Thinkphp5 association one-to-one add operation display save non-object

I read the manual and wrote down the user rights operation plan to use the association model
use a table to associate admin and permissions table before contacting

create table admin_group(
    id int auto_increment primary key ,
    admin_id int ,  -sharp
    auth_group_id int-sharp 
)

1. The administrator table model wants to associate this admin_group table. When inserting an administrator data, it will automatically insert a piece of data into the admin_group

.
class Admin extends Model  {
public function admin_group(){
        $this->hasOne("admin_group","admin_id","id");
    }
}

2. This is the AdminGroupmo model

class AdminGroupmo extends Model  {
        -sharp
}

3. Then call

through other methods in the Admin model
class Admin extends Model  {
public function t2($sqldata){
        $this->insert($sqldata);-sharpadmin 
        $this->admin_group()->save(["auth_group_id"=>"6"]); -sharp
    }
}

the result is Call to a member function save () on a non-object

could you tell me which step I made a mistake or? I also want to point it out

.
Php
Mar.22,2021

return $this- > hasOne ('admin_group','admin_id','id');
forgot to add

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