4 files changed, 41 insertions, 4 deletions
diff --git a/activerecord/CHANGELOG b/activerecord/CHANGELOG
index 67b0291bcd..a8a4170a20 100644
--- a/activerecord/CHANGELOG
+++ b/activerecord/CHANGELOG
@@ -1,5 +1,7 @@
 *SVN*
 
+* MySQL: detect when a NOT NULL column without a default value is misreported as default ''.  Can't detect for string, text, and binary columns since '' is a legitimate default.  #6156 [simon@redhillconsulting.com.au, obrie, Jeremy Kemper]
+
 * Simplify association proxy implementation by factoring construct_scope out of method_missing.  #6643 [martin]
 
 * Oracle: automatically detect the primary key.  #6594 [vesaria, Michael Schoen]
diff --git a/activerecord/lib/active_record/connection_adapters/mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/mysql_adapter.rb
index 6c75896233..574196e351 100755
--- a/activerecord/lib/active_record/connection_adapters/mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql_adapter.rb
@@ -1,4 +1,5 @@
 require 'active_record/connection_adapters/abstract_adapter'
+require 'set'
 
 module MysqlCompat
   # add all_hashes method to standard mysql-c bindings or pure ruby version
@@ -84,12 +85,30 @@ module ActiveRecord
 
   module ConnectionAdapters
     class MysqlColumn < Column #:nodoc:
+      TYPES_ALLOWING_EMPTY_STRING_DEFAULT = Set.new([:binary, :string, :text])
+
+      def initialize(name, default, sql_type = nil, null = true)
+        super
+        self.default = nil if missing_default_forged_as_empty_string?
+      end
+
       private
         def simplified_type(field_type)
           return :boolean if MysqlAdapter.emulate_booleans && field_type.downcase.index("tinyint(1)")
           return :string  if field_type =~ /enum/i
           super
         end
+
+        # MySQL misreports NOT NULL column default when none is given.
+        # We can't detect this for columns which may have a legitimate ''
+        # default (string, text, binary) but we can for others (integer,
+        # datetime, boolean, and the rest).
+        #
+        # Test whether the column has default '', is not null, and is not
+        # a type allowing default ''.
+        def missing_default_forged_as_empty_string?
+          !null && default == '' && !TYPES_ALLOWING_EMPTY_STRING_DEFAULT.include?(type)
+        end
     end
 
     # The MySQL adapter will work with both Ruby/MySQL, which is a Ruby-based MySQL adapter that comes bundled with Active Record, and with
diff --git a/activerecord/test/defaults_test.rb b/activerecord/test/defaults_test.rb
index aba3c66de4..f15f422dc7 100644
--- a/activerecord/test/defaults_test.rb
+++ b/activerecord/test/defaults_test.rb
@@ -1,8 +1,24 @@
 require 'abstract_unit'
 require 'fixtures/default'
+require 'fixtures/entrant'
 
-if current_adapter?(:PostgreSQLAdapter, :SQLServerAdapter)
-  class DefaultsTest < Test::Unit::TestCase
+class DefaultTest < Test::Unit::TestCase
+  def test_nil_defaults_for_not_null_columns
+    column_defaults =
+      if current_adapter?(:MysqlAdapter)
+        { 'id' => nil, 'name' => '', 'course_id' => 0 }
+      else
+        { 'id' => nil, 'name' => nil, 'course_id' => nil }
+      end
+
+    column_defaults.each do |name, default|
+      column = Entrant.columns_hash[name]
+      assert !column.null, "#{name} column should be NOT NULL"
+      assert_equal default, column.default, "#{name} column should be DEFAULT #{default.inspect}"
+    end
+  end
+
+  if current_adapter?(:PostgreSQLAdapter, :SQLServerAdapter, :FirebirdAdapter, :OpenBaseAdapter)
     def test_default_integers
       default = Default.new
       assert_instance_of Fixnum, default.positive_integer
diff --git a/activerecord/test/fixtures/db_definitions/postgresql.sql b/activerecord/test/fixtures/db_definitions/postgresql.sql
index ce2c775aba..b5412c7400 100644
--- a/activerecord/test/fixtures/db_definitions/postgresql.sql
+++ b/activerecord/test/fixtures/db_definitions/postgresql.sql
@@ -130,8 +130,8 @@ CREATE TABLE auto_id_tests (
 
 CREATE TABLE entrants (
   id serial,
-  name text,
-  course_id integer
+  name text not null,
+  course_id integer not null
 );
 
 CREATE TABLE colnametests (