Equivalent of `pluck` that Returns a Relation

Question

In my Rails app, I've written a method that generates an array of unique names from a set of records in the database:

class Book < ActiveRecord::Base
  attr_accessible :title, :author

  def self.all_authors
    self.uniq.pluck(:author)
  end
end

This method works as expected, but this application may end up having a large number of authors so now I want to paginate this query in the controller.

class AuthorsController < ApplicationController
  def index
    @authors = Book.all_authors.limit(10).offset(params[:page]*10)
  end
end

Obviously, this doesn't work because pluck(:authors) returns an array and not an ActiveRecord::Relation. Is there an alternative to pluck that would allow me to use Arel method call chains? Or perhaps a way to make pluck return an ActiveRecord::Relation instead of an array?

Answer 1

Try this:

@authors = Book.limit(10).offset(params[:page]*10).all_authors
# => ["first pair of authors", "second pair of authors", ...]

You just need to call the pluck method at the end of the chain.

Otherwise you can use select which will only return the specified columns from the database:

@authors = Book.select(:author).limit(10).offset(params[:page]*10)
# => [#<Book author: "first pair of authors">, #<Book author: "second pair of authors">, ...]