How to Map Company -> Estate --> Division - OpenERP ver 7

[tictic]

I'm going to create above mapping.need to many2one fieds & if i select company then need to come to second combo relevant estate for that companies.please help me to sort this issue.I checked 'related' filed type also in developer guide.but haven't idea to implement in that way.check this code & advice me to sort this issue.thanks\[code\]class bpl_company_registration(osv.osv): _name = "bpl.company.registration" _description = "BPL Company registration" _columns = { 'name': fields.char('Company', size=50, help='Company'), }bpl_company_registration()class bpl_estate_registration(osv.osv): _name = "bpl.estate.registration" _description = "BPL Estate registration" _columns = { 'name': fields.char('Estate', size=50, help='Estate'), }bpl_estate_registration()class bpl_division_registration(osv.osv): _name = "bpl.division.registration" _description = "BPL Division registration" _columns = { 'name': fields.char('Division', size=50, help='Division'), }bpl_division_registration()class company_estate_division(osv.osv): _name = "bpl.company.estate.division" _description = "Company Estate Division Mapping" _columns = { 'bpl_company_id':fields.many2one('bpl.company.registration', 'Company', help='Company'), 'bpl_estate_id':fields.many2one('bpl.estate.registration', 'Estate', help='Estate'), 'bpl_division_id':fields.many2one('bpl.division.registration', 'Division', help='Division'), }company_estate_division()\[/code\]