Handle nil in add_index :length option in MySQL

Our schema.rb is being generated with an `add_index` line similar to this:

    add_index "foo", ["foo", "bar"], :name => "xxx", :length => {"foo"=>8, "bar=>nil}

This is the same as it was on Rails 3.1.3, however, now when that
schema.rb is evaluated, its generating bad SQL in MySQL:

    Mysql::Error: You have an error in your SQL syntax; check the manual
    that corresponds to your MySQL server version for the right syntax
    to use near '))' at line 1: CREATE UNIQUE INDEX
    `xxx` ON `foo` (`foo`(8), `bar`())

This commit adds a check for nil on the length attribute to prevent the
empty parens from being output.

Conflicts:

	activerecord/test/cases/migration/index_test.rb

Signed-off-by: José Valim <jose.valim@gmail.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/activerecord/test/cases/migration_test.rb b/activerecord/test/cases/migration_test.rb
index 1e68911ab3..c4ccce9126 100644
--- a/activerecord/test/cases/migration_test.rb
+++ b/activerecord/test/cases/migration_test.rb
@@ -161,6 +161,11 @@ if ActiveRecord::Base.connection.supports_migrations?
       Person.connection.remove_index("people", :name => good_index_name)
     end
 
+    def test_add_index_attribute_length_limit
+      connection.add_index :testings, [:foo, :bar], :length => {:foo => 10, :bar => nil}
+      assert connection.index_exists?(:testings, [:foo, :bar])
+    end
+
     def test_remove_nonexistent_index
       # we do this by name, so OpenBase is a wash as noted above
       unless current_adapter?(:OpenBaseAdapter)