ActiveRecord(三)

查询

从数据库中获取数据

• find
• first
• take
• last

• find_each

  User.find_each(batch_size: 3) do |user|
    puts user.id
  end

• find_in_batches

  User.find_in_batches(batch_size: 3) do |users|
    puts users.count
  end

条件查询

>where

User.where("name = 'mazi'")

User.where("name = 'mazi' AND id = 39")

str = 'maizi'
User.where("name = ?", str)

User.where("id > 52")

num = 55
User.where("id < ?", num)

User.where("id < ? AND name = ?", num, str)

User.where(:name => 'maizi', :id => 55)

User.where.not(:name => 'maizi', :id => 55)

排序

升序

User.where.not(:name => 'maizi', :id => 55).order(:created_at => :ASC)

降序

User.where.not(:name => 'maizi', :id => 55).order(:created_at => :DESC)

查询指定字段

User.where.not(:name => 'maizi', :id => 55).order(:created_at => :DESC).select(:name)

分页查询

• limit
• offset

User.where.not(:name => 'maizi').order(:created_at => :DESC).select(:name, :created_at).limit(1).offset(1)

time = 0
User.where.not(:name => 'maizi').order(:created_at => :DESC).select(:name, :created_at).limit(10).offset(time)

time = 1
User.where.not(:name => 'maizi').order(:created_at => :DESC).select(:name, :created_at).limit(10).offset(time)

time = 2
User.where.not(:name => 'maizi').order(:created_at => :DESC).select(:name, :created_at).limit(10).offset(time)

分组查询

i = 0
User.find_each do |user|
  if i%2 == 0
    user.gender = '女'
  else
    user.gender = '男'
  end
  user.money = i+20
  user.save
  i += 1
end

result = User.select("gender", "sum(money) as total_money").group("gender")

result.first

result.first.total_money

result = User.select("gender", "sum(money) as total_money").group("gender")

result.first

result.first.total_money

result = User.select("gender", "sum(money) as total_money").group("gender").having("sum(money) > ?", 120)

条件覆盖

• unscope
• only
• reorder
• reverse_order
• rewhere

  User.select("id", "created_at").limit(5).unscope(:limit)
  
  User.select("id").limit(5).offset(2).only(:limit, :offset)
  
  User.select("id").order(:id => :DESC)
  
  User.select("id").order(:id => :DESC).reorder(:id)
  
  
  User.select("id").order(:id => :DESC).reorder(:id)
  
  User.select("id").order(:id => :DESC).reverse_order

只读对象

User.readonly.first