reproduction for https://github.com/rails/rails/issues/10693

test.rb

require 'logger'
require 'active_record'

ActiveRecord::Base.logger = Logger.new(STDERR)
ActiveRecord::Base.establish_connection(
  adapter:  "sqlite3",
  database: ":memory:"
)

ActiveRecord::Schema.define do
  create_table :states do |table|
    table.column :name, :string
    table.column :code, :string
  end
  add_index :states, :code

  create_table :cities do |table|
    table.column :name, :string
    table.column :state_code, :string
  end
  add_index :cities, :state_code
end

class State < ActiveRecord::Base
  has_many :cities, foreign_key: :state_code
end

class City < ActiveRecord::Base
  belongs_to :state, foreign_key: :state_code, primary_key: :code
end

state = State.create!(name: "A", code: "A")
state.cities.create!(name: "1")
state.cities.create!(name: "2")
new_city = City.create!(name: "3")

begin
  state.cities = [new_city]
rescue => e
  puts '===', e.message, '===', ''
end
state.save!
state.reload
p state.cities