count.rb

require 'bundler'
Bundler.setup(:default)

require 'active_record'
require 'minitest/autorun'
require 'logger'

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :posts do |t|
    t.string :name
    t.timestamps
  end

  create_table :comments do |t|
    t.integer :post_id
  end
end

class Post < ActiveRecord::Base
  has_many :comments
end

class BugTest < Minitest::Test
  def test_count_stuff
    post = Post.create! name: "first"
    post = Post.create! name: "second"
    post = Post.create! name: "third"

    assert_equal 3, Post.select("posts.name").count

    assert_raises(ActiveRecord::StatementInvalid) do
      Post.select("posts.name, posts.created_at").count
    end
   
    assert_equal 3, Post.select("posts.name, posts.created_at").count(:all)
    assert_equal 3, Post.select("posts.name, posts.created_at").unscope(:select).count
  end
end