CollectionAssociation#merge_target_lists trouble

scenario.rb

require 'active_record'

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

ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :widgets do |t|
    t.string :name
    t.integer :gadget_id
  end

  create_table :gadgets do |t|
  end
end

class Widget < ActiveRecord::Base
end

class Gadget < ActiveRecord::Base
  has_many :widgets
end

gadget = Gadget.create!
gadget.widgets.create!(name: "foo")

gadget.widgets.where(name: "foo").destroy_all

raise "boo!" unless gadget.widgets == []
# These work fine:
# raise "boo!" unless gadget.widgets(true) == []
# raise "boo!" unless gadget.widgets.all == []

sql.log

-- create_table(:widgets)
D, [2015-03-27T20:25:41.639110 #18052] DEBUG -- :    (0.4ms)  CREATE TABLE "widgets" ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, "name" varchar, "gadget_id" integer) 
   -> 0.0025s
-- create_table(:gadgets)
D, [2015-03-27T20:25:41.639627 #18052] DEBUG -- :    (0.1ms)  CREATE TABLE "gadgets" ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL) 
   -> 0.0004s
D, [2015-03-27T20:25:41.670625 #18052] DEBUG -- :    (0.1ms)  begin transaction
D, [2015-03-27T20:25:41.685707 #18052] DEBUG -- :   SQL (0.2ms)  INSERT INTO "gadgets" DEFAULT VALUES
D, [2015-03-27T20:25:41.686117 #18052] DEBUG -- :    (0.1ms)  commit transaction
D, [2015-03-27T20:25:41.693149 #18052] DEBUG -- :    (0.0ms)  begin transaction
D, [2015-03-27T20:25:41.698404 #18052] DEBUG -- :   SQL (0.2ms)  INSERT INTO "widgets" ("name", "gadget_id") VALUES (?, ?)  [["name", "foo"], ["gadget_id", 1]]
D, [2015-03-27T20:25:41.698847 #18052] DEBUG -- :    (0.1ms)  commit transaction
D, [2015-03-27T20:25:41.701873 #18052] DEBUG -- :   Widget Load (0.2ms)  SELECT "widgets".* FROM "widgets" WHERE "widgets"."gadget_id" = ? AND "widgets"."name" = ?  [["gadget_id", 1], ["name", "foo"]]
D, [2015-03-27T20:25:41.703228 #18052] DEBUG -- :    (0.1ms)  begin transaction
D, [2015-03-27T20:25:41.703965 #18052] DEBUG -- :   SQL (0.1ms)  DELETE FROM "widgets" WHERE "widgets"."id" = ?  [["id", 1]]
D, [2015-03-27T20:25:41.704155 #18052] DEBUG -- :    (0.0ms)  commit transaction
D, [2015-03-27T20:25:41.705938 #18052] DEBUG -- :   Widget Load (0.1ms)  SELECT "widgets".* FROM "widgets" WHERE "widgets"."gadget_id" = ?  [["gadget_id", 1]]