Support modifying the querying strategy on a per-query basis

Fixes CanCanCommunity#668

CHANGELOG.md

@@ -1,3 +1,7 @@
+## Unreleased
+
+* [#675](https://github.com/CanCanCommunity/cancancan/pull/675): Support modifying the `accessible_by` querying strategy on a per-query basis. ([@ghiculescu][])
+
 ## 3.2.1
 
 * [#674](https://github.com/CanCanCommunity/cancancan/pull/674): Fix accidental dependency on ActiveRecord in 3.2.0. ([@ghiculescu][])

lib/cancan/config.rb

@@ -21,7 +21,9 @@ def self.valid_accessible_by_strategies
   #                `distinct` is not reliable in some cases. See
   #                https://github.com/CanCanCommunity/cancancan/pull/605
   def self.accessible_by_strategy
-    @accessible_by_strategy || default_accessible_by_strategy
+    return @accessible_by_strategy if @accessible_by_strategy
+
+    @accessible_by_strategy = default_accessible_by_strategy
   end
 
   def self.default_accessible_by_strategy
@@ -36,9 +38,7 @@ def self.default_accessible_by_strategy
   end
 
   def self.accessible_by_strategy=(value)
-    unless valid_accessible_by_strategies.include?(value)
-      raise ArgumentError, "accessible_by_strategy must be one of #{valid_accessible_by_strategies.join(', ')}"
-    end
+    validate_accessible_by_strategy!(value)
 
     if value == :subquery && does_not_support_subquery_strategy?
       raise ArgumentError, 'accessible_by_strategy = :subquery requires ActiveRecord 5 or newer'
@@ -47,6 +47,26 @@ def self.accessible_by_strategy=(value)
     @accessible_by_strategy = value
   end
 
+  def self.with_accessible_by_strategy(value)
+    return yield if value == accessible_by_strategy
+
+    validate_accessible_by_strategy!(value)
+
+    begin
+      strategy_was = accessible_by_strategy
+      @accessible_by_strategy = value
+      yield
+    ensure
+      @accessible_by_strategy = strategy_was
+    end
+  end
+
+  def self.validate_accessible_by_strategy!(value)
+    return if valid_accessible_by_strategies.include?(value)
+
+    raise ArgumentError, "accessible_by_strategy must be one of #{valid_accessible_by_strategies.join(', ')}"
+  end
+
   def self.does_not_support_subquery_strategy?
     !defined?(CanCan::ModelAdapters::ActiveRecordAdapter) ||
       CanCan::ModelAdapters::ActiveRecordAdapter.version_lower?('5.0.0')

lib/cancan/model_additions.rb

@@ -20,8 +20,10 @@ module ClassMethods
       #   @articles = Article.accessible_by(current_ability, :update)
       #
       # Here only the articles which the user can update are returned.
-      def accessible_by(ability, action = :index)
-        ability.model_adapter(self, action).database_records
+      def accessible_by(ability, action = :index, strategy: CanCan.accessible_by_strategy)
+        CanCan.with_accessible_by_strategy(strategy) do
+          ability.model_adapter(self, action).database_records
+        end
       end
     end
 

