在 rails3 中使用范围进行 Eagerloading [英] Eagerloading with scoping in rails3

问题描述

我一直在尝试基于 Rails3 应用程序中的某个范围急切加载关联，但找不到任何解决方案.

I have been trying to eager load associations based on some scope in my rails3 app, but could not find any solution.

我的应用有以下模型:

class Project
 has_many :entries
 has_many :to_dos

class ToDo
 has_may :entries
 has_many :tasks
 belongs_to :project

class Task
 has_many :entries
 belongs_to :to_do

class Entry
belongs_to :project
belongs_to :to_do
belongs_to :task

# options format: {:from_date=>(Date.today-1.week), :to_date=>(Date.today+1.week), :user_id=>60}
scope :filtered_list, lambda { |options|
  condition = options[:user_id].nil? ? "true" : "user_id = #{options[:user_id]}"
  condition += options[:from_date].nil? ? "" : " AND entry_date >= '#{options[:from_date]}'"
  condition += options[:to_date].nil? ? "" : " AND entry_date <= '#{options[:to_date]}'"
  where(condition)
}

在 projects#index 我有以下代码来获取用户的所有项目:

And in projects#index i have following code to get all projects of an user:

@projects = current_user.projects.includes(:entries, :to_dos =>[:entries, :tasks => :entries])

它获取用户的所有项目，并预先加载关联.因此，当我执行以下循环以获取项目中的所有条目时，不会触发任何新查询.

It fetches all projects of the user, along with eager loading the associations. So when i perform following loop to get all the entries within the project, no new query gets fired.

def all_entries(options)
  entries = self.entries
  self.to_dos.each do |d|
    entries += d.entries
    d.tasks.each do |t|
      entries += t.entries
    end
  end
end

由于这种急切加载会获取所有条目，因此数据比我实际需要的要多得多.所以我尝试对预先加载的条目应用一些条件，但找不到任何解决方案.我正在寻找类似的东西:

As this eager loading fetches all entries, it is way too much data than what I actually needed. So I tried to apply some conditions to the entries eager loaded, but could not find any solution. I was looking for something like:

@projects = current_user.projects.includes(:entries.filtered_list(options), :to_dos =>[:entries.filtered_list(options), :tasks => :entries.filtered_list(options)])

这样只有满足某些条件的条目才会被加载.

So that only the entries satisfying some conditions get loaded.

我们不能在急切加载中使用作用域吗?请帮助我在确定范围的同时使用 Eagerloading.

Can't we use scoping with eager loading? Please help me out use eagerloading alongside scoping.

推荐答案

据我所知，作用域不能应用于这样的包含关联.但是，您可以指定仅应用于急切加载查询的条件.因此，通过一些重构，您可以拥有一个仅创建您当前在范围内定义的条件的方法:

As far as I know, scopes cannot be applied to included associations like this. However, you can specify conditions that should only be applied to the eager loading queries. So with a bit of refactoring, you could have a method that only created the conditions you currently define in your scope:

def self.filter_by(options)
  condition = options[:user_id].nil? ? "true" : "entries.user_id = #{options[:user_id]}"
  condition += options[:from_date].nil? ? "" : " AND entries.entry_date >= '#{options[:from_date]}'"
  condition += options[:to_date].nil? ? "" : " AND entries.entry_date <= '#{options[:to_date]}'
  condition
end

或者更红一点:

def self.filter_by(options)
  conditions = []
  conditions << "entries.user_id = #{options[:user_id]}" unless options[:user_id].nil?
  conditions << "entries.entry_date >= '#{options[:from_date]}'" unless options[:from_date].nil?
  conditions << "entries.entry_date <= '#{options[:to_date]}'" unless options[:to_date].nil?
  conditions.join(" AND ")
end

然后将该方法链接到您的急切加载:

and then chain that method to your eager loading:

@projects = current_user.projects.includes(:entries, :to_dos =>[:entries, :tasks => :entries].where(Entry.filter_by(options))

如果您需要单独使用它，也可以在您的范围内重复使用它:

and also reuse it in your scope if you need it independently:

scope :filtered_list, lambda { |options| where(Entry.filter_by(options)) }

免责声明:这些都没有用您的实际模型定义进行过测试，但它适用于我周围的一些非常等效的模型.

Disclaimer: None of this is tested with your actual model definitions, but it works fine with some pretty equivalent ones that I had lying around.

另请注意，如果过滤器选项最终来自客户端，则您的条件容易受到 SQL 注入的影响.

Also note that if the filter options ultimately come from the client side, your condition is vulnerable to SQL injection.

在幕后，Rails 使用 JOIN 来加载相关数据，因此需要注意这一点.这可能是一件好事(少一些查询)或一件坏事(如果您的索引不是最理想的).这可能就是 guide 这么说的原因:

Behind the scenes, Rails uses a JOIN to load the relevant data, so that is something to be aware of. It might be a good thing (a few less queries) or a bad thing (if your indexing is suboptimal). That's probably why the guide has this to say:

即使 Active Record 允许你在eager上指定条件加载关联就像连接一样，推荐的方法是使用改为加入.

Even though Active Record lets you specify conditions on the eager loaded associations just like joins, the recommended way is to use joins instead.

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