Rails - Form problems

In my app I have 3 models like that:

class Person
has_many :groups
has_many :group_memberships, :foreign_key => "member_id"
end

class Group_Membership
 belongs_to :member, :class_name => 'Person'
 belongs_to :group
end

class Group
belongs_to :person
has_many :group_memberships
has_many :members, :class_name => "Person", :through => "group_memberships", :foreign_key => "member_id"

What I need is to create a button in the group#show page, that allows the Person to ask for the group_membership. By suggestion from a previous question I made a def create_membership on my Group_Membership controller:

def create_membership
  @group = Group.find params[:id]
  if @group.group_memberships.create( :member_id => params[:member_id])
     redirect_to @group
  else
     render :action => 'show'
  end
end

So I moved on to my form, on the group#show view, and I did as follow:

<%= form_for :group_membership, @group_membership, :url => { :action => "create_membership" } do |f| %>
<%= submit_tag "Create" %>
<% end %>

But everytime I click on the button, it redirects me to the form to create a new Group, not Group_Membership. Any hints? Thanks in advance.

########EDIT##########

Well, I kinda changed my code to something like this:

def create
@person = Person.find(current_person)
@group = Group.find(params[:group_id])
params[:group_membership] = {:member_id => @person.id, :group_id => @group.id}
@group_membership = Group_Membership.create(params[:group_membership])
if @group_membership.save
redirect_to @person
else
redirect_to @person
end
end

And my form is right now like that:

<% form_for(:group_membership, :url => group_memberships_path(:member_id => current_person, :group_id => @group.id)) do |f| %> <%= submit_tag "Join!" %>

Now I'm able to actually create a group_membership model, but both id's are added blank in database column member_id and group_id.

If I change the line @group_membership = Group_Membership.create(params[:group_membership]) to @group_membership = Group.Group_Membership.create(params[:group_membership]), I'm able to get group id saved. Same goes if I change to Person..

just to clean up a bit

def create
  @group_membership = Group_Membership.create(:group => Group.find(params[:group_id]), :member => Person.find(current_person))
  if @group_membership.save
    redirect_to @person
  else
    redirect_to @person
  end
end

this code should work in rails 3 I'm not sure about earlier versions