Sifters don't work on on-the-fly tables

[pelletencate]

Issue

BabySqueel[:table] returns an object that does not respond to sift.

Now that I'm writing this, I realize it makes some sense, BabySqueel doesn't know anything about which model belongs to a table. However, would it be possible to add a model's sifter to a BabySqueel-initiated table object?

Reproduction

require 'bundler/inline'
require 'minitest/spec'
require 'minitest/autorun'

gemfile true do
  source 'https://rubygems.org'
  gem 'activerecord', '~> 5.0.0' # which Active Record version?
  gem 'sqlite3'
  gem 'baby_squeel', github: 'rzane/baby_squeel'
end

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')

ActiveRecord::Schema.define do
  create_table :dogs, force: true do |t|
    t.string :name
  end

  create_table :people, force: true do |t|
    t.string :name
    t.integer :age
    t.string :dog_name
  end
end

class Dog < ActiveRecord::Base
end

class Person < ActiveRecord::Base
  sifter(:female) { name.in? %w(angela brittany carol) }
end

class BabySqueelTest < Minitest::Spec
  it 'does not raise an exception' do
    people = BabySqueel[:people]
    # Find all dogs that have a female adult boss
    scope = Dog.joining { people.on(people.dog_name == name) }.where.has { people.sift(:female) & (people.age >= 18) }

	# FIXME: This SQL may be off, the point is whether or not it raises an exception. I'm not an expert with minitest.
    scope.to_sql.must_equal %{
      SELECT "dogs".* FROM "dogs"
        INNER JOIN "people" ON "people"."dog_name" = "dogs"."name"
        WHERE "people"."name" IN ('angela', 'brittany', 'carol')
          AND "people"."age" >= 18;
    }.squish
  end
end