Question

Using sqlite3 and Django I want to change to PostgreSQL and keep all data intact. I used ./manage.py dumpdata > dump.json to dump the data, and changed settings to use PostgreSQL.

With an empty database ./manage.py loaddata dump.json resulted in errors about tables not existing, so I ran ./manage.py syncdb and tried again. That results in this error:

Problem installing fixture 'dump.json': Traceback (most recent call last):
  File "/usr/lib/python2.6/site-packages/django/core/management/commands/loaddata.py", line 163, in handle
    obj.save()
  File "/usr/lib/python2.6/site-packages/django/core/serializers/base.py", line 163, in save
    models.Model.save_base(self.object, raw=True)
  File "/usr/lib/python2.6/site-packages/django/db/models/base.py", line 495, in save_base
    rows = manager.filter(pk=pk_val)._update(values)
  File "/usr/lib/python2.6/site-packages/django/db/models/query.py", line 448, in _update
    return query.execute_sql(None)
  File "/usr/lib/python2.6/site-packages/django/db/models/sql/subqueries.py", line 124, in execute_sql
    cursor = super(UpdateQuery, self).execute_sql(result_type)
  File "/usr/lib/python2.6/site-packages/django/db/models/sql/query.py", line 2347, in execute_sql
    cursor.execute(sql, params)
  File "/usr/lib/python2.6/site-packages/django/db/backends/util.py", line 19, in execute
    return self.cursor.execute(sql, params)
IntegrityError: duplicate key value violates unique constraint "django_content_type_app_label_key"

Is this not the correct way to move data from one database to another? How should I switch database backend safely?

Answer 1

The problem is simply that you’re getting the content types defined twice – once when you do syncdb, and once from the exported data you’re trying to import. Since you may well have other items in your database that depend on the original content type definitions, I would recommend keeping those.

So, after running syncdb, do manage.py dbshell and in your database do TRUNCATE django_content_type; to remove all the newly-defined content types. Then you shouldn’t get any conflicts – on that part of the process, in any case.