Unique index can have a null value, the primary key index can not be empty
In some scenarios, you can not take the index
limit_choices_to = {} in effect admin
db_constraint = True whether to create a foreign key constraint in the database,
Unique index can have a null value, the primary key index can not be empty
In some scenarios, you can not take the index
limit_choices_to = {} in effect admin
db_constraint = True whether to create a foreign key constraint in the database,