from_db_value(value, expression, connection, context)
Converts a value as returned by the database to a Python object. It is the reverse of get_prep_value().
This method is not used for most built-in fields as the database backend already returns the correct Python type, or the backend itself does the conversion.
See Converting values to Python objects for usage.
Note
For performance reasons, from_db_value is not implemented as a no-op on fields which do not require it (all Django fields). Consequently you may not call super in your definition.
When saving, pre_save() and get_db_prep_save() are used:
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