Ruby on Rails的：从默认选择列prevent [英] Ruby on Rails: prevent from selecting a column by default

问题描述

我有输入表与内容字段可能包含大量的文字。在大多数情况下，我并不需要获得该领域，所以它似乎是资源的巨大浪费，每次从数据库中加载了大量未使用的数据（SELECT * FROM项其中id = 1）。

I have entries table with a content field which might contain a lot of text. In most cases I don't need to access that field, so it seems to be a big waste of resources to every time load a huge amount of unused data from the database (select * from entries where id = 1).

我怎么能指定default_scope，所有除了内容领域将被从数据库加载？

How could I specify the default_scope, that all the fields apart from content would be loaded from database?

推荐答案

假设的Rails 3和看起来像这样一个模式：

Assuming Rails 3 and a schema that looks like this:

create_table "entries", :force => true do |t|
  t.string   "title"
  t.text     "content"
  t.datetime "created_at"
  t.datetime "updated_at"
end

您可以使用选择的方法来限制​​返回类似这样的字段：

You can use the select method to limit the fields that are returned like this:

class Entry < ActiveRecord::Base
  default_scope select([:id, :title])
end

在轨控制台，你会看到这样的事情：

In the rails console, you should see something like this:

puts Entry.where(:id => 1).to_sql  # => SELECT id, title FROM "entries"  WHERE "entries"."id" = 1

当你要选择所有的字段，您可以使用无范围这样的方法：

When you do want to select all of the fields, you can use the unscoped method like this:

puts Entry.unscoped.where(:id => 1).to_sql  # => SELECT * FROM "entries"  WHERE "entries"."id" = 1

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