Determine agent connections with cached COUNT queries

app/controllers/agents_controller.rb

@@ -6,7 +6,7 @@ class AgentsController < ApplicationController
   def index
     set_table_sort sorts: %w[name created_at last_check_at last_event_at last_receive_at], default: { created_at: :desc }
 
-    @agents = current_user.agents.preload(:scenarios, :controllers, :links_as_receiver, :links_as_source, :control_links_as_controller).reorder(table_sort).page(params[:page])
+    @agents = current_user.agents.preload(:scenarios, :controllers).reorder(table_sort).page(params[:page])
 
     if show_only_enabled_agents?
       @agents = @agents.where(disabled: false)

app/controllers/scenarios_controller.rb

@@ -26,7 +26,7 @@ class ScenariosController < ApplicationController
     @scenario = current_user.scenarios.find(params[:id])
 
     set_table_sort sorts: %w[name last_check_at last_event_at last_receive_at], default: { name: :asc }
-    @agents = @scenario.agents.preload(:scenarios, :controllers, :links_as_receiver, :links_as_source, :control_links_as_controller).reorder(table_sort).page(params[:page])
+    @agents = @scenario.agents.preload(:scenarios, :controllers).reorder(table_sort).page(params[:page])
 
     respond_to do |format|
       format.html

app/helpers/agent_helper.rb

@@ -58,10 +58,10 @@ module AgentHelper
     end
   end
 
-  def agent_type_icon(agent)
-    receiver_count = agent.links_as_receiver.length
-    control_count = agent.control_links_as_controller.length
-    source_count = agent.links_as_source.length
+  def agent_type_icon(agent, agents)
+    receiver_count = links_counter_cache(agents)[:links_as_receiver][agent.id] || 0
+    control_count  = links_counter_cache(agents)[:control_links_as_controller][agent.id] || 0
+    source_count   = links_counter_cache(agents)[:links_as_source][agent.id] || 0
 
     if control_count > 0 && receiver_count > 0
       content_tag('span') do
@@ -81,4 +81,22 @@ module AgentHelper
       icon_tag('glyphicon-unchecked')
     end
   end
+
+  private
+
+  def links_counter_cache(agents)
+    @counter_cache ||= {}
+    @counter_cache[agents.__id__] ||= {}.tap do |cache|
+      agent_ids = agents.map(&:id)
+      cache[:links_as_receiver] = Hash[Link.where(receiver_id: agent_ids)
+                                           .group(:receiver_id)
+                                           .pluck('receiver_id', 'count(receiver_id) as id')]
+      cache[:links_as_source]   = Hash[Link.where(source_id: agent_ids)
+                                           .group(:source_id)
+                                           .pluck('source_id', 'count(source_id) as id')]
+      cache[:control_links_as_controller] = Hash[ControlLink.where(controller_id: agent_ids)
+                                                            .group(:controller_id)
+                                                            .pluck('controller_id', 'count(controller_id) as id')]
+    end
+  end
 end

app/views/agents/_table.html.erb

@@ -16,7 +16,7 @@
     <% @agents.each do |agent| %>
       <tr>
         <td class='<%= "agent-unavailable" if agent.unavailable? %>'>
-          <%= agent_type_icon(agent) %>
+          <%= agent_type_icon(agent, @agents) %>
         </td>
         <td class='<%= "agent-unavailable" if agent.unavailable? %>'>
           <%= link_to agent.name, agent_path(agent, return: (defined?(return_to) && return_to) || request.path) %>