Create followable concern, follow model. Add followable to proposal model.

app/controllers/follows_controller.rb

@@ -0,0 +1,28 @@
+class FollowsController < ApplicationController
+  before_action :authenticate_user!
+  load_and_authorize_resource
+
+  def create
+    @followable = find_followable
+    @follow = Follow.create(user: current_user, followable: @followable)
+    render :refresh_follow_button
+  end
+
+  def destroy
+    @follow = Follow.find(params[:id])
+    @followable = @follow.followable
+    @follow.destroy
+    render :refresh_follow_button
+  end
+
+  private
+
+  def find_followable
+    params.each do |name, value|
+      if name =~ /(.+)_id$/
+        return $1.classify.constantize.find(value)
+      end
+    end
+    nil
+  end
+end