ActiveRecord tips: when to use select in place of pluck

As you probably know ActiveRecord allows arrays as arguments of conditions. I remember this feature at least since Rails 2…

So if you write:

Product.where tag_code: [1,2,3]

It produces:

SELECT * FROM products WHERE tag_code IN (1,2,3);

It’s ok, but it’s not a wonderful solution if you have a lot of arguments. You can even have an error because you’re passing too many arguments.

Then Rails 4 added the .pluck method, and since then I’m seeing a lot of code like:

Product.where tag_code: Tag.pluck(:code)

But pluck returns an array, so it executes the query right away. In the end you are doing two queries:

SELECT code FROM tags; # => [1,2,3]
SELECT * FROM products WHERE tag_code IN (1,2,3);

DON’T DO THIS. It’s bad. In this case you want to use .select:

Product.where tag_code: Tag.select(:code)

.select returns an ActiveRecord relation object, that can be included in other queries. In fact the above code will produce:

SELECT * FROM products WHERE tag_code IN (SELECT code FROM tags);