Add support for scopes to MiqPreloader#preload

We want to use preloaded data to populate a record's associations
Rails currently accepts a scope for preloading, but it runs the scope for every input record

We are using this to load VimPerformanceState values for a single record
We also want the list of records after it is done.

Before
======

```ruby
ems = ExtManagementSystem.take(5).to_a
vms = Vm.where(:ems => ems.map(&:id)).load

MiqPreloader.preload(ems, :vms, vms)      # => 5 queries
MiqPreloader.preload(ems, :vms, vms.to_a) # error
```

After
=====

```ruby
ems = ExtManagementSystem.take(5).to_a
vms = Vm.where(:ems => ems.map(&:id)).load

MiqPreloader.preload(ems, :vms, vms)      # => 5 queries
MiqPreloader.preload(ems, :vms, vms.to_a) # error
```

lib/extensions/ar_preloader.rb

@@ -0,0 +1,18 @@
+module ActiveRecordPreloadScopes
+  # based upon active record 6.1
+  def records_for(ids)
+    # use our logic if passing in [ActiveRecord::Base] or passing in a loaded Relation/scope
+    unless (preload_scope.kind_of?(Array) && preload_scope.first.kind_of?(ActiveRecord::Base)) ||
+            preload_scope.try(:loaded?)
+      return super
+    end
+
+    preload_scope.each do |record|
+      owner = owners_by_key[convert_key(record[association_key_name])].first
+      association = owner.association(reflection.name)
+      association.set_inverse_instance(record)
+    end
+  end
+end
+
+ActiveRecord::Associations::Preloader::Association.prepend(ActiveRecordPreloadScopes)

spec/lib/miq_preloader_spec.rb

@@ -36,6 +36,45 @@
       expect { preload(emses, :vms, vms) }.to make_database_queries(:count => 1)
     end
 
+    it "preloads with a loaded relation (records is a relation)" do
+      ems = FactoryBot.create(:ems_infra)
+      FactoryBot.create_list(:vm, 2, :ext_management_system => ems)
+
+      emses = ExtManagementSystem.all.load
+      vms   = Vm.where(:ems_id => emses.select(:id)).load
+
+      expect { preload(emses, :vms, vms) }.not_to make_database_queries
+      expect { preload(emses, :vms, vms) }.not_to make_database_queries
+      expect { expect(emses.first.vms.size).to eq(2) }.not_to make_database_queries
+      expect { expect(vms.first.ext_management_system).to eq(ems) }.not_to make_database_queries
+    end
+
+    it "preloads with an array (records is a relation)" do
+      ems = FactoryBot.create(:ems_infra)
+      FactoryBot.create_list(:vm, 2, :ext_management_system => ems)
+
+      emses = ExtManagementSystem.all.load
+      vms = Vm.where(:ems_id => emses.select(:id)).to_a
+
+      expect { preload(emses, :vms, vms) }.not_to make_database_queries
+      expect { preload(emses, :vms, vms) }.not_to make_database_queries
+      expect { expect(emses.first.vms.size).to eq(2) }.not_to make_database_queries
+      expect { expect(vms.first.ext_management_system).to eq(ems) }.not_to make_database_queries
+    end
+
+    it "preloads with an array (records is an array)" do
+      ems = FactoryBot.create(:ems_infra)
+      FactoryBot.create_list(:vm, 2, :ext_management_system => ems)
+
+      emses = ExtManagementSystem.all.load.to_a
+      vms   = Vm.where(:ems_id => emses.map(&:id)).to_a
+
+      expect { preload(emses, :vms, vms) }.not_to make_database_queries
+      expect { preload(emses, :vms, vms) }.not_to make_database_queries
+      expect { expect(emses.first.vms.size).to eq(2) }.not_to make_database_queries
+      expect { expect(vms.first.ext_management_system).to eq(ems) }.not_to make_database_queries
+    end
+
     def preload(*args)
       MiqPreloader.preload(*args)
     end