Hello All 

first of all , thanks in advanced :)

I have the Following 2 fields:

freight_type = fields.Selection(
    [('Ocean', 'Ocean'), ('Air', 'air'), ('Land', 'Land')],
    default='',
    required=True
)

ship_type = fields.Selection(
    [('FCL', 'fcl'), ('LCL', 'lcl'), ('Air', 'air'), ('Land', 'land')],
    default='',
    required=True
)

What I tried:

@api.onchange('freight_type')
def onchange_freight_type(self):
for rec in self:
if rec.freight_type == 'Ocean':
rec.ship_type = 'FCL'
elif rec.freight_type == 'Air':
rec.ship_type = 'Air'
elif rec.freight_type == 'Land':
rec.ship_type = 'Land'
else:
rec.ship_type = ''

Not what I need, cause  rec.ship_type may have  2  values 

[('FCL', 'fcl'), ('LCL', 'lcl')] or one value
[('Air', 'air')]

What I Need :

When I choice value 'Ocean' from field  freight_type I want to filter ship_type to show only values

[('FCL', 'fcl'), ('LCL', 'lcl')]

When I choice value 'Air' from field  freight_type I want to filter ship_type to show only values

[('Air', 'air')]

When I choice value 'Land' from field  freight_type I want to filter ship_type to show only values

[('Land', 'land')]

So, the field ship_type  will show values which depends on field  freight_type 

regards ,