Hello Michael

I guess the method inside which you use create call have old api argument.

Example:

def btn_send(self,cr,uid,ids,context=None):

     self.create({name:'AAAA'}) #you cant use new api call in old api like this

This is wrong

Now, what you have to do is :

@api.v8

def btn_send(self):

     self.write({name:'AAAA'}) 

You can use any method but you cant use create method because you are trying to create the record in which you are already in.

Complete Example:

class a(models.Model)

_name = 'a.a'

con = fields.Char('Country')

class B(models.Model)

_name = 'b.b'

lnam = fields.Char('lnam')

@api.v8

def btn_join(self):

  self.env['a.a'].create({con:self.lnam}) #result : value_of_Lnam in Country

You can refer guidline of How to use Environment in new api click here 

Hope that this answer will help you.

Please don't hesitate to ask any question regarding this.

Thank You