Create a blacklist to disallow mutator methods to be delegated to `Array`.

This change was necessary because the whitelist wouldn't work.
It would be painful for users trying to update their applications.

This blacklist intent to prevent odd bugs and confusion in code that call mutator
methods directely on the `Relation`.

4 files changed, 59 insertions, 30 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index d1db4db8d8..c90ed03ac4 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,20 @@
+*   `Relation` no longer has mutator methods like `#map!` and `#delete_if`. Convert
+    to an `Array` by calling `#to_a` before using these methods.
+
+    It intends to prevent odd bugs and confusion in code that call mutator
+    methods directly on the `Relation`.
+
+    Example:
+
+        # Instead of this
+        Author.where(name: 'Hank Moody').compact!
+
+        # Now you have to do this
+        authors = Author.where(name: 'Hank Moody').to_a
+        authors.compact!
+
+    *Lauro Caetano*
+
 *   Better support for `where()` conditions that use a `belongs_to`
     association name.
 
@@ -80,21 +97,6 @@
 
     *arthurnn*
 
-*   Create a whitelist of delegable methods to `Array`.
-
-    Currently `Relation` directly delegates methods to `Array`. With this change,
-    only the methods present in this whitelist will be delegated.
-
-    The whitelist contains:
-
-        #&, #+, #[], #all?, #collect, #detect, #each, #each_cons, #each_with_index,
-        #flat_map, #group_by, #include?, #length, #map, #none?, :one?, #reverse, #sample,
-        #second, #sort, #sort_by, #to_ary, #to_set, #to_xml, #to_yaml
-
-    To use any other method, instead first call `#to_a` on the association.
-
-    *Lauro Caetano*
-
 *   Use the right column to type cast grouped calculations with custom expressions.
 
     Fixes #13230.
diff --git a/activerecord/lib/active_record/relation/delegation.rb b/activerecord/lib/active_record/relation/delegation.rb
index 246c5db5bd..21beed332f 100644
--- a/activerecord/lib/active_record/relation/delegation.rb
+++ b/activerecord/lib/active_record/relation/delegation.rb
@@ -1,3 +1,4 @@
+require 'set'
 require 'active_support/concern'
 require 'active_support/deprecation'
 
@@ -36,18 +37,13 @@ module ActiveRecord
     # may vary depending on the klass of a relation, so we create a subclass of Relation
     # for each different klass, and the delegations are compiled into that subclass only.
 
-    # TODO: This is not going to work. Brittle, painful. We'll switch to a blacklist
-    # to disallow mutator methods like map!, pop, and delete_if instead.
-    ARRAY_DELEGATES = [
-      :+, :-, :|, :&, :[],
-      :all?, :collect, :detect, :each, :each_cons, :each_with_index,
-      :exclude?, :find_all, :flat_map, :group_by, :include?, :length,
-      :map, :none?, :one?, :partition, :reject, :reverse,
-      :sample, :second, :sort, :sort_by, :third,
-      :to_ary, :to_set, :to_xml, :to_yaml
-    ]
+    BLACKLISTED_ARRAY_METHODS = [
+      :compact!, :flatten!, :reject!, :reverse!, :rotate!, :map!,
+      :shuffle!, :slice!, :sort!, :sort_by!, :delete_if,
+      :keep_if, :pop, :shift, :delete_at, :compact
+    ].to_set # :nodoc:
 
-    delegate(*ARRAY_DELEGATES, to: :to_a)
+    delegate :to_xml, :to_yaml, :length, :collect, :map, :each, :all?, :include?, :to_ary, to: :to_a
 
     delegate :table_name, :quoted_table_name, :primary_key, :quoted_primary_key,
              :connection, :columns_hash, :to => :klass
@@ -119,14 +115,21 @@ module ActiveRecord
 
     def respond_to?(method, include_private = false)
       super || @klass.respond_to?(method, include_private) ||
+        array_delegable?(method) ||
         arel.respond_to?(method, include_private)
     end
 
     protected
 
+    def array_delegable?(method)
+      Array.method_defined?(method) && BLACKLISTED_ARRAY_METHODS.exclude?(method)
+    end
+
     def method_missing(method, *args, &block)
       if @klass.respond_to?(method)
         scoping { @klass.public_send(method, *args, &block) }
+      elsif array_delegable?(method)
+        to_a.public_send(method, *args, &block)
       elsif arel.respond_to?(method)
         arel.public_send(method, *args, &block)
       else
diff --git a/activerecord/test/cases/relation/delegation_test.rb b/activerecord/test/cases/relation/delegation_test.rb
index f295ccbc6a..9b2bfed039 100644
--- a/activerecord/test/cases/relation/delegation_test.rb
+++ b/activerecord/test/cases/relation/delegation_test.rb
@@ -26,15 +26,22 @@ module ActiveRecord
   end
 
   module DelegationWhitelistBlacklistTests
-    ActiveRecord::Delegation::ARRAY_DELEGATES.each do |method|
+    ARRAY_DELEGATES = [
+      :+, :-, :|, :&, :[],
+      :all?, :collect, :detect, :each, :each_cons, :each_with_index,
+      :exclude?, :find_all, :flat_map, :group_by, :include?, :length,
+      :map, :none?, :one?, :partition, :reject, :reverse,
+      :sample, :second, :sort, :sort_by, :third,
+      :to_ary, :to_set, :to_xml, :to_yaml
+    ]
+
+    ARRAY_DELEGATES.each do |method|
       define_method "test_delegates_#{method}_to_Array" do
         assert_respond_to target, method
       end
     end
 
-    [:compact!, :flatten!, :reject!, :reverse!, :rotate!,
-     :shuffle!, :slice!, :sort!, :sort_by!, :delete_if,
-     :keep_if, :pop, :shift, :delete_at, :compact].each do |method|
+    ActiveRecord::Delegation::BLACKLISTED_ARRAY_METHODS.each do |method|
       define_method "test_#{method}_is_not_delegated_to_Array" do
         assert_raises(NoMethodError) { call_method(target, method) }
       end
diff --git a/guides/source/upgrading_ruby_on_rails.md b/guides/source/upgrading_ruby_on_rails.md
index ca5623bf73..60b27aa6e5 100644
--- a/guides/source/upgrading_ruby_on_rails.md
+++ b/guides/source/upgrading_ruby_on_rails.md
@@ -156,6 +156,23 @@ end
 ActiveRecord::FixtureSet.context_class.send :include, FixtureFileHelpers
 ```
 
+### Mutator methods called on Relation
+
+`Relation` no longer has mutator methods like `#map!` and `#delete_if`. Convert
+to an `Array` by calling `#to_a` before using these methods.
+
+It intends to prevent odd bugs and confusion in code that call mutator
+methods directly on the `Relation`.
+
+```ruby
+# Instead of this
+Author.where(name: 'Hank Moody').compact!
+
+# Now you have to do this
+authors = Author.where(name: 'Hank Moody').to_a
+authors.compact!
+```
+
 Upgrading from Rails 3.2 to Rails 4.0
 -------------------------------------