config.yml

SilverStripe\Core\Injector\Injector:
  NullableBoolean:
    class: App\ORM\FieldType\DBNullableBoolean

DBNullableBoolean.php

<?php

namespace App\ORM\FieldType;

use SilverStripe\Forms\DropdownField;
use SilverStripe\Forms\OptionsetField;
use SilverStripe\ORM\DB;
use SilverStripe\ORM\FieldType\DBBoolean;

class DBNullableBoolean extends DBBoolean
{
    public function __construct($name = null, $defaultVal = null)
    {
        parent::__construct($name);

        $this->defaultVal = null;
        if ($defaultVal !== null) {
            $this->defaultVal = ($defaultVal) ? 1 : 0;
        }
    }

    public function requireField()
    {
        $spec = 'tinyint(1) unsigned';
        if (isset($this->defaultVal)) {
            $spec .= ' default ' . DB::get_conn()->quoteString($this->defaultVal);
        }

        DB::require_field($this->tableName, $this->name, $spec);
    }

    public function Nice()
    {
        if ($this->value === null) {
            return 'Not set';
        }

        return ($this->value)
            ? _t('SilverStripe\\ORM\\FieldType\\DBBoolean.YESANSWER', 'Yes')
            : _t('SilverStripe\\ORM\\FieldType\\DBBoolean.NOANSWER', 'No');
    }

    public function NiceAsBoolean()
    {
        if ($this->value === null) {
            return 'null';
        }

        return ($this->value) ? 'true' : 'false';
    }

    public function saveInto($dataObject)
    {
        $fieldName = $this->name;
        if ($fieldName) {
            if ($this->value === null) {
                $dataObject->$fieldName = null;
            } else {
                $dataObject->$fieldName = ($this->value) ? 1 : 0;
            }
        } else {
            $class = static::class;
            user_error("DBField::saveInto() Called on a nameless '$class' object", E_USER_ERROR);
        }
    }

    public function scaffoldFormField($title = null, $params = null)
    {
        return new OptionsetField($this->name, $title, [
            1 => _t('SilverStripe\\ORM\\FieldType\\DBBoolean.YESANSWER', 'Yes'),
            0 => _t('SilverStripe\\ORM\\FieldType\\DBBoolean.NOANSWER', 'No')
        ]);
    }

    public function scaffoldSearchField($title = null)
    {
        $anyText = _t('SilverStripe\\ORM\\FieldType\\DBBoolean.ANY', 'Any');
        $source = [
            1 => _t('SilverStripe\\ORM\\FieldType\\DBBoolean.YESANSWER', 'Yes'),
            0 => _t('SilverStripe\\ORM\\FieldType\\DBBoolean.NOANSWER', 'No')
        ];

        $field = new DropdownField($this->name, $title, $source);
        $field->setEmptyString("($anyText)");
        return $field;
    }

    public function nullValue()
    {
        return null;
    }

    public function writeToManipulation(&$manipulation)
    {
        $manipulation['fields'][$this->name] = $this->prepValueForDB($this->value);
    }

    public function prepValueForDB($value)
    {
        if (is_bool($value)) {
            return $value ? 1 : 0;
        } elseif (is_string($value)) {
            switch (strtolower($value)) {
                case 'false':
                case 'f':
                    return 0;
                case 'true':
                case 't':
                    return 1;
                case 'null':
                    return null;
            }
        }

        if ($value === null) {
            return null;
        }

        return $value ? 1 : 0;
    }
}