Make sure nested through associations are read only

activerecord/lib/active_record/associations.rb

@@ -65,6 +65,12 @@ module ActiveRecord
     end
   end
   
+  class HasManyThroughNestedAssociationsAreReadonly < ActiveRecordError #:nodoc
+    def initialize(owner, reflection)
+      super("Cannot modify association '#{owner.class.name}##{reflection.name}' because it goes through more than one other association.")
+    end
+  end
+
   class HasAndBelongsToManyAssociationWithPrimaryKeyError < ActiveRecordError #:nodoc:
     def initialize(reflection)
       super("Primary key is not allowed in a has_and_belongs_to_many join table (#{reflection.options[:join_table]}).")

activerecord/lib/active_record/associations/has_many_through_association.rb

@@ -8,6 +8,11 @@ module ActiveRecord
     class HasManyThroughAssociation < HasManyAssociation #:nodoc:
       include ThroughAssociationScope
 
+      def build(attributes = {}, &block)
+        ensure_not_nested
+        super
+      end
+
       alias_method :new, :build
 
       def create!(attrs = nil)
@@ -37,6 +42,7 @@ module ActiveRecord
 
       protected
         def create_record(attrs, force = true)
+          ensure_not_nested
           ensure_owner_is_not_new
 
           transaction do
@@ -60,6 +66,8 @@ module ActiveRecord
         end
 
         def insert_record(record, force = true, validate = true)
+          ensure_not_nested
+          
           if record.new_record?
             if force
               record.save!
@@ -75,6 +83,8 @@ module ActiveRecord
 
         # TODO - add dependent option support
         def delete_records(records)
+          ensure_not_nested
+          
           klass = @reflection.through_reflection.klass
           records.each do |associate|
             klass.delete_all(construct_join_attributes(associate))

activerecord/lib/active_record/associations/has_one_through_association.rb

@@ -14,6 +14,8 @@ module ActiveRecord
       private
 
       def create_through_record(new_value) #nodoc:
+        ensure_not_nested
+        
         klass = @reflection.through_reflection.klass
 
         current_object = @owner.send(@reflection.through_reflection.name)

activerecord/lib/active_record/associations/through_association_scope.rb

@@ -8,15 +8,18 @@ module ActiveRecord
       protected
 
       def construct_scope
-        { :create => construct_owner_attributes(@reflection),
-          :find   => { :conditions  => construct_conditions,
+        scope = {}
+        scope[:find] = {
+          :conditions => construct_conditions,
           :joins      => construct_joins,
           :include    => @reflection.options[:include] || @reflection.source_reflection.options[:include],
           :select     => construct_select,
           :order      => @reflection.options[:order],
           :limit      => @reflection.options[:limit],
-                       :readonly    => @reflection.options[:readonly],
-           } }
+          :readonly   => @reflection.options[:readonly]
+        }
+        scope[:create] = construct_owner_attributes(@reflection) unless @reflection.nested?
+        scope
       end
 
       # Build SQL conditions from attributes, qualified by table name.
@@ -299,6 +302,12 @@ module ActiveRecord
       end
 
       alias_method :sql_conditions, :conditions
+      
+      def ensure_not_nested
+        if @reflection.nested?
+          raise HasManyThroughNestedAssociationsAreReadonly.new(@owner, @reflection)
+        end
+      end
     end
   end
 end

activerecord/lib/active_record/reflection.rb

@@ -396,6 +396,10 @@ module ActiveRecord
         end
       end
       
+      def nested?
+        through_reflection_chain.length > 2
+      end
+
       # Gets an array of possible <tt>:through</tt> source reflection names:
       #
       #   [:singularized, :pluralized]

activerecord/test/cases/associations/nested_has_many_through_associations_test.rb

@@ -364,6 +364,48 @@ class NestedHasManyThroughAssociationsTest < ActiveRecord::TestCase
     assert !scope.where("comments.type" => "SubSpecialComment").empty?
   end
 
+  def test_nested_has_many_through_writers_should_raise_error
+    david = authors(:david)
+    subscriber = subscribers(:first)
+    
+    assert_raises(ActiveRecord::HasManyThroughNestedAssociationsAreReadonly) do
+      david.subscribers = [subscriber]
+    end
+    
+    assert_raises(ActiveRecord::HasManyThroughNestedAssociationsAreReadonly) do
+      david.subscriber_ids = [subscriber.id]
+    end
+    
+    assert_raises(ActiveRecord::HasManyThroughNestedAssociationsAreReadonly) do
+      david.subscribers << subscriber
+    end
+    
+    assert_raises(ActiveRecord::HasManyThroughNestedAssociationsAreReadonly) do
+      david.subscribers.delete(subscriber)
+    end
+    
+    assert_raises(ActiveRecord::HasManyThroughNestedAssociationsAreReadonly) do
+      david.subscribers.clear
+    end
+    
+    assert_raises(ActiveRecord::HasManyThroughNestedAssociationsAreReadonly) do
+      david.subscribers.build
+    end
+    
+    assert_raises(ActiveRecord::HasManyThroughNestedAssociationsAreReadonly) do
+      david.subscribers.create
+    end
+  end
+  
+  def test_nested_has_one_through_writers_should_raise_error
+    groucho = members(:groucho)
+    founding = member_types(:founding)
+    
+    assert_raises(ActiveRecord::HasManyThroughNestedAssociationsAreReadonly) do
+      groucho.nested_member_type = founding
+    end
+  end
+  
   private
   
     def assert_includes_and_joins_equal(query, expected, association)