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django / django / db / backends / mysql / validation.py

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from django.db.backends import BaseDatabaseValidation

class DatabaseValidation(BaseDatabaseValidation):
    def validate_field(self, errors, opts, f):
        """
        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 models
        varchar_fields = (models.CharField, models.CommaSeparatedIntegerField,
                models.SlugField)
        if isinstance(f, varchar_fields) and f.max_length > 255 and f.unique:
            msg = '"%(name)s": %(cls)s cannot have a "max_length" greater than 255 when using "unique=True".'
            errors.add(opts, msg % {'name': f.name, 'cls': f.__class__.__name__})