python - django migrate has error: Specify a USING expression to perform the conversion -

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i change model field charfiled() genericipaddressfield()

ip = models.genericipaddressfield()

and use django 1.7 migrate 

./manage.py makemigrations core ./manage.py migrate

but there error: 

return self.cursor.execute(sql, params) django.db.utils.programmingerror: column "ip" cannot cast automatically type inet hint:  specify using expression perform conversion.

i try this,but not work:

alter table core_message alter column ip type inet using (ip::inet);

error:

error:  invalid input syntax type inet: ""

what can now?
please me thank you!

one quick fix drop , create field:

  1. delete migration changing field type.
  2. delete/comment field ip
  3. make migrations
  4. get back/uncomment field ip new field type
  5. make migrations
  6. migrate

i did in production , restored data previous csv backup , python script of few lines code.


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