Rails: Updating Nested attributes - undefined method `to_sym' for nil:NilClass - ruby-on-rails

Question

Edit: Added the update action, and on what line the error occurs
Model:
class Match < ActiveRecord::Base
has_and_belongs_to_many :teams
has_many :match_teams
has_many :teams, :through => :match_teams
accepts_nested_attributes_for :match_teams, :allow_destroy => true
end
Controller:
def new
#match = Match.new
#match_teams = 2.times do
#match.match_teams.build
end
respond_to do |format|
format.html # new.html.erb
format.json { render json: #match }
end
end
def update
#match = Match.find(params[:id])
respond_to do |format|
if #match.update_attributes(params[:match])
format.html { redirect_to #match, notice: 'Match was successfully updated.' }
format.json { head :ok }
else
format.html { render action: "edit" }
format.json { render json: #match.errors, status: :unprocessable_entity }
end
end
end
Nested model:
class MatchTeam < ActiveRecord::Base
belongs_to :match
belongs_to :team
end
Association:
class Team < ActiveRecord::Base
has_and_belongs_to_many :matches
end
View:
<%= form_for(#match) do |f| %>
<%= f.fields_for :match_teams, #match_teams do |builder| %>
<%= builder.collection_select :team_id, Team.all, :id, :name, :include_blank => true %>
<% end %>
<% unless #match.new_record? %>
<div class="field">
<%= f.label :winning_team_id %><br />
<%= f.collection_select :winning_team_id, #match.teams, :id, :representation %>
</div>
<% end %>
<div class="actions">
<%= f.submit %>
</div>
<% end %>
Params:
Processing by MatchesController#update as HTML
Parameters: {"utf8"=>"Ô£ô", "authenticity_token"=>"QIJChzkYOPZ1hxbzTZS8H3AXc7i
BzkKv3Z5daRmlOsQ=", "match"=>{"match_teams_attributes"=>{"0"=>{"team_id"=>"1", "
id"=>""}, "1"=>{"team_id"=>"3", "id"=>""}}, "winning_team_id"=>"3"}, "commit"=>"
Update Match", "id"=>"2"}
Creating a new match with 2 teams work fine, the edit view also shows the correct values, but the update action gives me this error.
undefined method `to_sym' for nil:NilClass
app/controllers/matches_controller.rb:65:in `block in update'
line 65: if #match.update_attributes(params[:match])

Answer 1

I've figured it out. I read that a join table like MatchTeams doesn't need an ID. I'm guessing this is true when not doing any nested forms. I redid my migration removing the exclusion of the id column, and now everything works fine. Don't we all love this stupid errors? :)

Answer 2

Without seeing the offending to_sym in your code, just know that the thing it's attached to has not been defined properly. If this is a variable such as #var.to_sym, you most likely:
Haven't set #var at all
Set it but it's returning nil because there are no matches (e.g. #var = #project.companies.first but #project has no companies tied to it).
You are missing a relevant bit of data in your params. If your to_sym is relying on data submitted through the form, it won't work if the user leaves out the bit of data you're assuming. In this case, you should test first to see if the data was entered before running .to_sym on it.