controller

class UserpostsController < ApplicationController
  # GET /userposts
  # GET /userposts.xml
  def index
    @userposts = Userpost.all

    respond_to do |format|
      format.html # index.html.erb
      format.xml  { render :xml => @userposts }
    end
  end

  # GET /userposts/1
  # GET /userposts/1.xml
  def show
    @userpost = Userpost.find(params[:id])

    respond_to do |format|
      format.html # show.html.erb
      format.xml  { render :xml => @userpost }
    end
  end

  # GET /userposts/new
  # GET /userposts/new.xml
  def new
    @userpost = Userpost.new

    respond_to do |format|
      format.html # new.html.erb
      format.xml  { render :xml => @userpost }
    end
  end

  # GET /userposts/1/edit
  def edit
    @userpost = Userpost.find(params[:id])
  end

  # POST /userposts
  # POST /userposts.xml
  def create
    @user = User.find(params[:user_id])
    @userpost = @user.userposts.create(params[:userpost])
    redirect_to user_path(@user)
  end

  # PUT /userposts/1
  # PUT /userposts/1.xml
  def update
    @userpost = Userpost.find(params[:id])

    respond_to do |format|
      if @userpost.update_attributes(params[:userpost])
        format.html { redirect_to(@userpost, :notice => 'Userpost was successfully updated.') }
        format.xml  { head :ok }
      else
        format.html { render :action => "edit" }
        format.xml  { render :xml => @userpost.errors, :status => :unprocessable_entity }
      end
    end
  end

  # DELETE /userposts/1
  # DELETE /userposts/1.xml
  def destroy
    @userpost = Userpost.find(params[:id])
    @userpost.destroy

    respond_to do |format|
      format.html { redirect_to(userposts_url) }
      format.xml  { head :ok }
    end
  end
end

routes

LoginApp::Application.routes.draw do
  
  resources :gymposts

  resources :userposts

  resources :gyms, :order => 'totalBench DESC'

  resources :users, :user_sessions
  match 'login' => 'user_sessions#new', :as => :login
  match 'logout' => 'user_sessions#destroy', :as => :logout
  
  resources :gyms do
    resources :gymposts, :users do
      resources :userposts do
      end
  end
end
  get '/leaderboard', :to => 'users#leaderboard'
end

show

<p id="notice"><%= notice %></p>

<p>
  <b>Username:</b>
  <%= @user.username %>
</p>

<p>
  <b>Email:</b>
  <%= @user.email %>
</p>
<!--
<p>
  <b>Crypted password:</b>
  <%= @user.crypted_password %>
</p>

<p>
  <b>Password salt:</b>
  <%= @user.password_salt %>
</p>

<p>
  <b>Persistence token:</b>
  <%= @user.persistence_token %>
</p>
-->
<p>
  <b>Age:</b>
  <%= @user.age %>
</p>

<p>
  <b>Weight:</b>
  <%= @user.weight %>
</p>

<p>
  <b>Max bench:</b>
  <%= @user.maxBench %>
</p>

<p>
  <b>Max Squat:</b>
  <%= @user.maxSquat %>
</p>

<p>
  <b>Gym:</b>
  <%= @user.gym.name %>
</p>

<p>
<%= image_tag @user.photo.url(:small) %>
</p>


<%= link_to 'Edit', edit_user_path(@user) %> |
<%= link_to 'Back', users_path %>

<h1>Listing userposts</h1>

<table>
  <tr>
    <th>Body</th>
    <th></th>
    <th></th>
    <th></th>
  </tr>

<% @user.userposts.each do |userpost| %>
  <tr>
    <td><%= userpost.body %></td>
    <td><%= link_to 'Show', userpost %></td>
    <td><%= link_to 'Edit', edit_userpost_path(userpost) %></td>
    <td><%= link_to 'Destroy', userpost, :confirm => 'Are you sure?', :method => :delete %></td>
  </tr>
<% end %>
</table>

<br />

<h2>Add a comment:</h2>
<%= form_for [@current_user, Userpost.new] do |f| %>

  <div class="field">
    <%= f.label :body %><br />
    <%= f.text_area :body %>
    <%= f.hidden_field :user_id %>
  </div>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

