替换Laravel deleted_at为NULL的默认行为，使deleted_at不能为NULL

LaravelCustomSoftDeletes.php

<?php
/**
 * 覆盖Laravel默认的SoftDeletingScope，用于替换软删除的默认行为，解决deleted_at必须为NULL的问题，为NULL的时候不能设置唯一键
 * 即默认是whereNull更改为判断timestamp的0值，即'0000-00-00 00:00:00'(空字符串在某些数据库版本也是可以的，但是考虑到兼容性还是写全吧)，以使deleted_at字段不为NULL
 * 使用时只需要把之前Model中的`use SoftDeletes`替换为`use LaravelCustomSoftDeletes`即可
 * 请自行替换本文件中的部分命名空间
 */

namespace Common;

use Illuminate\Database\Eloquent\Builder;
use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\SoftDeletes;
use Illuminate\Database\Eloquent\SoftDeletingScope;

/**
 * 
 * Class CustomSoftDeletingScope
 * @package Common
 */
class CustomSoftDeletingScope extends SoftDeletingScope
{
    public function apply(Builder $builder, Model $model)
    {
        $builder->where($model->getQualifiedDeletedAtColumn(), '0000-00-00 00:00:00');

        $this->extend($builder);
    }
}

/**
 * 复写Laravel自带的SoftDeletes中的部分方法
 *
 * Trait ComposedSoftDelete
 * @package Common
 */
trait ComposedSoftDelete
{
    /**
     * Boot the soft deleting trait for a model.
     *
     * @return void
     */
    public static function bootSoftDeletes()
    {
        static::addGlobalScope(new CustomSoftDeletingScope);
    }

    /**
     * Restore a soft-deleted model instance.
     *
     * @return bool|null
     */
    public function restore()
    {
        // If the restoring event does not return false, we will proceed with this
        // restore operation. Otherwise, we bail out so the developer will stop
        // the restore totally. We will clear the deleted timestamp and save.
        if ($this->fireModelEvent('restoring') === false) {
            return false;
        }

        $this->{$this->getDeletedAtColumn()} = '0000-00-00 00:00:00';

        // Once we have saved the model, we will fire the "restored" event so this
        // developer will do anything they need to after a restore operation is
        // totally finished. Then we will return the result of the save call.
        $this->exists = true;

        $result = $this->save();

        $this->fireModelEvent('restored', false);

        return $result;
    }

    /**
     * Determine if the model instance has been soft-deleted.
     *
     * @return bool
     */
    public function trashed()
    {
        return $this->{$this->getQualifiedDeletedAtColumn()} != '0000-00-00 00:00:00';
    }

    /**
     * Get a new query builder that includes soft deletes.
     *
     * @return \Illuminate\Database\Eloquent\Builder|static
     */
    public static function withTrashed()
    {
        return (new static)->newQueryWithoutScope(new CustomSoftDeletingScope);
    }

    /**
     * Get a new query builder that only includes soft deletes.
     *
     * @return \Illuminate\Database\Eloquent\Builder|static
     */
    public static function onlyTrashed()
    {
        $instance = new static;

        $column = $instance->getQualifiedDeletedAtColumn();

        return $instance->newQueryWithoutScope(new CustomSoftDeletingScope)->where($column, '!=', '0000-00-00 00:00:00');
    }
}

/**
 * 只需要继承这个SoftDeletes即可实现替换
 *
 * Trait CustomSoftDeletes
 * @package Common
 */
trait LaravelCustomSoftDeletes
{
    use SoftDeletes, ComposedSoftDelete {
        ComposedSoftDelete::bootSoftDeletes insteadof SoftDeletes;
        ComposedSoftDelete::restore insteadof SoftDeletes;
        ComposedSoftDelete::trashed insteadof SoftDeletes;
        ComposedSoftDelete::withTrashed insteadof SoftDeletes;
        ComposedSoftDelete::onlyTrashed insteadof SoftDeletes;
    }
}