NoMethodError on collection_select input - ruby-on-rails

Question

I have been trying different things to fix this and it's driving me nuts...
I have an input field:
<%= collection_select(:stakeholder, :user_id, User.all, :id, :first_name) %>
Each stakeholder has a user_id attribute, which is an integer.
When I try to render the form, I get this NoMethodError:
undefined method `id' for 2:Fixnum
Any ideas?
Edit: adding full form view
<%= simple_form_for([#project, #stakeholder]) do |f| %>
<%= f.error_notification %>
<div class="form-inputs">
<%= f.input :first_name %>
<%= f.input :last_name %>
<%= f.input :email %>
<%= f.input :address %>
<%= f.input :city %>
<%= f.input :sentiment, %>
<%= f.label :tags, "Tags (separated by commas)" %>
<%= collection_select(:stakeholder, :user_id, User.all, :id, :first_name) %>
<%= f.button :submit %>
</div>
user.rb
class User < ActiveRecord::Base
belongs_to :account
has_many :stakeholders
def set_default_role
self.role ||= :user
end
# Include default devise modules. Others available are:
# :confirmable, :lockable, :timeoutable and :omniauthable
devise :invitable, :database_authenticatable, :registerable, :confirmable,
:recoverable, :rememberable, :trackable, :validatable
end
stakeholder.rb
class Stakeholder < ActiveRecord::Base
belongs_to :project
has_many :notes
belongs_to :user
acts_as_taggable
def full_name
full_name = "#{first_name.capitalize} #{last_name.capitalize}"
end
end

Answer 1

Take a look at this documentation.
collection_select(object, method, collection, value_method,
text_method, options = {}, html_options = {}) public
Returns <select> and <option> tags for the collection of existing
return values of method for object's class. The value returned from
calling method on the instance object will be selected.
You seem to have an extra argument in your collection_select tag which is causing Rails to call :id as a method when it's actually a number (2).
user_id is a field in some Stakeholder object, right? So unless I'm misunderstanding how you've set up your relationships, stakeholder isn't relevant (in the sense that all you want is a user ID from your form).
This line should get you what you want.
<%= f.collection_select(:user_id, #users, :id, :first_name)