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.

3 files changed, 38 insertions, 3 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 815902a129..f630be6f70 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,5 +1,28 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   `ActiveRecord::AttributeMethods#[]` raises `ActiveModel::MissingAttributeError`
+    error if the given attribute is missing. Fixes #5433.
+
+        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
+
+    *Francesco Rodriguez*
+
 *   Small binary fields use the `VARBINARY` MySQL type, instead of `TINYBLOB`.
 
     *Victor Costan*
@@ -51,7 +74,7 @@
 
     *Scott Willson*
 
-*   Fix bug where sum(expression) returns string '0' for no matching records
+*   Fix bug where sum(expression) returns string '0' for no matching records.
     Fixes #7439
 
     *Tim Macfarlane*
diff --git a/activerecord/lib/active_record/attribute_methods.rb b/activerecord/lib/active_record/attribute_methods.rb
index 9a4fcdfda3..101c641877 100644
--- a/activerecord/lib/active_record/attribute_methods.rb
+++ b/activerecord/lib/active_record/attribute_methods.rb
@@ -266,16 +266,22 @@ module ActiveRecord
 
     # Returns the value of the attribute identified by <tt>attr_name</tt> after it has been typecast (for example,
     # "2004-12-12" in a data column is cast to a date object, like Date.new(2004, 12, 12)).
-    # (Alias for the protected <tt>read_attribute</tt> method).
+    # (Alias for the protected <tt>read_attribute</tt> method). It raises an <tt>ActiveModel::MissingAttributeError</tt>
+    # error if the identified attribute is missing.
     #
     #   class Person < ActiveRecord::Base
+    #     belongs_to :organization
     #   end
     #
     #   person = Person.new(name: 'Francesco', age: '22'
     #   person[:name] # => "Francesco"
     #   person[:age]  # => 22
+    #
+    #   person = Person.select('id').first
+    #   person[:name]            # => ActiveModel::MissingAttributeError: missing attribute: name
+    #   person[:organization_id] # => ActiveModel::MissingAttributeError: missing attribute: organization_id
     def [](attr_name)
-      read_attribute(attr_name)
+      read_attribute(attr_name) { |n| missing_attribute(n, caller) }
     end
 
     # Updates the attribute identified by <tt>attr_name</tt> with the specified +value+.
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