Deprecate using method_missing for attributes that are columns.

This shouldn't ever happen unless people are doing something
particularly weird, but adding a deprecation in case there are bugs not
caught by our tests.

activerecord/lib/active_record/attribute_methods.rb

@@ -1,4 +1,5 @@
 require 'active_support/core_ext/enumerable'
+require 'active_support/deprecation'
 
 module ActiveRecord
   # = Active Record Attribute Methods
@@ -77,6 +78,20 @@ module ActiveRecord
       end
     end
 
+    def attribute_missing(match, *args, &block)
+      if self.class.columns_hash[match.attr_name]
+        ActiveSupport::Deprecation.warn(
+          "The method `#{match.method_name}', matching the attribute `#{match.attr_name}' has " \
+          "dispatched through method_missing. This shouldn't happen, because `#{match.attr_name}' " \
+          "is a column of the table. If this error has happened through normal usage of Active " \
+          "Record (rather than through your own code or external libraries), please report it as " \
+          "a bug."
+        )
+      end
+
+      super
+    end
+
     def respond_to?(name, include_private = false)
       self.class.define_attribute_methods unless self.class.attribute_methods_generated?
       super

activerecord/test/cases/attribute_methods_test.rb

@@ -675,6 +675,21 @@ class AttributeMethodsTest < ActiveRecord::TestCase
     assert subklass.method_defined?(:id), "subklass is missing id method"
   end
 
+  def test_dispatching_column_attributes_through_method_missing_deprecated
+    Topic.define_attribute_methods
+
+    topic = Topic.new(:id => 5)
+    topic.id = 5
+
+    topic.method(:id).owner.send(:remove_method, :id)
+
+    assert_deprecated do
+      assert_equal 5, topic.id
+    end
+  ensure
+    Topic.undefine_attribute_methods
+  end
+
   private
   def cached_columns
     @cached_columns ||= (time_related_columns_on_topic + serialized_columns_on_topic).map(&:name)