Added the new strategy: double_exist_subquery

lib/cancan/config.rb

@@ -3,7 +3,7 @@
 module CanCan
   def self.valid_accessible_by_strategies
     strategies = [:left_join]
-    strategies << :subquery unless does_not_support_subquery_strategy?
+    strategies.push(:double_exist_subquery, :subquery) unless does_not_support_subquery_strategy?
     strategies
   end
 

lib/cancan/model_adapters/active_record_5_adapter.rb

@@ -21,17 +21,57 @@ def self.matches_condition?(subject, name, value)
 
       private
 
+      delegate :connection, :quoted_primary_key, to: :@model_class
+      delegate :quote_table_name, to: :connection
+
       def build_joins_relation(relation, *where_conditions)
         case CanCan.accessible_by_strategy
         when :subquery
-          inner = @model_class.unscoped do
-            @model_class.left_joins(joins).where(*where_conditions)
-          end
-          @model_class.where(@model_class.primary_key => inner)
-
+          build_joins_relation_subquery(where_conditions)
         when :left_join
           relation.left_joins(joins).distinct
+        when :double_exist_subquery
+          build_joins_relation_double_exist_subquery(where_conditions)
+        end
+      end
+
+      def build_joins_relation_subquery(where_conditions)
+        inner = @model_class.unscoped do
+          @model_class.left_joins(joins).where(*where_conditions)
         end
+        @model_class.where(@model_class.primary_key => inner)
+      end
+
+      def build_joins_relation_double_exist_subquery(where_conditions)
+        @model_class.where("EXISTS (#{double_exists_query_sql(where_conditions)})")
+      end
+
+      def double_exists_inner_query(where_conditions)
+        @model_class
+          .unscoped
+          .select('1')
+          .left_joins(joins)
+          .where(*where_conditions)
+          .where(
+            "#{quoted_table_name}.#{quoted_primary_key} = " \
+            "#{quoted_aliased_table_name}.#{quoted_primary_key}"
+          )
+      end
+
+      def double_exists_query_sql(where_conditions)
+        'SELECT 1 ' \
+        "FROM #{quoted_table_name} AS #{quoted_aliased_table_name} " \
+        'WHERE ' \
+        "#{quoted_aliased_table_name}.#{quoted_primary_key} = #{quoted_table_name}.#{quoted_primary_key} AND " \
+        "EXISTS (#{double_exists_inner_query(where_conditions).to_sql})"
+      end
+
+      def quoted_aliased_table_name
+        @quoted_aliased_table_name ||= quote_table_name('aliased_table')
+      end
+
+      def quoted_table_name
+        @quoted_table_name ||= quote_table_name(@model_class.table_name)
       end
 
       def sanitize_sql(conditions)

spec/cancan/model_adapters/has_and_belongs_to_many_spec.rb

@@ -46,6 +46,37 @@ class House < ActiveRecord::Base
   end
 
   unless CanCan::ModelAdapters::ActiveRecordAdapter.version_lower?('5.0.0')
+    describe 'fetching of records - double_exist_subquery strategy' do
+      before do
+        CanCan.accessible_by_strategy = :double_exist_subquery
+      end
+
+      it 'it retreives the records correctly' do
+        houses = House.accessible_by(ability)
+        expect(houses).to match_array [@house2, @house1]
+      end
+
+      if CanCan::ModelAdapters::ActiveRecordAdapter.version_greater_or_equal?('5.0.0')
+        it 'generates the correct query' do
+          expect(ability.model_adapter(House, :read))
+            .to generate_sql("SELECT \"houses\".*
+                            FROM \"houses\"
+                            WHERE (EXISTS (SELECT 1
+                              FROM \"houses\" AS \"aliased_table\"
+                              WHERE
+                                \"aliased_table\".\"id\" = \"houses\".\"id\" AND
+                                EXISTS (SELECT 1
+                                  FROM \"houses\"
+                                  LEFT OUTER JOIN \"houses_people\" ON \"houses_people\".\"house_id\" = \"houses\".\"id\"
+                                  LEFT OUTER JOIN \"people\" ON \"people\".\"id\" = \"houses_people\".\"person_id\"
+                                  WHERE
+                                    \"people\".\"id\" = #{@person1.id} AND
+                                    (\"houses\".\"id\" = \"aliased_table\".\"id\"))))
+                            ")
+        end
+      end
+    end
+
     describe 'fetching of records - subquery strategy' do
       before do
         CanCan.accessible_by_strategy = :subquery