1、擷取資料
擷取第一條、最後一條記錄
複製代碼 代碼如下:
Model.first
Model.first(options)
Model.find(:first, options)
Model.last
Model.last(options)
Model.find(:last, options)
通過id擷取記錄
複製代碼 代碼如下:
Model.find(1, 10, options)
Model.find([1, 10], options)
.find all
複製代碼 代碼如下:
Model.all(options)
對一組資料進行相同操作
複製代碼 代碼如下:
User.all.each do |user|
NewsLetter.weekly_deliver(user)
end
如果表記錄數比較大，這種方式比較耗資源，因為它會一次載入整個表的資料。改用以下這種方式，它每次只載入1000行，然後逐步yield完整個表
複製代碼 代碼如下:
User.find_each do |user|
NewsLetter.weekly_deliver(user)
end
自訂方式，find_each接受和find同樣的options
複製代碼 代碼如下:
User.find_each(:batch_size => 5000, :start => 2000) do |user|
NewsLetter.weekly_deliver(user)
end
find_in_batches，和find_each相似，但它yield時傳遞的是model對象數組，而不是單個model對象
複製代碼 代碼如下:
Invoice.find_in_batches(:include => :invoice_lines) do |invoices|
export.add_invoices(invoices)
end
2、查詢條件
通過替換？來傳遞條件值，可避免SQL注入
複製代碼 代碼如下:
Client.first(:conditions => ["orders_count = ?", params[:orders])
symbol佔位條件
複製代碼 代碼如下:
Client.all(:conditions => ["created_at >= :start_date AND created_at <= :end_date", {:start_date => params[:start_date], :end_date => params[:end_date] }])
範圍條件 in(集合)
複製代碼 代碼如下:
Client.all(:conditions => ["created_at IN (?)", (params[:start_date].to_date)..(params[:end_date].to_date])
產生sql
複製代碼 代碼如下:
SELECT * FROM users WHERE (created_at IN ('2007-12-31','2008-01-01','2008-01-02','2008-01-03','2008-01-04','2008-01-05', '2008-01-06','2008-01-07','2008-01-08'))
如果要產生日期時間，再加上.to_time
複製代碼 代碼如下:
params[:start_date].to_date.to_time，產生2007-12-01 00:00:00格式
有上資料庫會在以上條件中報錯，如Mysql會報查詢語句過長的錯誤，此時可以改成created_at > ? AND created_at < ?的形式
Hash條件
複製代碼 代碼如下:
Client.all(:conditions => {:locked => true })
帶範圍條件
複製代碼 代碼如下:
Client.all(:conditons => {:created => (Time.now.midnight - 1.day)..Time.now.midnight})
產生sql
複製代碼 代碼如下:
SELECT * FROM clients WHERE (clients.created_at BETWEEN '2008-12-21 00:00:00' AND '2008-12-22 00:00:00')
集合條件
複製代碼 代碼如下:
Client.all(:conditons => {:orders_count => [1,3,5])
產生sql
複製代碼 代碼如下:
SELECT * FROM clients WHERE (clients.orders_count IN (1,3,5))
3、查詢選項
排序
複製代碼 代碼如下:
#單個排序
Client.all(:order => "created_at ASC")
#多個排序
Client.all(:order => "orders_count ASC, created_at DESC")
返回指定欄位
複製代碼 代碼如下:
Client.all(:select => "viewable_by, locked")
#使用函數
Client.all(:select => "DISTINCT(name)")
限定和位移Limit and Offset
複製代碼 代碼如下:
Client.all(:limit => 5)
#產生
SELECT * FROM clients LIMIT 5
Client.all(:limit => 5, :offset => 5)
#產生
SELECT * FROM clients LIMIT 5, 5
Group分組
複製代碼 代碼如下:
Order.all(:group => "date(created_at)", :order => "created_at")
產生sql
複製代碼 代碼如下:
SELECT * FROM orders GROUP BY date(created_at)
Having
複製代碼 代碼如下:
Order.all(:group => "date(created_at)", :having => ["created_at > ?", 1.month.ago)
產生sql
複製代碼 代碼如下:
SELECT * FROM orders GROUP BY date(created_at) HAVING created_at > '2009-01-15'
唯讀
複製代碼 代碼如下:
client = Client.first(:readonly => true)
client.locked = false
client.save
#對唯讀對象進行儲存將會觸發ActiveRecord::ReadOnlyRecord異常
更新時鎖定記錄
樂觀鎖Optimistic Locking
為使用樂觀鎖，須在表裡建一個lock_version的欄位，每次更新記錄時，ActiveRecord自動遞增lock_version的值，
複製代碼 代碼如下:
c1 = Client.find(1) c2 = Client.find(1) c1.name = "Michael" c1.save c2.name = "should fail" c2.save # Raises a ActiveRecord::StaleObjectError
備忘：You must ensure that your database schema defaults the lock_version column to 0.
