Synch Rails Database for a Model with has_many association

Question

Because of a fire on our Datacenter (for real), we setup a fresh server without any Datas in it, and our user started to use it despite it missing millions of datas.

We may be able to get access to our backup quickly and I'm started to prepare the merging process between the Real and backup Production Db and the new and temp Production Db.

For making it simple, let's use this convention: Real and backup Production => production_db new and temp Production => migrate_from_db

We got a User model with multiple has_one Association and multiple has_many Association.

We could set an exemple like this:

class User < ApplicationRecord
    devise :database_authenticatable, :recoverable, :rememberable, :validatable, :trackable, :registerable, :omniauthable, omniauth_providers: %i[facebook]
    validates :user_name, presence: true, uniqueness: true, format: { with: /\A([\p{L}0-9\_\-]{4,127}\z)/u, message: I18n.t("devise.registrations.new.username_invalid") }, if: :should_validate_user_name?

    has_many :tokens
    has_many :scenes
    has_many :related_users, through: :related_scenes, class_name: "User", source: "user"
    has_many :notifications, foreign_key: "target_user_id"

    belongs_to :track_color, class_name: "UserColor", optional: true     
        
    has_one_attached :profile_picture  
  end

So I started to make it with my current knowledge with something like:

User.all.each do |user|
  ActiveRecord::Base.establish_connection :production
  targetedUser = User.where("id = ? OR email = ? OR user_name = ?", user.id, user.email, user.user_name)
  if !targetedUser.exists?
    targetedUser = User.create user.attributes
  end
  # How can I after this, insert all has_many associations to targetedUser ?
end

Does exists a way to automatically fetch and insert the associations (has_many, has_one_attached, belongs_to) for the new (or existing) targetedUser ?

May be it could be done with some tools such as Capistrano, but i'm pretty new on Rails.

Thanks a lot by advance for any helps