Accessible by strategy: Exists subquery

lib/cancan.rb

@@ -22,6 +22,8 @@
   require 'cancan/model_adapters/active_record_4_adapter'
   require 'cancan/model_adapters/active_record_5_adapter'
   require 'cancan/model_adapters/strategies/base'
+  require 'cancan/model_adapters/strategies/joined_alias_each_rule_as_exists_subquery'
+  require 'cancan/model_adapters/strategies/joined_alias_exists_subquery'
   require 'cancan/model_adapters/strategies/left_join'
   require 'cancan/model_adapters/strategies/subquery'
 end

lib/cancan/config.rb

@@ -3,7 +3,11 @@
 module CanCan
   def self.valid_accessible_by_strategies
     strategies = [:left_join]
-    strategies << :subquery unless does_not_support_subquery_strategy?
+
+    unless does_not_support_subquery_strategy?
+      strategies.push(:joined_alias_exists_subquery, :joined_alias_each_rule_as_exists_subquery, :subquery)
+    end
+
     strategies
   end
 

lib/cancan/model_adapters/strategies/joined_alias_each_rule_as_exists_subquery.rb

@@ -0,0 +1,91 @@
+module CanCan
+  module ModelAdapters
+    class Strategies
+      class JoinedAliasEachRuleAsExistsSubquery < Base
+        def execute!
+          model_class
+            .joins(
+              "JOIN #{quoted_table_name} AS #{quoted_aliased_table_name} ON " \
+              "#{quoted_aliased_table_name}.#{quoted_primary_key} = #{quoted_table_name}.#{quoted_primary_key}"
+            )
+            .where(double_exists_sql)
+        end
+
+        def double_exists_sql
+          double_exists_sql = ''
+
+          compressed_rules.each_with_index do |rule, index|
+            double_exists_sql << ' OR ' if index > 0
+            double_exists_sql << "EXISTS (#{sub_query_for_rule(rule).to_sql})"
+          end
+
+          double_exists_sql
+        end
+
+        def sub_query_for_rule(rule)
+          conditions_extractor = ConditionsExtractor.new(model_class)
+          rule_where_conditions = extract_multiple_conditions(conditions_extractor, [rule])
+          joins_hash, left_joins_hash = extract_joins_from_rule(rule)
+          sub_query_for_rules_and_join_hashes(rule_where_conditions, joins_hash, left_joins_hash)
+        end
+
+        def sub_query_for_rules_and_join_hashes(rule_where_conditions, joins_hash, left_joins_hash)
+          model_class
+            .select('1')
+            .joins(joins_hash)
+            .left_joins(left_joins_hash)
+            .where(
+              "#{quoted_table_name}.#{quoted_primary_key} = " \
+              "#{quoted_aliased_table_name}.#{quoted_primary_key}"
+            )
+            .where(rule_where_conditions)
+            .limit(1)
+        end
+
+        def extract_joins_from_rule(rule)
+          joins = {}
+          left_joins = {}
+
+          extra_joins_recursive([], rule.conditions, joins, left_joins)
+          [joins, left_joins]
+        end
+
+        def extra_joins_recursive(current_path, conditions, joins, left_joins)
+          conditions.each do |key, value|
+            if value.is_a?(Hash)
+              current_path << key
+              extra_joins_recursive(current_path, value, joins, left_joins)
+              current_path.pop
+            else
+              extra_joins_recursive_merge_joins(current_path, value, joins, left_joins)
+            end
+          end
+        end
+
+        def extra_joins_recursive_merge_joins(current_path, value, joins, left_joins)
+          hash_joins = current_path_to_hash(current_path)
+
+          if value.nil?
+            left_joins.deep_merge!(hash_joins)
+          else
+            joins.deep_merge!(hash_joins)
+          end
+        end
+
+        # Converts an array like [:child, :grand_child] into a hash like {child: {grand_child: {}}
+        def current_path_to_hash(current_path)
+          hash_joins = {}
+          current_hash_joins = hash_joins
+
+          current_path.each do |path_part|
+            new_hash = {}
+            current_hash_joins[path_part] = new_hash
+            current_hash_joins = new_hash
+          end
+
+          hash_joins
+        end
+      end
+    end
+  end
+end

lib/cancan/model_adapters/strategies/joined_alias_exists_subquery.rb

@@ -0,0 +1,29 @@
+module CanCan
+  module ModelAdapters
+    class Strategies
+      class JoinedAliasExistsSubquery < Base
+        def execute!
+          model_class
+            .joins(
+              "JOIN #{quoted_table_name} AS #{quoted_aliased_table_name} ON " \
+              "#{quoted_aliased_table_name}.#{quoted_primary_key} = #{quoted_table_name}.#{quoted_primary_key}"
+            )
+            .where("EXISTS (#{joined_alias_exists_subquery_inner_query.to_sql})")
+        end
+
+        def joined_alias_exists_subquery_inner_query
+          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}"
+            )
+            .limit(1)
+        end
+      end
+    end
+  end
+end

spec/cancan/model_adapters/accessible_by_has_many_through_spec.rb

@@ -89,7 +89,7 @@ class Editor < ActiveRecord::Base
       if CanCan::ModelAdapters::ActiveRecordAdapter.version_greater_or_equal?('5.0.0')
         describe 'selecting custom columns' do
           it 'extracts custom columns correctly' do
-            posts = Post.accessible_by(ability).where(published: true).select('title as mytitle')
+            posts = Post.accessible_by(ability).where(published: true).select('posts.title as mytitle')
             expect(posts[0].mytitle).to eq 'post1'
           end
         end

spec/cancan/model_adapters/has_and_belongs_to_many_spec.rb

@@ -46,6 +46,62 @@ class House < ActiveRecord::Base
   end
 
   unless CanCan::ModelAdapters::ActiveRecordAdapter.version_lower?('5.0.0')
+    describe 'fetching of records - joined_alias_subquery strategy' do
+      before do
+        CanCan.accessible_by_strategy = :joined_alias_exists_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\"
+                             JOIN \"houses\" AS \"houses_alias\" ON \"houses_alias\".\"id\" = \"houses\".\"id\"
+                             WHERE (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\" = \"houses_alias\".\"id\") LIMIT 1))
+                            ")
+        end
+      end
+    end
+
+    describe 'fetching of records - joined_alias_each_rule_as_exists_subquery strategy' do
+      before do
+        CanCan.accessible_by_strategy = :joined_alias_each_rule_as_exists_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\"
+                             JOIN \"houses\" AS \"houses_alias\" ON \"houses_alias\".\"id\" = \"houses\".\"id\"
+                             WHERE (EXISTS (SELECT 1
+                               FROM \"houses\"
+                               INNER JOIN \"houses_people\" ON \"houses_people\".\"house_id\" = \"houses\".\"id\"
+                               INNER JOIN \"people\" ON \"people\".\"id\" = \"houses_people\".\"person_id\"
+                               WHERE (\"houses\".\"id\" = \"houses_alias\".\"id\") AND
+                               (\"people\".\"id\" = #{@person1.id})
+                               LIMIT 1))
+                             ")
+        end
+      end
+    end
+
     describe 'fetching of records - subquery strategy' do
       before do
         CanCan.accessible_by_strategy = :subquery