AR::AttributeMethods#[] raises AM::AttributeMissingError for missing attributes.

This fixes the following behaviour:

    class Person < ActiveRecord::Base
      belongs_to :company
    end

    # Before:
    person = Person.select('id').first
    person[:name]       # => nil
    person.name         # => ActiveModel::MissingAttributeError: missing_attribute: name
    person[:company_id] # => nil
    person.company      # => nil

    # After:
    person = Person.select('id').first
    person[:name]       # => ActiveModel::MissingAttributeError: missing_attribute: name
    person.name         # => ActiveModel::MissingAttributeError: missing_attribute: name
    person[:company_id] # => ActiveModel::MissingAttributeError: missing_attribute: company_id
    person.company      # => ActiveModel::MissingAttributeError: missing_attribute: company_id

Fixes #5433.

1 files changed, 6 insertions, 0 deletions

diff --git a/activerecord/test/cases/attribute_methods_test.rb b/activerecord/test/cases/attribute_methods_test.rb
index c2b58fd7d1..8b82b79219 100644
--- a/activerecord/test/cases/attribute_methods_test.rb
+++ b/activerecord/test/cases/attribute_methods_test.rb
@@ -287,6 +287,12 @@ class AttributeMethodsTest < ActiveRecord::TestCase
     assert_equal "Don't change the topic", topic[:title]
   end
 
+  def test_read_attribute_raises_missing_attribute_error_when_not_exists
+    computer = Computer.select('id').first
+    assert_raises(ActiveModel::MissingAttributeError) { computer[:developer] }
+    assert_raises(ActiveModel::MissingAttributeError) { computer[:extendedWarranty] }
+  end
+
   def test_read_attribute_when_false
     topic = topics(:first)
     topic.approved = false