在不更改数据库的情况下导出Django import_export csv文件之前，如何对其进行更改

from import_export import resources

class CustomResource(resources.ModelResource):
    def before_export(self, queryset, *args, **kwargs):
        # 在导出之前对数据进行修改
        for obj in queryset:
            # 对每个对象进行修改
            obj.field = "modified value"

from django.http import HttpResponse
from import_export import resources, fields
from import_export.admin import ExportMixin

class MyModelResource(resources.ModelResource):
    # 定义需要导出的字段
    field = fields.Field(attribute='field', column_name='Field')

    class Meta:
        model = MyModel
        fields = ('field',)

class MyModelAdmin(ExportMixin, admin.ModelAdmin):
    resource_class = MyModelResource

    def export_csv(self, request, queryset):
        # 创建自定义资源类的实例
        resource = CustomResource()
        # 调用before_export()方法对数据进行修改
        resource.before_export(queryset)
        # 导出csv文件
        dataset = resource.export(queryset)
        response = HttpResponse(dataset.csv, content_type='text/csv')
        response['Content-Disposition'] = 'attachment; filename="export.csv"'
        return response

    actions = [export_csv]