select

test.rb

require 'active_record'
require 'sqlite3'
require 'squeel'

ActiveRecord::Base.establish_connection(:adapter => 'sqlite3', :database => 'test.db')

ActiveRecord::Base.connection.tap do |c| # !> `&' interpreted as argument prefix
  c.create_table(:posts, force: true) do |t| # !> method redefined; discarding old eval_scope
    t.string :title
    t.text :body
    t.string :status
  end

  c.create_table(:events, force: true) do |t|
    t.string :title
    t.timestamp :when
    t.string :status
  end
 # !> method redefined; discarding old where
  c.create_table(:quotes, force: true) do |t|
    t.string :author # !> `&' interpreted as argument prefix
    t.string :body
  end # !> shadowing outer local variable - reflection

  c.create_table(:comments, force: true) do |t|
    t.text :body
    t.references :commentable, polymorphic: true
  end
end

class Post < ActiveRecord::Base
  has_many :comments, as: :commentable
end

class Event < ActiveRecord::Base
  has_many :comments, as: :commentable
end

class Quote < ActiveRecord::Base
  has_many :comments, as: :commentable
end

class Comment < ActiveRecord::Base
  belongs_to :commentable, polymorphic: true

  def self.active_posts_and_events
    where do
      [Post, Event].map do |model|
        (commentable_type == model.name) & (commentable_id.in model.where(status: 'active'))
      end.reduce(:|)
    end.preload(:commentable)
  end
end

Comment.create(body: 'comment-1', commentable: Event.new(title: 'event-1', status: 'inactive'))
Comment.create(body: 'comment-2', commentable: Event.new(title: 'event-2', status: 'active'))
Comment.create(body: 'comment-3', commentable: Post.new(title: 'post-1', status: 'inactive'))
Comment.create(body: 'comment-4', commentable: Post.new(title: 'post-2', status: 'active'))
Comment.create(body: 'comment-5', commentable: Quote.new(body: 'quote-1'))
Comment.create(body: 'comment-6', commentable: Quote.new(body: 'quote-2')) # !> assigned but unused variable - aliased_table

Comment.create(body: 'comment-7', commentable: Event.new(title: 'event-3', status: 'active'))
Comment.create(body: 'comment-8', commentable: Post.new(title: 'post-3', status: 'active')) # !> `&' interpreted as argument prefix

Comment.active_posts_and_events
# => [#<Comment id: 2, body: "comment-2", commentable_id: 2, commentable_type: "Event">,
#     #<Comment id: 4, body: "comment-4", commentable_id: 2, commentable_type: "Post">,
#     #<Comment id: 7, body: "comment-7", commentable_id: 3, commentable_type: "Event">,
#     #<Comment id: 8, body: "comment-8", commentable_id: 3, commentable_type: "Post">]
puts Comment.active_posts_and_events.to_sql
# >> SELECT "comments".* FROM "comments"  WHERE 
#   ((("comments"."commentable_type" = 'Post' AND "comments"."commentable_id" IN (SELECT "posts"."id" FROM "posts"  WHERE "posts"."status" = 'active')) 
#  OR ("comments"."commentable_type" = 'Event' AND "comments"."commentable_id" IN (SELECT "events"."id" FROM "events"  WHERE "events"."status" = 'active'))))