my-first-blog/myvenv/Lib/site-packages/django/db/backends/mysql/schema.py

from django.db.backends.base.schema import BaseDatabaseSchemaEditor
from django.db.models import NOT_PROVIDED


class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):

    sql_rename_table = "RENAME TABLE %(old_table)s TO %(new_table)s"

    sql_alter_column_null = "MODIFY %(column)s %(type)s NULL"
    sql_alter_column_not_null = "MODIFY %(column)s %(type)s NOT NULL"
    sql_alter_column_type = "MODIFY %(column)s %(type)s"

    # No 'CASCADE' which works as a no-op in MySQL but is undocumented
    sql_delete_column = "ALTER TABLE %(table)s DROP COLUMN %(column)s"

    sql_rename_column = "ALTER TABLE %(table)s CHANGE %(old_column)s %(new_column)s %(type)s"

    sql_delete_unique = "ALTER TABLE %(table)s DROP INDEX %(name)s"

    sql_delete_fk = "ALTER TABLE %(table)s DROP FOREIGN KEY %(name)s"

    sql_delete_index = "DROP INDEX %(name)s ON %(table)s"

    sql_create_pk = "ALTER TABLE %(table)s ADD CONSTRAINT %(name)s PRIMARY KEY (%(columns)s)"
    sql_delete_pk = "ALTER TABLE %(table)s DROP PRIMARY KEY"

    sql_create_index = 'CREATE INDEX %(name)s ON %(table)s (%(columns)s)%(extra)s'

    def quote_value(self, value):
        self.connection.ensure_connection()
        quoted = self.connection.connection.escape(value, self.connection.connection.encoders)
        if isinstance(value, str):
            quoted = quoted.decode()
        return quoted

    def _is_limited_data_type(self, field):
        db_type = field.db_type(self.connection)
        return db_type is not None and db_type.lower() in self.connection._limited_data_types

    def skip_default(self, field):
        return self._is_limited_data_type(field)

    def add_field(self, model, field):
        super().add_field(model, field)

        # Simulate the effect of a one-off default.
        # field.default may be unhashable, so a set isn't used for "in" check.
        if self.skip_default(field) and field.default not in (None, NOT_PROVIDED):
            effective_default = self.effective_default(field)
            self.execute('UPDATE %(table)s SET %(column)s = %%s' % {
                'table': self.quote_name(model._meta.db_table),
                'column': self.quote_name(field.column),
            }, [effective_default])

    def _field_should_be_indexed(self, model, field):
        create_index = super()._field_should_be_indexed(model, field)
        storage = self.connection.introspection.get_storage_engine(
            self.connection.cursor(), model._meta.db_table
        )
        # No need to create an index for ForeignKey fields except if
        # db_constraint=False because the index from that constraint won't be
        # created.
        if (storage == "InnoDB" and
                create_index and
                field.get_internal_type() == 'ForeignKey' and
                field.db_constraint):
            return False
        return not self._is_limited_data_type(field) and create_index

    def _delete_composed_index(self, model, fields, *args):
        """
        MySQL can remove an implicit FK index on a field when that field is
        covered by another index like a unique_together. "covered" here means
        that the more complex index starts like the simpler one.
        http://bugs.mysql.com/bug.php?id=37910 / Django ticket #24757
        We check here before removing the [unique|index]_together if we have to
        recreate a FK index.
        """
        first_field = model._meta.get_field(fields[0])
        if first_field.get_internal_type() == 'ForeignKey':
            constraint_names = self._constraint_names(model, [first_field.column], index=True)
            if not constraint_names:
                self.execute(self._create_index_sql(model, [first_field], suffix=""))
        return super()._delete_composed_index(model, fields, *args)

    def _set_field_new_type_null_status(self, field, new_type):
        """
        Keep the null property of the old field. If it has changed, it will be
        handled separately.
        """
        if field.null:
            new_type += " NULL"
        else:
            new_type += " NOT NULL"
        return new_type

    def _alter_column_type_sql(self, model, old_field, new_field, new_type):
        new_type = self._set_field_new_type_null_status(old_field, new_type)
        return super()._alter_column_type_sql(model, old_field, new_field, new_type)

    def _rename_field_sql(self, table, old_field, new_field, new_type):
        new_type = self._set_field_new_type_null_status(old_field, new_type)
        return super()._rename_field_sql(table, old_field, new_field, new_type)