Rails4：如何在活动记录中添加计算列

Question

We have landed ourself with a column 'data' on User table that has a huge json dump in it.

Now every time we load a bunch of users we get all this data into the memory resulting into out of memory errors.

We want to be able to write some computed columns that we can use in our select statements.

eg:
Instead of doing this 
user.data['profile_data']['data']['image']

We would like to add a column :image and then write a query like:

Here :name and :email are actual columns on the table and :image is a computed column:
Users.where(SOME_CONDITION).select(:name,:email,:image)

The main use case is the index page where we display all users which basically loads data column for all users

This will avoid loading the huge data column in memory and help us load the fields we want from the data column

Whats the best way to do this in Rails4?

Updates:

• We use postgres on Heroku.

Answer 1

I would move the data column to another table, but if that's not an option try the lazy_columns gem.

class User < ActiveRecord::Base
  lazy_load :data
end

Now the data column will be excluded during the initial load, but if you try to access the .data it will be retrieved from the database.

Answer 2

Add the column :image using the migration . Then add the below code :

class User < ActiveRecord::Base
  before_save :extract_image

  private

  def extract_image
    self.image = self.data['profile_data']['data']['image']
    self.save
  end
end

before_save : Is called before Base.save (regardless of whether it's a create or update save).

Answer 3

Postgresql support two json data type , as described in the documentation

There are two JSON data types: json and jsonb. They accept almost identical sets of values as input. The major practical difference is one of efficiency. The json data type stores an exact copy of the input text, which processing functions must reparse on each execution; while jsonb data is stored in a decomposed binary format that makes it slightly slower to input due to added conversion overhead, but significantly faster to process, since no reparsing is needed. jsonb also supports indexing, which can be a significant advantage.

So to solve your problem, you need to change the type of the data column to jsonb through a migration:

# This should use the up and down methods, because change_column 
# is not reversible     
class ChangeUsersDataColumnTypeToJsonb < ActiveRecord::Migration
   def up
      change_column :users, :data, :jsonb
   end
   def down
      change_column :users, :data, :text # or whatever datatype it was
   end
end

than to query the image field with you use the functions that postgres provides to query the json data type :

Users.where(SOME_CONDITION).select(:name,:email,"data::json->'image' as image")

than you access the image attribute like any other attribute.

You have also to define the :data attribute as a lazy loading column , like , so that column do not get loaded when the user object is instantiated.

class User < ActiveRecord::Base
  lazy_load :data
end