Apply distinct on LAN/switch selection

Several hosts might be connected to the same switch, sharing the same
LAN. Therefore it is required to apply 'DISTINCT' on the result of the
join of these tables.

app/models/manageiq/providers/infra_manager.rb

@@ -7,17 +7,17 @@ class InfraManager < BaseManager
 
     include AvailabilityMixin
 
-    has_many :host_hardwares,         :through => :hosts, :source => :hardware
-    has_many :host_operating_systems, :through => :hosts, :source => :operating_system
-    has_many :host_storages,          :through => :hosts
-    has_many :host_switches,          :through => :hosts
-    has_many :host_networks,          :through => :hosts, :source => :networks
-    has_many :snapshots,              :through => :vms_and_templates
-    has_many :operating_systems,      :through => :vms_and_templates
-    has_many :switches,               :through => :hosts
-    has_many :lans,                   :through => :hosts
-    has_many :networks,               :through => :hardwares
-    has_many :guest_devices,          :through => :hardwares
+    has_many :host_hardwares,             :through => :hosts, :source => :hardware
+    has_many :host_operating_systems,     :through => :hosts, :source => :operating_system
+    has_many :host_storages,              :through => :hosts
+    has_many :host_switches,              :through => :hosts
+    has_many :host_networks,              :through => :hosts, :source => :networks
+    has_many :snapshots,                  :through => :vms_and_templates
+    has_many :operating_systems,          :through => :vms_and_templates
+    has_many :switches, -> { distinct },  :through => :hosts
+    has_many :lans, -> { distinct },      :through => :hosts
+    has_many :networks,                   :through => :hardwares
+    has_many :guest_devices,              :through => :hardwares
 
     class << model_name
       define_method(:route_key) { "ems_infras" }