This behavior can be turned off by setting ActiveRecord::Base.lock_optimistically = false.
指定樂觀鎖欄位名
複製代碼 代碼如下:
class Client < ActiveRecord::Base set_locking_column :lock_client_column end
悲觀鎖Pessimistic Locking
悲觀鎖定由資料庫直接提供
複製代碼 代碼如下:
Item.transaction do
i = Item.first(:lock => true)
i.name = 'Jones'
i.save
end
Mysql執行返回
複製代碼 代碼如下:
SQL (0.2ms) BEGIN Item Load (0.3ms) SELECT * FROM `items` LIMIT 1 FOR UPDATE Item Update (0.4ms) UPDATE `items` SET `updated_at` = '2009-02-07 18:05:56', `name` = 'Jones' WHERE `id` = 1 SQL (0.8ms) COMMIT
為特定資料庫加入原始的lock聲明
為Mysql的鎖定聲明為共用模式，即鎖定時仍然可讀
複製代碼 代碼如下:
Item.transaction do i = Item.find(1, :lock => "LOCK IN SHARE MODE") i.increment!(:views) end
4、關聯表
複製代碼 代碼如下:
Client.all(:joins => "LEFT OUTER JOIN address ON addresses.client_id = clients.id')
產生sql
複製代碼 代碼如下:
SELECT clients.* FROM clients LEFT OUTER JOIN addresses ON addresses.client_id = clients.id
使用Array、Hash、Named Associations關聯表
有如下model
複製代碼 代碼如下:
class Category < ActiveRecord::Base
has_many :posts
end
class Post < ActiveRecord::Base
belongs_to :category
has_many :comments
has_many :tags
end
class Comments <ActiveRecord::Base
belongs_to :post
has_one :guest
end
class Guest < ActiveRecord::Base
belongs_to :comment
end
複製代碼 代碼如下:
#關聯一個關係
Category.all :joins => :posts
#關聯多個關係
Post.all :joins => [:category, :comments]
#嵌套關聯
Category.all :joins => {:posts => [{:comments => :guest}, :tags]}
為關聯查詢結果設定條件
複製代碼 代碼如下:
time_range = (Time.now.midnight - 1.day)..Time.now.midnight Client.all :joins => :orders, :conditions => {'orders.created_at' => time_ran
#或者
time_range = (Time.now.midnight - 1.day)..Time.now.midnight Client.all :joins => :orders, :conditions => {:orders => {:created_at => time_range}}
5、最佳化載入
以下代碼，需要執行1 + 10次sql
複製代碼 代碼如下:
clients = Client.all(:limit => 10) clients.each do |client|
puts client.address.postcode
end
最佳化：
複製代碼 代碼如下:
clients = Client.all(:include => :address, :limit => 10)
clients.each do |client|
puts client.address.postcode
end
一次性載入post的所有分類和評論
複製代碼 代碼如下:
Post.all :include => [:category, :comments]
載入category為1的所有post和cooment及tag
複製代碼 代碼如下:
Category.find 1, :include => {:posts => [{:comments => :guest}, :tags]}
6、動態查詢
複製代碼 代碼如下:
Client.find_by_name("Ryan")
Client.find_all_by_name("Ryan")
#！方法，沒有記錄時拋出ActiveRecord::RecordNotFound異常
Client.find_by_name!("Ryan")
#查詢多個欄位
Client.find_by_name_and_locked("Ryan", true)
#查詢不到時就建立並儲存
Client.find_or_create_by_name(params[:name])
#查詢不到時建立一個執行個體，但不儲存
Client.find_or_initialize_by_name('Ryan')
7、find_by_sql
複製代碼 代碼如下:
Client.find_by_sql("SELECT * FROM clients INNER JOIN orders ON clients.id = orders.client_id ORDER clients.created_at desc")
8、select_all
和find_by_sql類似，但不會用model執行個體化返回記錄，你會得到一個hash數組
複製代碼 代碼如下:
Client.connection.select_all("SELECT * FROM clients WHERE id = '1'")
9、判斷記錄是否存在
複製代碼 代碼如下:
#通過id來查詢
Client.exists?(1)
Client.exists?(1, 2, 3)
#or
Client.exists?([1,2,3])
#通過其他條件來查詢
Client.exists?(:conditions => "first_name = 'Ryan'")
#沒有參數時，則：表是空的 ? false : true
Client.exists?
10、計算
複製代碼 代碼如下:
#求結果集條數
Client.count(:conditons => "first_name = 'Ryan'")
#求某個欄位非空白的條數
Client.count(:age)
#平均值
Client.average("orders_count")
#求最小值
Client.minimum("age")
#求最大值
Client.maximum("age")
#求和
Client.sum("orders_count")
