class CreateSimpleModels < ActiveRecord::Migration
  def self.up
    create_table :simple_models do |t|
      t.string :name
      t.integer :user_number
      t.timestamps
    end
    execute "CREATE SEQUENCE simple_models_user_number_seq OWNED BY
simple_models.user_number INCREMENT BY 1 START WITH 1"
  end

  def self.down
    drop_table :simple_models
    execute "DELETE SEQUENCE simple_models_user_number_seq"
  end
end

你可以直接修改你的mysql表，试试这个：
Alter table Tablename modify column ColumnName int(11) auto_increment;
您可以引用Auto increment a non-primary key field in Ruby on Rails以rails方式执行它。
你也可以想出一个非常愚蠢的方法，比如在你的模型中，在create方法中：

def self.create(user, user_number)
 user = User.new #update fields
 user.user_number += 1
 user.save
end

但是我仍然会问为什么不使用id本身作为user_number？

对于Rails 6 +

add_column :table_name, :column_name, :type, autoincrement: true