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from django.core import checks
from django.db.backends import BaseDatabaseValidation
class DatabaseValidation(BaseDatabaseValidation):
def check_field(self, field, **kwargs):
"""
MySQL has the following field length restriction:
No character (varchar) fields can have a length exceeding 255
characters if they have a unique index on them.
"""
from django.db import connection
errors = super(DatabaseValidation, self).check_field(field, **kwargs)
try:
field_type = field.db_type(connection)
except AttributeError:
# If the field is a relative field and the target model is
# missing, then field.rel.to is not a model and doesn't have
# `_meta` attribute.
field_type = ''
if (field_type.startswith('varchar') and field.unique
and (field.max_length is None or int(field.max_length) > 255)):
errors.append(
checks.Error(
('Under mysql backend, the field cannot have a "max_length" '
'greated than 255 when it is unique.'),
hint=None,
obj=field,
id='E047',
)
)
return errors
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