spec/cancan/model_adapters/active_record_adapter_spec.rb

@@ -817,6 +817,101 @@ class Transaction < ActiveRecord::Base
     end
   end
 
+  if CanCan::ModelAdapters::ActiveRecordAdapter.version_greater_or_equal?('5.0.0')
+    context 'switching strategies' do
+      before do
+        CanCan.accessible_by_strategy = :left_join # default - should be ignored in these tests
+      end
+
+      it 'allows you to switch strategies with a keyword argument' do
+        u = User.create!(name: 'pippo')
+        Article.create!(mentioned_users: [u])
+
+        ability = Ability.new(u)
+        ability.can :read, Article, mentions: { user: { name: u.name } }
+
+        subquery_sql = Article.accessible_by(ability, strategy: :subquery).to_sql
+        left_join_sql = Article.accessible_by(ability, strategy: :left_join).to_sql
+
+        expect(subquery_sql.strip.squeeze(' ')).to eq(%(
+    SELECT "articles".*
+    FROM "articles"
+    WHERE "articles"."id" IN
+    (SELECT "articles"."id"
+      FROM "articles"
+      LEFT OUTER JOIN "legacy_mentions" ON "legacy_mentions"."article_id" = "articles"."id"
+      LEFT OUTER JOIN "users" ON "users"."id" = "legacy_mentions"."user_id"
+      WHERE "users"."name" = 'pippo')
+    ).gsub(/\s+/, ' ').strip)
+
+        expect(left_join_sql.strip.squeeze(' ')).to eq(%(
+  SELECT DISTINCT "articles".*
+  FROM "articles"
+  LEFT OUTER JOIN "legacy_mentions" ON "legacy_mentions"."article_id" = "articles"."id"
+  LEFT OUTER JOIN "users" ON "users"."id" = "legacy_mentions"."user_id"
+  WHERE "users"."name" = 'pippo').gsub(/\s+/, ' ').strip)
+      end
+
+      it 'allows you to switch strategies with a block' do
+        u = User.create!(name: 'pippo')
+        Article.create!(mentioned_users: [u])
+
+        ability = Ability.new(u)
+        ability.can :read, Article, mentions: { user: { name: u.name } }
+
+        subquery_sql = CanCan.with_accessible_by_strategy(:subquery) { Article.accessible_by(ability).to_sql }
+        left_join_sql = CanCan.with_accessible_by_strategy(:left_join) { Article.accessible_by(ability).to_sql }
+
+        expect(subquery_sql.strip.squeeze(' ')).to eq(%(
+    SELECT "articles".*
+    FROM "articles"
+    WHERE "articles"."id" IN
+    (SELECT "articles"."id"
+      FROM "articles"
+      LEFT OUTER JOIN "legacy_mentions" ON "legacy_mentions"."article_id" = "articles"."id"
+      LEFT OUTER JOIN "users" ON "users"."id" = "legacy_mentions"."user_id"
+      WHERE "users"."name" = 'pippo')
+    ).gsub(/\s+/, ' ').strip)
+
+        expect(left_join_sql.strip.squeeze(' ')).to eq(%(
+  SELECT DISTINCT "articles".*
+  FROM "articles"
+  LEFT OUTER JOIN "legacy_mentions" ON "legacy_mentions"."article_id" = "articles"."id"
+  LEFT OUTER JOIN "users" ON "users"."id" = "legacy_mentions"."user_id"
+  WHERE "users"."name" = 'pippo').gsub(/\s+/, ' ').strip)
+      end
+
+      it 'allows you to switch strategies with a block, and to_sql called outside the block' do
+        u = User.create!(name: 'pippo')
+        Article.create!(mentioned_users: [u])
+
+        ability = Ability.new(u)
+        ability.can :read, Article, mentions: { user: { name: u.name } }
+
+        subquery_sql = CanCan.with_accessible_by_strategy(:subquery) { Article.accessible_by(ability) }.to_sql
+        left_join_sql = CanCan.with_accessible_by_strategy(:left_join) { Article.accessible_by(ability) }.to_sql
+
+        expect(subquery_sql.strip.squeeze(' ')).to eq(%(
+    SELECT "articles".*
+    FROM "articles"
+    WHERE "articles"."id" IN
+    (SELECT "articles"."id"
+      FROM "articles"
+      LEFT OUTER JOIN "legacy_mentions" ON "legacy_mentions"."article_id" = "articles"."id"
+      LEFT OUTER JOIN "users" ON "users"."id" = "legacy_mentions"."user_id"
+      WHERE "users"."name" = 'pippo')
+    ).gsub(/\s+/, ' ').strip)
+
+        expect(left_join_sql.strip.squeeze(' ')).to eq(%(
+  SELECT DISTINCT "articles".*
+  FROM "articles"
+  LEFT OUTER JOIN "legacy_mentions" ON "legacy_mentions"."article_id" = "articles"."id"
+  LEFT OUTER JOIN "users" ON "users"."id" = "legacy_mentions"."user_id"
+  WHERE "users"."name" = 'pippo').gsub(/\s+/, ' ').strip)
+      end
+    end
+  end
+
   CanCan.valid_accessible_by_strategies.each do |strategy|
     context "when a model has renamed primary_key with #{strategy} strategy" do
       before :each do