docs: add guide for using custom model names with acts_as helpers (#171)

While integrating RubyLLM with an existing Rails application that
already had a `Chat` model, I discovered that the framework supports
using custom model names with the `acts_as` helpers. This isn't
explicitly documented, but is a valuable feature for real-world Rails
applications where naming conflicts or existing conventions might exist.

Let me know if I'm missing something on this behaviour

---------

Co-authored-by: Carmine Paolino <carmine@paolino.me>

Author: matheuscumpian

docs/guides/rails.md

@@ -163,7 +163,7 @@ Include the RubyLLM helpers in your ActiveRecord models:
 class Chat < ApplicationRecord
   # Includes methods like ask, with_tool, with_instructions, etc.
   # Automatically persists associated messages and tool calls.
-  acts_as_chat # Assumes Message and ToolCall model names
+  acts_as_chat # Defaults to Message and ToolCall model names
 
   # --- Add your standard Rails model logic below ---
   belongs_to :user, optional: true # Example
@@ -173,7 +173,7 @@ end
 # app/models/message.rb
 class Message < ApplicationRecord
   # Provides methods like tool_call?, tool_result?
-  acts_as_message # Assumes Chat and ToolCall model names
+  acts_as_message # Defaults to Chat and ToolCall model names
 
   # --- Add your standard Rails model logic below ---
   # Note: Do NOT add "validates :content, presence: true"
@@ -187,7 +187,7 @@ end
 # app/models/tool_call.rb (Only if using tools)
 class ToolCall < ApplicationRecord
   # Sets up associations to the calling message and the result message.
-  acts_as_tool_call # Assumes Message model name
+  acts_as_tool_call # Defaults to Message model name
 
   # --- Add your standard Rails model logic below ---
 end
@@ -496,6 +496,45 @@ This setup allows for:
 
 Your `Chat`, `Message`, and `ToolCall` models are standard ActiveRecord models. You can add any other associations, validations, scopes, callbacks, or methods as needed for your application logic. The `acts_as` helpers provide the core persistence bridge to RubyLLM without interfering with other model behavior.
 
+You can use custom model names by passing parameters to the `acts_as` helpers. For example, if you prefer `Conversation` over `Chat`, you could use `acts_as_chat` in your `Conversation` model and then specify `chat_class: 'Conversation'` in your `Message` model's `acts_as_message` call.
+
+### Using Custom Model Names
+
+If your application uses different model names, you can configure the `acts_as` helpers accordingly:
+
+```ruby
+# app/models/conversation.rb (instead of Chat)
+class Conversation < ApplicationRecord
+  # Specify custom model names if needed (not required if your models
+  # are called Message and ToolCall)
+  acts_as_chat message_class: 'ChatMessage', tool_call_class: 'AIToolCall'
+  
+  belongs_to :user, optional: true
+  # ... your custom logic
+end
+
+# app/models/chat_message.rb (instead of Message)
+class ChatMessage < ApplicationRecord
+  # Let RubyLLM know to use your Conversation model instead of the default Chat
+  acts_as_message chat_class: 'Conversation', tool_call_class: 'AIToolCall'
+  # You can also customize foreign keys if needed:
+  # chat_foreign_key: 'conversation_id'
+  
+  # ... your custom logic
+end
+
+# app/models/ai_tool_call.rb (instead of ToolCall)
+class AIToolCall < ApplicationRecord 
+  acts_as_tool_call message_class: 'ChatMessage'
+  # Optionally customize foreign keys:
+  # message_foreign_key: 'chat_message_id'
+  
+  # ... your custom logic
+end
+```
+
+This flexibility allows you to integrate RubyLLM with existing Rails applications that may already have naming conventions established.
+
 Some common customizations include:
 
 ```ruby