generating meaningful usernames in devise - ruby-on-rails

Question

I'm trying to have my app automatically generate usernames to be used as the url. Typically they will be the users first and last name added together however when there already exists a user with the same first and last name it will append a number to the name that increases for each.
This is the method i created:
def full_name
first_name + ' ' + last_name
end
def user_name
t_user_name = first_name.downcase.strip.gsub(' ', '').gsub(/[^\w-]/, '') + last_name.downcase.strip.gsub(' ', '').gsub(/[^\w-]/, '')
user_name = t_user_name
num = 1
while User.find_by_user_name(user_name).count > 0
num += 1
user_name = "#{t_user_name}#{num}"
end
end
I'm currently getting the error:
undefined method `find_by_user_name'
Which i thought would work automatically?
I was also trying to examples shown in this post:
generate unique username (omniauth + devise)
but I kept getting the error:
Mysql2::Error: Unknown column 'users.login' in 'where clause': SELECT COUNT(*) FROM `users` WHERE `users`.`login` = 'bobweir'
Even though I added
t.string :login, null: false, default: ""
to the users table
edit2: schema.rb
ActiveRecord::Schema.define(version: 20150611033237) do
create_table "users", force: :cascade do |t|
t.string "first_name", limit: 255
t.string "last_name", limit: 255
t.string "email", limit: 255, default: "", null: false
t.string "encrypted_password", limit: 255, default: "", null: false
t.string "reset_password_token", limit: 255
t.datetime "reset_password_sent_at"
t.datetime "remember_created_at"
t.integer "sign_in_count", limit: 4, default: 0, null: false
t.datetime "current_sign_in_at"
t.datetime "last_sign_in_at"
t.string "current_sign_in_ip", limit: 255
t.string "last_sign_in_ip", limit: 255
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
end
add_index "users", ["email"], name: "index_users_on_email", unique: true, using: :btree
add_index "users", ["reset_password_token"], name: "index_users_on_reset_password_token", unique: true, using: :btree
end

Answer 1

Rails achieves find_by_sth magic by inferring an attribute name in your method call, thus returning an ActiveRecordRelation.
As you have no user_name attribute in your model, Active Record will not be able to perform a search on it (just imagine it has to be translated into an SQL query).
On the other hand, I'd re-think the logic of your user_name method if I were you. Consider persisting the user_name on your database.
By the way, in the second error you mention, notice that the example you have is working with a login attribute, which you simply don't have.