In a database with a schema, if you saved a string to an integer field, the database could either complain or coerce the string to a integer. Either way, whenever you read a row, you know what type the value will be whenever you read data a row from the database. Another issue with schemaless is simply mis-typing the key (column) name. If you have a field called referrer, but you perform a query using referer, MongoDB won’t know you have done anything wrong. In a SQL database you will get an error on insert and on querying. An ORM can instead be the one to tell you about the error.