Bump sqlalchemy from 1.4.15 to 1.4.21

[dependabot[bot]]

Bumps sqlalchemy from 1.4.15 to 1.4.21.

Release notes

Sourced from sqlalchemy's releases.

1.4.21

Released: July 14, 2021

orm

• [orm] [usecase] Modified the approach used for history tracking of scalar object relationships that are not many-to-one, i.e. one-to-one relationships that would otherwise be one-to-many. When replacing a one-to-one value, the "old" value that would be replaced is no longer loaded immediately, and is instead handled during the flush process. This eliminates an historically troublesome lazy load that otherwise often occurs when assigning to a one-to-one attribute, and is particularly troublesome when using "lazy='raise'" as well as asyncio use cases.

  This change does cause a behavioral change within the _orm.AttributeEvents.set() event, which is nonetheless currently documented, which is that the event applied to such a one-to-one attribute will no longer receive the "old" parameter if it is unloaded and the _orm.relationship.active_history flag is not set. As is documented in _orm.AttributeEvents.set(), if the event handler needs to receive the "old" value when the event fires off, the active_history flag must be established either with the event listener or with the relationship. This is already the behavior with other kinds of attributes such as many-to-one and column value references.

  The change additionally will defer updating a backref on the "old" value in the less common case that the "old" value is locally present in the session, but isn't loaded on the relationship in question, until the next flush occurs. If this causes an issue, again the normal _orm.relationship.active_history flag can be set to True on the relationship.

  References: #6708

• [orm] [bug] [regression] Fixed regression caused in 1.4.19 due to #6503 and related involving _orm.Query.with_entities() where the new structure used would be inappropriately transferred to an enclosing _orm.Query when making use of set operations such as _orm.Query.union(), causing the JOIN instructions within to be applied to the outside query as well.

  References: #6698

• [orm] [bug] [regression] Fixed regression which appeared in version 1.4.3 due to #6060 where rules that limit ORM adaptation of derived selectables interfered with other ORM-adaptation based cases, in this case when applying adaptations for a _orm.with_polymorphic() against a mapping which uses a _orm.column_property() which in turn makes use of a scalar select that includes a _orm.aliased() object of the mapped table.

... (truncated)

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Superseded by #357.