web2py：根据另一个表字段值更新字段值

Question

在下面的模型中，如何在编辑过程中将services.interest_rate的值分配给loans.interest_rate？

db.define_table('services',
                Field('service_name',requires=[IS_NOT_EMPTY(),IS_NOT_IN_DB(db,'services.service_name')]),
                Field('service_type','reference service_types',requires=IS_IN_DB(db,db.service_types.id,
                                                                                '%(type_name)s',
                                                                                error_message='not in table',
                                                                                zero=None),
                                                                                ondelete='RESTRICT',
                                                                                ),
                Field('interest_rate','decimal(15,2)',requires=IS_DECIMAL_IN_RANGE(0,100)),
                Field('max_term','integer'),
                auth.signature,
                format='%(service_name)s',
    )

if db(db.service_types).count() < 1:
    db.service_types.insert(type_name='Loan')
    db.service_types.insert(type_name='Contribution')
    db.service_types.insert(type_name='Other')

db.define_table('members',
    Field('member_name'),                                       
    )

db.define_table('loan',
    Field('service','reference services',requires=IS_IN_DB(db,db.services.id,
                                            '%(service_name)s',
                                            error_message='not in table',
                                            zero=None),
                                            ondelete='RESTRICT',),
    Field('member_id','reference members',requires=IS_IN_DB(db,db.members.id,
                                            '%(member_name)s',
                                            error_message='not in table',
                                            zero=None),
                                            ondelete='RESTRICT',),
    Field('amount','decimal(15,2)',requires=IS_DECIMAL_IN_RANGE(1000)),
    Field('interest_rate','decimal(6,2)'),
    )   

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