ActiveRecord select attributes without AR objects

I have a table with MANY rows, I need just the IDs of certain rows. The slow way is to call

SomeARClass.find(:all, :conditions => {:foo => true}, :select => :id)

This returns AR Objects...

Is there a way to call a select on a class and have it return a plain old ruby data structure. Something like this:

SomeARClass.select(:id, :conditions => {:foo => true})
-&g开发者_JAVA百科t; [1,2,3]

ActiveRecord::Base.connection.select_all(sql)

or

SomeARClass.connection.select_all(sql)

This is what you want to use. It returns an array of hashes. It should be used sparingly though. Hand coded sql is what ActiveRecord was built to replace. I only use in in really performance critical areas where constructing and returning AR objects is too slow.

The pluck method is exactly what you want, without compromising the SQL bad practice:

SomeARClass.where(:foo => true).pluck(:id)

I believe this should be the selected answer!

I don't think there is anything like SomeARClass.select(:id, :conditions => {:foo => true}) but you have two options

1. SomeARClass.find(:all, :conditions => {:foo => true}, :select => :id).map(&:id)
   #=> [1,2,3,4]
   

2. id_hash = ActiveRecord::Base.connection.select_all('select id from tablename')
   #=>  [{"id"=>"1"}, {"id"=>"2"}, {"id"=>"3"}, {"id"=>"4"}]
   
   id_hash.map(&:values).flatten
   #=>  ["1", "2", "3", "4"]
   

The second option returns only a hash and not Active record objects but it does looks a bit hackish.

Short answer:

• Use .ids to fetch only ids
• Use pluck to fetch ONLY some columns, but values will be PARSED by Active Record
• Use ActiveRecord::Base.connection.select_all to fetch UNPARSED values.

Notes:

There is small difference between pluck and select_all:

If you pluck data - ActiveRecord map data to Ruby objects, like Dates, Enums, Models, etc, and it could be different than you expect.

You can notice that the result below is a string:

2.5.1 :001 > ActiveRecord::Base.connection.select_all("select created_at from some_ar_class where id = 1 limit 1").rows.first
   (0.6ms)  select created_at from some_ar_classes where id = 1 limit 1
 => ["2018-10-01 01:12:31.758161"]

While pluck return Dates

2.5.1 :002 > SomeARClass.where(id: 1).pluck(:created_at).first.class
   (0.4ms)  SELECT "some_ar_classes"."created_at" FROM "some_ar_classes" WHERE "some_ar_classes"."id" = $1  [["id", 1]]
 => ActiveSupport::TimeWithZone

The same happens with Enums(int in database, but :symbols in Ruby)

And all this parsing/mapping operations also takes time. It's not a lot, but still. So if you asking for the most fast way to fetch data - its definitely raw sql query with connection.select_all, but there are very small situations when you can get some significant performance increase on that.

So my recommendation is using pluck.