I was having this error in Development but not in Test:
ActiveRecord::SerializationTypeMismatch: cambiable_attributes was supposed to be a Hash, but was a String
Which was the problem? I was using MySQL in Development and SQLite3 in Test. Field 'cambiable_attributes' was type String in the DB schema and had to be type Text. I changed the data type to Text and it worked well in both environments.
