Rails-6: Mass import HABTM produces thousands inserts

After hours of googling for best practice, I came up here.

I have a new Foo object, and want to mass-assign thousands of bars to it, which I've pre-loaded:

@foo = Foo.create
@bars = Bar.find_all_by_some_attribute(:a)

What's the fastest way to do this? I've tried:

@foo.bars = @bars
@foo.bars << @bars

This results in thousands of insert queries being created.

Is there a Rails 6 (Insert_all) way to solve my issue or do I have to solve it with the "m2m_fast_insert" gem or hardcoded SQL?

If you have a Join table setup, like:

class Item < ActiveRecord::Base
  has_and_belongs_to_many :orders
end

class Order < ActiveRecord::Base
  has_and_belongs_to_many :items
end

class ItemsOrders < ActiveRecord::Base
  belongs_to :item
  belongs_to :order
end

I believe you could insert many ItemOrders in this case (or whatever your join model is) with insert_all, which is new in Rails 6. If not your best bet is some amount of custom SQL. I did find another answer on SO that seemed to have a good starting point for the SQL: What is the fastest way to create mass HABTM associations in Rails?