiveRecord::Base

    has_and_belongs_to_many :people

end

#SalaryRange Model and Migration 

rails g model salary_range min_salary:float max_salary:float job:references 

class CreateSalaryRanges < ActiveRecord::Migration

    def change

        create_table :salary_ranges do |t|

            t.float :min_salary

            t.float :max_salary

            t.references :job,  index: true, foreign_key: true 

            t.timestamps null: false

        end

    end

 end

#model 

class Person < ActiveRecord::Base

    has_many :jobs

    has_many :approx_salaries, through: :jobs, source: :salary_range

    def max_salary

        approx_salaries.maximum(:max_salary)

    end

end

class Job < ActiveRecord::Base

    belongs_to :person

    has_one :salary_range

end 

class SalaryRange < ActiveRecord::Base

    belongs_to :job

end