Valium

Suffering from ActiveRecord instantiation anxiety? Try Valium. It saves your CPU and memory for more important things, retrieving just the values you're interested in seeing.

Usage

In your Gemfile:

gem 'valium'

In your code:

You can select a single value...

Post.where(:published => true).value_of :title
# => ["First Post", "Another Awesome Post", ...]

... or several ...

Employee.where(:title => 'Sr. Monkey Wrangler').values_of :first_name, :last_name, :hired_at
# => [["Ernie", "Miller", 2009-09-21 08:00:00 -0400],
      ["Herb", "Myers", 2002-02-13 09:00:00 -0400], ...]

Values returned by Valium will be the data types you'd expect, just as though you instantiated the ActiveRecord object and used the accessor. This includes serialized attributes:

class Animal < ActiveRecord::Base
  serialize :extra_info
end

Animal.where(:genus => 'felis').values_of :species, :extra_info
# => [["catus", {:domestic => true}], ["lolcatus", {:can_has_cheezburger => true}], ...]

Why would I use this?

It's not uncommon for Rails apps to need only one or two attributes from a bunch of ActiveRecord objects. They'll have code like this:

MyModel.some_scope.map(&:id)

Or, if the developer is a bit more clever about saving memory, he might use code like this:

MyModel.some_scope.select(:id).map(&:id)

This helps a good deal with memory usage, but even if we cut down on the memory usage a bit, the truth is that no matter what we try, instantiating ActiveRecord objects is slow.

This is because ActiveRecord provides all kinds of awesome stuff, none of which we need, if we're just looking to grab one or two values and print them out, or perform a quick calculation, or something.

Check out this gist of a benchmark script and results to see just how much it can hurt when you're instantiating unnecessary ActiveRecord objects.

TL;DR: It hurts a lot. For retrieving a single value, using Valium is nearly 10x faster than mapping over ActiveRecord objects. For multiple values, it's about 5x faster. Even if you're deserializing attributes, where more time, proportionally, gets spent in the deserialization process than with normal attributes, Valium can be up to twice as fast as mapping over ActiveRecord objects, in my tests.

Limitations

Valium will only retrieve columns from the model you are querying against. Joined associations may be used to limit the scope of the query, but their attribute values can't be selected. There's a bit of discussion on issue #2 about why this is, if you're interested in reading more.

Conclusion

You knew everything I mentioned above, already. In fact, you're probably mocking me right now:

"ZOMG OBJECTS USE MEMORY AND INSTANTIATION TAKES CPU CYCLES! I R GENIUS!"

Yeah. It's not rocket science. This is just a quick little (seriously, around 100 LOC) gem that provides some intuitive syntax around a common pattern and doesn't stomp on any existing ActiveRecord functionality. It "just works."

Give it a try. Your code will thank you for it.

ernie/valium

ernie

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