Rails: Unable to destroy association - ruby-on-rails

Question

In my app, Users and Conversations are tied through a has_many :through relationship, the association table being conversation_users.
How do I delete an association (a User leaves the Conversation) from the Conversations controller? I tried:
def destroy_assoc
Conversation.find(params[:id]).conversation_users.find_by_user_id(current_user.id).destroy
redirect_to :back
end
But I got a undefined method 'eq' for nil:NilClass error. Here is the log when I call that action:
Processing by ConversationsController#destroy_assoc as HTML
Parameters: {"authenticity_token"=>"8AT1wx5MUVz1Gsqpp+u+z0vk8uj4F0mAWz5wpn+CjCU=", "username"=>"albert", "id"=>"14"}
User Load (0.2ms) SELECT "users".* FROM "users" WHERE "users"."id" = 8 LIMIT 1
Conversation Load (0.2ms) SELECT "conversations".* FROM "conversations" WHERE "conversations"."id" = 14 LIMIT 1
ConversationUser Load (0.2ms) SELECT "conversation_users".* FROM "conversation_users" WHERE "conversation_users"."user_id" = 8 AND ("conversation_users".conversation_id = 14) LIMIT 1
Completed 500 Internal Server Error in 109ms
NoMethodError (undefined method 'eq' for nil:NilClass):
app/controllers/conversations_controller.rb:34:in `destroy_assoc'
Rendered /Library/Ruby/Gems/1.8/gems/actionpack-3.0.9/lib/action_dispatch/middleware/templates/rescues/_trace.erb (1.0ms)
Rendered /Library/Ruby/Gems/1.8/gems/actionpack-3.0.9/lib/action_dispatch/middleware/templates/rescues/_request_and_response.erb (7.1ms)
Rendered /Library/Ruby/Gems/1.8/gems/actionpack-3.0.9/lib/action_dispatch/middleware/templates/rescues/diagnostics.erb within rescues/layout (12.9ms)
The record is found, but destroy raises an error. What am I doing wrong?
Thanks.
Full stack trace:
activesupport (3.0.9) lib/active_support/whiny_nil.rb:48:in `method_missing'
activerecord (3.0.9) lib/active_record/persistence.rb:83:in `destroy'
activerecord (3.0.9) lib/active_record/locking/optimistic.rb:110:in `destroy'
activerecord (3.0.9) lib/active_record/callbacks.rb:256:in `destroy'
activesupport (3.0.9) lib/active_support/callbacks.rb:414:in `_run_destroy_callbacks'
activerecord (3.0.9) lib/active_record/callbacks.rb:256:in `destroy'
activerecord (3.0.9) lib/active_record/transactions.rb:235:in `destroy'
activerecord (3.0.9) lib/active_record/transactions.rb:292:in `with_transaction_returning_status'
activerecord (3.0.9) lib/active_record/connection_adapters/abstract/database_statements.rb:139:in `transaction'
activerecord (3.0.9) lib/active_record/transactions.rb:207:in `transaction'
activerecord (3.0.9) lib/active_record/transactions.rb:290:in `with_transaction_returning_status'
activerecord (3.0.9) lib/active_record/transactions.rb:235:in `destroy'
app/controllers/conversations_controller.rb:34:in `destroy_assoc'
actionpack (3.0.9) lib/action_controller/metal/implicit_render.rb:4:in `send_action'
actionpack (3.0.9) lib/action_controller/metal/implicit_render.rb:4:in `send_action'
actionpack (3.0.9) lib/abstract_controller/base.rb:150:in `process_action'
actionpack (3.0.9) lib/action_controller/metal/rendering.rb:11:in `process_action'
actionpack (3.0.9) lib/abstract_controller/callbacks.rb:18:in `process_action'
activesupport (3.0.9) lib/active_support/callbacks.rb:441:in `_run__2060746170__process_action__2040097307__callbacks'
activesupport (3.0.9) lib/active_support/callbacks.rb:410:in `send'
activesupport (3.0.9) lib/active_support/callbacks.rb:410:in `_run_process_action_callbacks'
activesupport (3.0.9) lib/active_support/callbacks.rb:94:in `send'
activesupport (3.0.9) lib/active_support/callbacks.rb:94:in `run_callbacks'
actionpack (3.0.9) lib/abstract_controller/callbacks.rb:17:in `process_action'
actionpack (3.0.9) lib/action_controller/metal/instrumentation.rb:30:in `process_action'
activesupport (3.0.9) lib/active_support/notifications.rb:52:in `instrument'
activesupport (3.0.9) lib/active_support/notifications/instrumenter.rb:21:in `instrument'
activesupport (3.0.9) lib/active_support/notifications.rb:52:in `instrument'
actionpack (3.0.9) lib/action_controller/metal/instrumentation.rb:29:in `process_action'
actionpack (3.0.9) lib/action_controller/metal/rescue.rb:17:in `process_action'
actionpack (3.0.9) lib/abstract_controller/base.rb:119:in `process'
actionpack (3.0.9) lib/abstract_controller/rendering.rb:41:in `process'
actionpack (3.0.9) lib/action_controller/metal.rb:138:in `dispatch'
actionpack (3.0.9) lib/action_controller/metal/rack_delegation.rb:14:in `dispatch'
actionpack (3.0.9) lib/action_controller/metal.rb:178:in `action'