trace

activerecord (3.0.5) lib/active_record/relation/finder_methods.rb:279:in `find_with_ids'
activerecord (3.0.5) lib/active_record/relation/finder_methods.rb:107:in `find'
activerecord (3.0.5) lib/active_record/base.rb:439:in `__send__'
activerecord (3.0.5) lib/active_record/base.rb:439:in `find'
app/controllers/userposts_controller.rb:43:in `create'
actionpack (3.0.5) lib/action_controller/metal/implicit_render.rb:4:in `send_action'
actionpack (3.0.5) lib/action_controller/metal/implicit_render.rb:4:in `send_action'
actionpack (3.0.5) lib/abstract_controller/base.rb:150:in `process_action'
actionpack (3.0.5) lib/action_controller/metal/rendering.rb:11:in `process_action'
actionpack (3.0.5) lib/abstract_controller/callbacks.rb:18:in `process_action'
activesupport (3.0.5) lib/active_support/callbacks.rb:440:in `_run__961832392__process_action__1623385099__callbacks'
activesupport (3.0.5) lib/active_support/callbacks.rb:409:in `send'
activesupport (3.0.5) lib/active_support/callbacks.rb:409:in `_run_process_action_callbacks'
activesupport (3.0.5) lib/active_support/callbacks.rb:93:in `send'
activesupport (3.0.5) lib/active_support/callbacks.rb:93:in `run_callbacks'
actionpack (3.0.5) lib/abstract_controller/callbacks.rb:17:in `process_action'
actionpack (3.0.5) lib/action_controller/metal/instrumentation.rb:30:in `process_action'
activesupport (3.0.5) lib/active_support/notifications.rb:52:in `instrument'
activesupport (3.0.5) lib/active_support/notifications/instrumenter.rb:21:in `instrument'
activesupport (3.0.5) lib/active_support/notifications.rb:52:in `instrument'
actionpack (3.0.5) lib/action_controller/metal/instrumentation.rb:29:in `process_action'
actionpack (3.0.5) lib/action_controller/metal/rescue.rb:17:in `process_action'
actionpack (3.0.5) lib/abstract_controller/base.rb:119:in `process'
actionpack (3.0.5) lib/abstract_controller/rendering.rb:41:in `process'
actionpack (3.0.5) lib/action_controller/metal.rb:138:in `dispatch'
actionpack (3.0.5) lib/action_controller/metal/rack_delegation.rb:14:in `dispatch'
actionpack (3.0.5) lib/action_controller/metal.rb:178:in `action'
actionpack (3.0.5) lib/action_dispatch/routing/route_set.rb:62:in `call'
actionpack (3.0.5) lib/action_dispatch/routing/route_set.rb:62:in `dispatch'
actionpack (3.0.5) lib/action_dispatch/routing/route_set.rb:27:in `call'
rack-mount (0.6.13) lib/rack/mount/route_set.rb:148:in `call'
rack-mount (0.6.13) lib/rack/mount/code_generation.rb:93:in `recognize'
rack-mount (0.6.13) lib/rack/mount/code_generation.rb:68:in `optimized_each'
rack-mount (0.6.13) lib/rack/mount/code_generation.rb:92:in `recognize'
rack-mount (0.6.13) lib/rack/mount/route_set.rb:139:in `call'
actionpack (3.0.5) lib/action_dispatch/routing/route_set.rb:492:in `call'
actionpack (3.0.5) lib/action_dispatch/middleware/best_standards_support.rb:17:in `call'
actionpack (3.0.5) lib/action_dispatch/middleware/head.rb:14:in `call'
rack (1.2.1) lib/rack/methodoverride.rb:24:in `call'
actionpack (3.0.5) lib/action_dispatch/middleware/params_parser.rb:21:in `call'
actionpack (3.0.5) lib/action_dispatch/middleware/flash.rb:182:in `call'
actionpack (3.0.5) lib/action_dispatch/middleware/session/abstract_store.rb:149:in `call'
actionpack (3.0.5) lib/action_dispatch/middleware/cookies.rb:302:in `call'
activerecord (3.0.5) lib/active_record/query_cache.rb:32:in `call'
activerecord (3.0.5) lib/active_record/connection_adapters/abstract/query_cache.rb:28:in `cache'
activerecord (3.0.5) lib/active_record/query_cache.rb:12:in `cache'
activerecord (3.0.5) lib/active_record/query_cache.rb:31:in `call'
activerecord (3.0.5) lib/active_record/connection_adapters/abstract/connection_pool.rb:354:in `call'
actionpack (3.0.5) lib/action_dispatch/middleware/callbacks.rb:46:in `call'
activesupport (3.0.5) lib/active_support/callbacks.rb:415:in `_run_call_callbacks'
actionpack (3.0.5) lib/action_dispatch/middleware/callbacks.rb:44:in `call'
rack (1.2.1) lib/rack/sendfile.rb:107:in `call'
actionpack (3.0.5) lib/action_dispatch/middleware/remote_ip.rb:48:in `call'
actionpack (3.0.5) lib/action_dispatch/middleware/show_exceptions.rb:47:in `call'
railties (3.0.5) lib/rails/rack/logger.rb:13:in `call'
rack (1.2.1) lib/rack/runtime.rb:17:in `call'
activesupport (3.0.5) lib/active_support/cache/strategy/local_cache.rb:72:in `call'
rack (1.2.1) lib/rack/lock.rb:11:in `call'
rack (1.2.1) lib/rack/lock.rb:11:in `synchronize'
rack (1.2.1) lib/rack/lock.rb:11:in `call'
actionpack (3.0.5) lib/action_dispatch/middleware/static.rb:30:in `call'
railties (3.0.5) lib/rails/application.rb:168:in `call'
railties (3.0.5) lib/rails/application.rb:77:in `send'
railties (3.0.5) lib/rails/application.rb:77:in `method_missing'
railties (3.0.5) lib/rails/rack/log_tailer.rb:14:in `call'
rack (1.2.1) lib/rack/content_length.rb:13:in `call'
rack (1.2.1) 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 (1.2.1) lib/rack/handler/webrick.rb:13:in `run'
rack (1.2.1) lib/rack/server.rb:213:in `start'
railties (3.0.5) lib/rails/commands/server.rb:65:in `start'
railties (3.0.5) lib/rails/commands.rb:30
railties (3.0.5) lib/rails/commands.rb:27:in `tap'
railties (3.0.5) lib/rails/commands.rb:27
script/rails:6:in `require'
script/rails:6

http://ruby.railstutorial.org/ruby-on-rails-tutorial-book

@userpost = Userpost.create(params[:userpost])
@user.userposts << @userpost

This is the reason it wasn't working: <%= f.hidden_field :user_id %>
You're setting a hidden field user_id, which is nil, because you're not setting it to anything.
Then when you @user.userposts.create(params[:userposts]), the params[:userposts][:user_id] (nil) overrides the user_id that would usually get set by association. Take out that hidden field, and you should be able to create it like you originally did: @user.userposts.create(params[:userpost])

Cheers,
Edward

Also, did you understand why the routes needed to change?

aaaaah i see. I think the routes were the main problem. Adding the hidden field was a desperate attempt to fix the error. I probably tried a hundred other things before that. I should have went right for the routes!