Editing a single attribute with more than one form field - ruby-on-rails

Question

Background: Users and communities share a
has_many :through
relationship. Each community has a "community_type" string that identifies it (ie "Gender", "City", etc.).
Objective: In my edit form, I'd like to allow the user to edit his :community_ids based on community type. Something like:
<%= form_for current_user do |f| %>
<%= f.collection_select(:community_ids, Community.filtered_by("Gender"), :id, :name) %>
<%= f.collection_select(:community_ids, Community.filtered_by("City"), :id, :name) %>
<% end %>
The issue is that the form only accepts the last form field value for :community_ids - in this case being the "City" - rather than merging all of them as one big :community_ids array.
Solution:
For those interested, I ended up refactoring my model code from the answers below to this:
%W[ community1 community2 community3 community4 ].each do |name|
define_method "#{name}" do
self.communities.filtered_by("#{name}").map(&:name)
end
define_method "#{name}_ids" do
self.communities.filtered_by("#{name}").map(&:id)
end
define_method "#{name}_ids=" do |val|
self.community_ids += val
end
end

Answer 1

Is there a reason you're using select boxes for a has_many relationship? It seems checkboxes would be more appropriate. If you want to go with select boxes, I don't think you can use FormHelper#select, because as far as I know, it's expecting a single value, and your community_ids is an array. This is why it's only picking one of the values you give it.
For a select box (or any field), you can combine the values across fields by adding [] to the parameter name which tells Rails that the parameter is an array of values. You can do this by using regular select_tag to create the fields, and setting the parameter name as follows:
<%= form_for current_user do |f| %>
<%= select_tag("user[community_ids][]", options_for_select(Community.filtered_by("Gender").map{|c| [c.name, c.id]}, :selected => current_user.communities.filtered_by("Gender").first.id) %>
<%= select_tag("user[community_ids][]", options_for_select(Community.filtered_by("City").map{|c| [c.name, c.id]}, :selected => current_user.communities.filtered_by("City").first.id) %>
<% end %>
You could also go with Ryan's approach of sending separate parameters, though one downside is your User model will have to be very aware of the types of communities that exist, and you'll have to write separate logic in the User model for each type of community. This will make your resources less modular. But if you do go that way, I'd suggest using pseudo-attributes instead of a before_save so that your community_ids get updated automatically from the params:
class User < ActiveRecord::Base
...
def community_gender_ids=(cg_ids)
self.community_ids ||= []
self.community_ids += cg_ids
end
def community_city_ids=(cc_ids)
self.community_ids ||= []
self.community_ids += cc_ids
end
...
end
And then your select_tag calls would look something like this:
<%= form_for current_user do |f| %>
<%= select_tag("user[community_gender_ids][]", options_for_select(Community.filtered_by("Gender").map{|c| [c.name, c.id]}, :selected => current_user.communities.filtered_by("Gender").first.id) %>
<%= select_tag("user[community_city_ids][]", options_for_select(Community.filtered_by("City").map{|c| [c.name, c.id]}, :selected => current_user.communities.filtered_by("City").first.id) %>
<% end %>

Answer 2

Updated to complete tsherif's (better than my original) answer.
view.rb
<%= form_for current_user do |f| %>
<%= f.collection_select(:community_gender_ids, Community.filtered_by("Gender"), :id, :name, {}, id: 'community-gender-options') %>
<%= f.collection_select(:community_city_ids, Community.filtered_by("City"), :id, :name, {}, id: 'community-city-options') %>
<% end %>
model.rb
def community_gender_ids=(cg_ids)
self.community_ids ||= []
self.community_ids += cg_ids
end
def community_city_ids=(cc_ids)
self.community_ids ||= []
self.community_ids += cc_ids
end
def community_gender_ids
self.communities.select(:id).where(:community_type => 'gender').map(&:id)
end
def community_city_ids
self.communities.select(:id).where(:community_type => 'city').map(&:id)
end
Alternatively, you could write some CoffeeScript/Javascript to bind to the select tags and add the IDs to a hidden value which is then submitted to the server with the form.