[web2py] defining constraints in model.

[goutham]

whats the diff between

####### case 1 #########
db.define_table('branch',
    Field('name','string', unique = True, notnull = True),
    Field('content','list:string'))


###### case 2 #######
db.define_table('branch',
    Field('name','string'),
    Field('content','list:string'))

db.branch.name.unique = True
db.branch.name.notnull = True
#############

in case 2 the database is not added with the unique and notnull
constraints .. why is this happening whats the diff..