ActiveRecord select attributes without AR objects - mysql

Question

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})
-> [1,2,3]

Answer 1

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.

Answer 2

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!

Answer 3

I don't think there is anything like SomeARClass.select(:id, :conditions => {:foo => true})
but you have two options
SomeARClass.find(:all, :conditions => {:foo => true}, :select => :id).map(&:id)
#=> [1,2,3,4]
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.

Answer 4

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.