Validators can fine-tune NHib metadata (Generate SQL constraints) when ApplyToDDL option is enabled.
Let it do backward task - if no validation defined explicitly, extract NHib mappings info and generate validator based on this info with AutoGenerateFromMapping option.
Entities mapped to DB with some NHib convention.
If length of some string field must be changed - specific validator declared and ApplyToDDL option used.
For properties where length is defined by convention - no specific validators declared, but validation attributes are created in run-time when AutoGenerateFromMapping option is enabled. .