外鍵
If a model has a ForeignKey, instances of that model will have access to the related (foreign) object via a simple attribute of the model. 如果一個(gè)模型有一個(gè)外鍵，（那它通過(guò)）模型的一個(gè)實(shí)例將可以通過(guò)一個(gè)模型屬性來(lái)訪問(wèn)（外鍵）關(guān)系對(duì)象
