Avoid unnecessary solr query

If only one property has been defined for a predicate,
there's no reason to run a query to filter the results by class.
This avoids a call to solr that is unnecessary in most situations.

lib/active_fedora/associations/rdf.rb

@@ -33,9 +33,15 @@ def filtered_results
         end
       end
 
-      # TODO Detect when this is the only relationship for this predicate, then skip the filtering.
       def filtering_required?
-        reflection.klass != ActiveFedora::Base
+        return false if reflection.klass == ActiveFedora::Base
+        reflections_with_same_predicate.count > 1
+      end
+
+      # Count the number of reflections that have the same predicate as the reflection
+      # for this association.
+      def reflections_with_same_predicate
+        owner.class.outgoing_reflections.select { |k, v| v.options[:predicate] == reflection.predicate }
       end
 
       # @return [Array<RDF::URI>]
@@ -48,8 +54,6 @@ def rdf_query
         owner.resource.query(subject: owner.rdf_subject, predicate: reflection.predicate).enum_statement
       end
 
-
-      # TODO this is a huge waste of time that can be completely avoided if the attributes aren't sharing predicates.
       # @return [Array<RDF::URI>]
       def filter_by_class(candidate_uris)
         return [] if candidate_uris.empty?

spec/integration/associations/rdf_spec.rb

@@ -0,0 +1,49 @@
+require 'spec_helper'
+
+describe "rdf associations" do
+  context "when there is one relationship for the predicate" do
+    before do
+      class Foo < ActiveFedora::Base
+      end
+      class Library < ActiveFedora::Base
+        has_and_belongs_to_many :foos, predicate: ::RDF::URI('http://example.com')
+      end
+    end
+    after do
+      Object.send(:remove_const, :Foo)
+      Object.send(:remove_const, :Library)
+    end
+
+    let(:library) { Library.new }
+
+    it "doesn't not bother to filter by class type" do
+      expect(library.association(:foo_ids)).not_to receive(:filter_by_class)
+      library.foos.to_a
+    end
+  end
+
+  context "when two relationships have the same predicate" do
+    before do
+      class Foo < ActiveFedora::Base
+      end
+      class Bar < ActiveFedora::Base
+      end
+      class Library < ActiveFedora::Base
+        has_and_belongs_to_many :foos, predicate: ::RDF::URI('http://example.com')
+        has_and_belongs_to_many :bars, predicate: ::RDF::URI('http://example.com')
+      end
+    end
+    after do
+      Object.send(:remove_const, :Foo)
+      Object.send(:remove_const, :Bar)
+      Object.send(:remove_const, :Library)
+    end
+
+    let(:library) { Library.new }
+
+    it "filters by class type" do
+      expect(library.association(:foo_ids)).to receive(:filter_by_class).and_call_original
+      library.foos.to_a
+    end
+  end
+end