actionpack (3.0.9) lib/action_dispatch/routing/route_set.rb:62:in `call'
actionpack (3.0.9) lib/action_dispatch/routing/route_set.rb:62:in `dispatch'
actionpack (3.0.9) lib/action_dispatch/routing/route_set.rb:27:in `call'
rack-mount (0.6.14) lib/rack/mount/route_set.rb:148:in `call'
rack-mount (0.6.14) lib/rack/mount/code_generation.rb:93:in `recognize'
rack-mount (0.6.14) lib/rack/mount/code_generation.rb:82:in `optimized_each'
rack-mount (0.6.14) lib/rack/mount/code_generation.rb:92:in `recognize'
rack-mount (0.6.14) lib/rack/mount/route_set.rb:139:in `call'
actionpack (3.0.9) lib/action_dispatch/routing/route_set.rb:493:in `call'
actionpack (3.0.9) lib/action_dispatch/middleware/best_standards_support.rb:17:in `call'
actionpack (3.0.9) lib/action_dispatch/middleware/head.rb:14:in `call'
rack (1.2.3) lib/rack/methodoverride.rb:24:in `call'
actionpack (3.0.9) lib/action_dispatch/middleware/params_parser.rb:21:in `call'
actionpack (3.0.9) lib/action_dispatch/middleware/flash.rb:182:in `call'
actionpack (3.0.9) lib/action_dispatch/middleware/session/abstract_store.rb:149:in `call'
actionpack (3.0.9) lib/action_dispatch/middleware/cookies.rb:302:in `call'
activerecord (3.0.9) lib/active_record/query_cache.rb:32:in `call'
activerecord (3.0.9) lib/active_record/connection_adapters/abstract/query_cache.rb:28:in `cache'
activerecord (3.0.9) lib/active_record/query_cache.rb:12:in `cache'
activerecord (3.0.9) lib/active_record/query_cache.rb:31:in `call'
activerecord (3.0.9) lib/active_record/connection_adapters/abstract/connection_pool.rb:354:in `call'
actionpack (3.0.9) lib/action_dispatch/middleware/callbacks.rb:46:in `call'
activesupport (3.0.9) lib/active_support/callbacks.rb:416:in `_run_call_callbacks'
actionpack (3.0.9) lib/action_dispatch/middleware/callbacks.rb:44:in `call'
rack (1.2.3) lib/rack/sendfile.rb:107:in `call'
actionpack (3.0.9) lib/action_dispatch/middleware/remote_ip.rb:48:in `call'
actionpack (3.0.9) lib/action_dispatch/middleware/show_exceptions.rb:47:in `call'
railties (3.0.9) lib/rails/rack/logger.rb:13:in `call'
rack (1.2.3) lib/rack/runtime.rb:17:in `call'
activesupport (3.0.9) lib/active_support/cache/strategy/local_cache.rb:72:in `call'
rack (1.2.3) lib/rack/lock.rb:11:in `call'
rack (1.2.3) lib/rack/lock.rb:11:in `synchronize'
rack (1.2.3) lib/rack/lock.rb:11:in `call'
actionpack (3.0.9) lib/action_dispatch/middleware/static.rb:30:in `call'
railties (3.0.9) lib/rails/application.rb:168:in `call'
railties (3.0.9) lib/rails/application.rb:77:in `send'
railties (3.0.9) lib/rails/application.rb:77:in `method_missing'
railties (3.0.9) lib/rails/rack/log_tailer.rb:14:in `call'
rack (1.2.3) lib/rack/content_length.rb:13:in `call'
rack (1.2.3) lib/rack/handler/webrick.rb:52:in `service'
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/webrick/httpserver.rb:104:in `service'
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/webrick/httpserver.rb:65:in `run'
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/webrick/server.rb:173:in `start_thread'
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/webrick/server.rb:162:in `start'
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/webrick/server.rb:162:in `start_thread'
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/webrick/server.rb:95:in `start'
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/webrick/server.rb:92:in `each'
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/webrick/server.rb:92:in `start'
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/webrick/server.rb:23:in `start'
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/webrick/server.rb:82:in `start'
rack (1.2.3) lib/rack/handler/webrick.rb:13:in `run'
rack (1.2.3) lib/rack/server.rb:217:in `start'
railties (3.0.9) lib/rails/commands/server.rb:65:in `start'
railties (3.0.9) lib/rails/commands.rb:30
railties (3.0.9) lib/rails/commands.rb:27:in `tap'
railties (3.0.9) lib/rails/commands.rb:27
script/rails:6:in `require'
script/rails:6

Answer 1

Check the answer to this question:
undefined method `eq' for nil:NilClass with rails 3 and ruby enterprise on ubuntu hardy
If you have no ID field in your association class, it can't be treated as a model, it appears. Are you using a a :has_and_belongs_to_many association with a join table without a primary key id?

Answer 2

What about
#conversation = current_user.conversation_users.find_by_conversation_id(params[:id])
#conversation.destroy
I guess
User has_many :conversation_users
User has_many :conversations :through => :conversation_users
Does that work? I think it's a cleaner approach at least, letting AR do the job.