[web2py] Problemas com Mysql

[Fernando J]

I have an error with mysql (about reference):

*"1452, u'Cannot add or update a child row"*

The table is created with

*db.define_table('navbar',
    Field("title", "string"),
    Field("url", "string", requires=IS_EMPTY_OR(IS_URL())), 
    Field("c", label="Controller"),
    Field("f", label="Function"), 
    Field("args", label="Arguments"), 
    Field("sortable", "integer"), 
    Field("parent_id","reference navbar"),
    Field('created_on','datetime',default=request.now,
          label=T('Created On'),writable=False,readable=False),
    Field('modified_on','datetime',default=request.now,
          label=T('Modified On'),writable=False,readable=False,
          update=request.now),
    Field('registration_key',default='',
          writable=False,readable=False),
    Field('reset_password_key',default='',
          writable=False,readable=False),
    Field('registration_id',default='',
          writable=False,readable=False),         
    format='%(title)s',
       migrate=False)*

Any suggestion, please?

Thanks in Advance!