Join on table query without foreign key association

In my project, I did not use foreign keys such as models ForeignKey. Instead, models were used BigIntegralField to associate with other tables
For example: customer-id=models BigIntegralField() stores the ID field of the customer table
But now what should I do if I want to use a join on query?
For example, TopUp. objects. select_delate ("customer-id").

Please help me,thank you!

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