Import with synchronize docs issue

[gvaibhav1734]

Issue

Documentation needs to call out the actual working of :synchronize.

Example

Let's say we have a ActiveRecord model Item and we are using something like:

class Item < ApplicationRecord
  def self.is_active?
    return @active if @active
    @active = self.active
  end
end

# In controller
def some_function
  @items = Item.all
  # some changes to @items which changes :active field
  Item.import(@items, synchronize: @items, synchronize_keys: [:shop_id, :cart_id], on_duplicate_key_update: :all)
end

The expectation after import is that @items got synchronized with import updates in place. However, the @active instance variable is not reset which should be explicitly called out in the documentation.

Code explanation

activerecord-import/lib/activerecord-import/synchronize.rb

Line 46 in 20c1d3a

instance.instance_variable_set :@attributes, matched_instance.instance_variable_get(:@attributes)

From this file (and line)

1. It is evident that the instance variables @attributes are being set in memory with fresh values from import call.
2. Its also clear that only the instance variables belonging to ActiveRecord (like @new_record @destroyed) are being changed.

Docs should explicitly call this out because if @items had its own custom instance variables they wont be replaced or reset to nil.