feat: Allow setting a cool id on a different column (#5)

README.md

@@ -35,9 +35,26 @@ and generate ids without creating a record
 # generate an id, e.g. for batch inserts or upserts
 User.generate_cool_id
 # => "usr_vktd1b5v84lr"
+```
+
+you can use cool_id with a separate field, keeping the default primary key:
+
+```ruby
+class Product < ActiveRecord::Base
+  include CoolId::Model
+  cool_id prefix: "prd", id_field: :public_id
+end
 
+product = Product.create!(name: "Cool Product")
+product.id  # => 1 (or another integer)
+product.public_id  # => "prd_vktd1b5v84lr"
+
+# You can still use CoolId.locate with the public_id
+CoolId.locate("prd_vktd1b5v84lr")  # => #<Product id: 1, public_id: "prd_vktd1b5v84lr", name: "Cool Product">
 ```
 
+this approach allows you to keep your primary key as an auto-incrementing integer while still benefiting from CoolId's functionality. it's particularly useful when you want to expose a public identifier that's separate from your internal primary key.
+
 it takes parameters to change the alphabet or length
 
 ```ruby

lib/cool_id.rb

@@ -15,10 +15,10 @@ class MaxRetriesExceededError < StandardError; end
   DEFAULT_LENGTH = 12
   DEFAULT_MAX_RETRIES = 1000
 
-  Id = Struct.new(:key, :prefix, :id, :model_class)
+  Id = Struct.new(:key, :prefix, :id, :model_class, :id_field)
 
   class << self
-    attr_accessor :separator, :alphabet, :length, :max_retries
+    attr_accessor :separator, :alphabet, :length, :max_retries, :id_field
 
     def configure
       yield self
@@ -29,6 +29,7 @@ def reset_configuration
       self.alphabet = DEFAULT_ALPHABET
       self.length = DEFAULT_LENGTH
       self.max_retries = DEFAULT_MAX_RETRIES
+      self.id_field = nil
     end
 
     def registry
@@ -54,6 +55,10 @@ def generate_id(config)
         end
       end
     end
+
+    def resolve_cool_id_field(model_class)
+      model_class.cool_id_config&.id_field || CoolId.id_field || model_class.primary_key
+    end
   end
 
   self.separator = DEFAULT_SEPARATOR
@@ -72,26 +77,31 @@ def register(prefix, model_class)
 
     def locate(id)
       parsed = parse(id)
-      parsed&.model_class&.find_by(id: id)
+      return nil unless parsed
+
+      id_field = CoolId.resolve_cool_id_field(parsed.model_class)
+      parsed.model_class.find_by(id_field => id)
     end
 
     def parse(id)
       prefix, key = id.split(CoolId.separator, 2)
       model_class = @prefix_map[prefix]
       return nil unless model_class
-      Id.new(key, prefix, id, model_class)
+      id_field = CoolId.resolve_cool_id_field(model_class)
+      Id.new(key, prefix, id, model_class, id_field)
     end
   end
 
   class Config
-    attr_reader :prefix, :length, :alphabet, :max_retries, :model_class
+    attr_reader :prefix, :length, :alphabet, :max_retries, :model_class, :id_field
 
-    def initialize(prefix:, model_class:, length: nil, alphabet: nil, max_retries: nil)
+    def initialize(prefix:, model_class:, length: nil, alphabet: nil, max_retries: nil, id_field: nil)
       @length = length
       @prefix = validate_prefix(prefix)
       @alphabet = validate_alphabet(alphabet)
       @max_retries = max_retries
       @model_class = model_class
+      @id_field = id_field
     end
 
     private
@@ -148,7 +158,8 @@ def inherited(subclass)
       private
 
       def set_cool_id
-        self.id = self.class.generate_cool_id if id.blank?
+        id_field = CoolId.resolve_cool_id_field(self.class)
+        self[id_field] = self.class.generate_cool_id if self[id_field].blank?
       end
 
       def ensure_cool_id_configured

spec/cool_id_spec.rb

@@ -4,16 +4,6 @@
 
 require "active_record"
 
-class User < ActiveRecord::Base
-  include CoolId::Model
-  cool_id prefix: "usr"
-end
-
-class Customer < ActiveRecord::Base
-  include CoolId::Model
-  cool_id prefix: "cus", alphabet: "ABCDEFGHIJKLMNOPQRSTUVWXYZ", length: 8, max_retries: 500
-end
-
 RSpec.describe CoolId do
   before(:each) do
     CoolId.reset_configuration
@@ -101,6 +91,16 @@ def self.exists?(id:)
   end
 
   describe CoolId::Model do
+    class User < ActiveRecord::Base
+      include CoolId::Model
+      cool_id prefix: "usr"
+    end
+
+    class Customer < ActiveRecord::Base
+      include CoolId::Model
+      cool_id prefix: "cus", alphabet: "ABCDEFGHIJKLMNOPQRSTUVWXYZ", length: 8, max_retries: 500
+    end
+
     before(:all) do
       ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
     end
@@ -219,6 +219,30 @@ class LimitedRetryModel < ActiveRecord::Base
       expect(CoolId.locate(user.id)).to eq(user)
       expect(CoolId.locate(customer.id)).to eq(customer)
     end
+
+    it "generates a cool_id for a custom id field" do
+      class Product < ActiveRecord::Base
+        include CoolId::Model
+        cool_id prefix: "prd", id_field: :public_id
+      end
+
+      ActiveRecord::Schema.define do
+        create_table :products do |t|
+          t.string :public_id
+          t.string :name
+        end
+      end
+
+      product = Product.create!(name: "Cool Product")
+      expect(product.id).to be_a(Integer)
+      expect(product.public_id).to match(/^prd_[0-9a-z]{12}$/)
+      expect(product.id).not_to match(/^prd_[0-9a-z]{12}$/)
+
+      located_product = CoolId.locate(product.public_id)
+      expect(located_product).to eq(product)
+
+      ActiveRecord::Base.connection.drop_table :products
+    end
   end
 
   describe "ensure_cool_id_setup behavior" do