Constraints are a feature available in Django 2.2+. They're a way of

listing explicit constraints on a table, for ensuring that a set of

fields are unique (potentially with a conditional query that must also

be met), or for ensuring that a condition passes when inserting data.

These are dependent on the support in the database backend.
This initial change introduces signature support for

serializing/deserializing information on conditions specified on a

model. They're not currently used as part of the evolution process, but

that will be arriving in an upcoming change.

Unit tests pass for all versions of Python and Django.