IsUnique Rule Changes Explained

If you have unique indexes on nullable columns, you’ve probably noticed some confusing
changes to the IsUnique application rule in CakePHP 4.

In CakePHP 3, IsUnique has an option named allowMultipleNulls that defaults to true.
This option controls whether NULL values are considered unique. If they are unique then
the rule returns true when any field contains a NULL value.

When CakePHP 4 was released, the allowMultipleNulls option was removed due to a miscommuncation.
This caused IsUnique to fail if a row matched the fields including NULL values.

However, this issue was caught and the option reintroduced in CakePHP 4.2 with a different implementation.
Due to backwards compatible rules for releases, the default was set to false because CakePHP 4.0 did
not include it.

Baking allowMultipleNulls in Application Rules

Bake 2.6 will automatically add the allowMultipleNulls option for any unique index that contains
a nullable column. This will be set to true for all databases except SQL Server. This should be easier
for users to understand the behavior and know where to change the option.

You should see unique rules generated something like this:

$rules->add($rules->isUnique(['field_1', 'field_2'], ['allowMultipleNulls' => true]), ['errorField' => 'field_1']);

There are always edge cases for rules like this. Some users might have nullable columns but maintain unique
null values. Some users might have a solution for SQL Server to consider NULL unique. Our goal is to
provide reasonable and reliable defaults.

Note

Bake 2.6 also includes support for baking unique rules with multiple fields.

CakePHP 5 will default allowMultipleNulls to true

We understand there is confusion caused by changing default behavior of options that are hidden,
but we think this needs to be corrected the right way.

CakePHP 5 will default this behavior back to allowing multiple NULL values.

With the above changes to Bake, this should not be a signifcant change in behavior.

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