Nullable Behavior

The framework core in some places treats the incoming value too literally, without introspecting the schema. The schema could say NOT NULL, meaning you do not have to provide a string value. But if you do that, you easily end up with two empty values: null and '' (empty string).

You should attach the following behavior for better data consistency:

// In your Table
$this->addBehavior('Shim.Nullable');

This makes sure empty strings posted will respect the nullable type of the table schema. Your queries then only have to check for one type, instead of this code smell:

// check for empty
->where(['OR' => ['field_name IS' => null, 'field_name' => '']])

// check for non-empty
->where(['field_name IS NOT' => null, 'field_name !=' => ''])

Now it can always be this for DEFAULT NOT NULL database fields:

// check for empty
->where(['field_name' => ''])

// check for non-empty
->where(['field_name !=' => ''])

And this for DEFAULT NULL database fields:

// check for empty
->where(['field_name IS' => null])

// check for non-empty
->where(['field_name IS NOT' => null])

INFO

See this ticket for details and the hope that this one day makes it into the core.

Callback options

You can define the time this should happen.

Default callback affects validation

Be careful with the default setting (beforeMarshal), as this can affect validation rules. Some rules use the empty-string existence to validate fields — through isset() checks. Those checks might not work as expected if the input is already back to null. There you would then need array_key_exists().

Another option is to apply the data-integrity cleanup on beforeSave instead:

// In your Table
$this->addBehavior('Shim.Nullable', ['on' => 'beforeSave']);