如何在Ecto连接表中关联两个现有模型？解决必填校验报错

问题背景 #

我有Practitioner和Product两个模型，二者为多对多关系，通过ProductPractitioners连接表关联，但保存关联时出现can't be blank错误，请问关联两个现有实体到连接表的正确方式是什么？

模型代码 #

products_practitioners #

10   schema "products_practitioners" do
 11     belongs_to :product, Product
 12     belongs_to :practitioner, Practitioner
 13
 14     timestamps()
 15   end

 18   def changeset(products_practitioners, attrs) do
 19     products_practitioners
 20     # TODO: Don´t accept invalid params here
 21     |> cast(attrs, [])
 22     |> validate_required([:product, :practitioner])
 23   end

products #

10   schema "products" do
 11     field :name, :string
 12     many_to_many :practitioners, Practitioner, join_through: ProductsPractitioners
 13
 14     timestamps()
 15   end

practitioners #

11   schema "practitioners" do
 12     field :first_name, :string
 13     field :last_name, :string
 14
 15     has_one :calendar, Calendar
 16     many_to_many :products, Product, join_through: ProductsPractitioners
 17
 18     timestamps()
 19   end

创建实体的IEX操作 #

iex(11)> {:ok, product} = Factory.product_factory |> Booking.Repo.insert


iex(12)> {:ok, practictioner} = Factory.practitioner_factory |> Booking.Repo.insert


iex(13)> attrs = %{product: product, practictioner: practictioner}
%{
  practictioner: %Booking.Practitioners.Practitioner{
    __meta__: #Ecto.Schema.Metadata<:loaded, "practitioners">,
    id: "52dc8f9e-d3b4-4ce5-9016-6a3ef07679e2",
    first_name: "Judd",
    last_name: "Klocko",
    calendar: #Ecto.Association.NotLoaded<association :calendar is not loaded>,
    products: #Ecto.Association.NotLoaded<association :products is not loaded>,
    inserted_at: ~N[2022-12-10 09:31:21],
    updated_at: ~N[2022-12-10 09:31:21]
  },
  product: %Booking.Products.Product{
    __meta__: #Ecto.Schema.Metadata<:loaded, "products">,
    id: "bcb98f0e-e5a2-40cb-bd9a-0eac62ddf0a8",
    name: "qui",
    practitioners: #Ecto.Association.NotLoaded<association :practitioners is not loaded>,
    inserted_at: ~N[2022-12-10 09:31:40],
    updated_at: ~N[2022-12-10 09:31:40]
  }
}

尝试关联的IEX操作及错误 #

iex(14)> %ProductsPractitioners{}|> ProductsPractitioners.changeset(attrs)|> Repo.insert()
{:error,
 #Ecto.Changeset<
   action: :insert,
   changes: %{},
   errors: [
     product: {"can't be blank", [validation: :required]},
     practitioner: {"can't be blank", [validation: :required]}
   ],
   data: #Booking.Products.ProductsPractitioners<>,
   valid?: false
 >}

错误原因及解决方案 #

错误原因 #

1. Changeset字段配置错误：连接表的belongs_to关联对应的数据库字段是product_id和practitioner_id，但原changeset的cast方法未包含这两个字段，导致传入参数无法被解析；同时validate_required检查的是关联名:product、:practitioner，而非实际存储的字段名，不符合Ecto验证逻辑。
2. 参数传递方式错误：直接传入完整模型结构体，而非关联对应的ID字段，结合错误的changeset配置，最终导致验证失败。

正确解决步骤 #

步骤1：修复连接表的Changeset

修改products_practitioners的changeset方法，将实际数据库字段加入cast和validate_required：

def changeset(products_practitioners, attrs) do
  products_practitioners
  |> cast(attrs, [:product_id, :practitioner_id])
  |> validate_required([:product_id, :practitioner_id])
end

步骤2：两种关联实现方式

方式一：直接插入连接表记录

传递实体的ID参数到连接表：

attrs = %{product_id: product.id, practitioner_id: practictioner.id}
%ProductsPractitioners{} |> ProductsPractitioners.changeset(attrs) |> Repo.insert()

方式二：通过Ecto多对多关联方法操作（推荐）

利用Ecto提供的put_assoc方法，通过主模型管理关联，更符合Ecto关联设计：

# 通过Product模型添加关联的Practitioner
product
|> Ecto.Changeset.change()
|> Ecto.Changeset.put_assoc(:practitioners, [practictioner])
|> Repo.update()

# 若要添加多个关联，扩展列表即可：
# |> Ecto.Changeset.put_assoc(:practitioners, [practictioner1, practictioner2])

内容的提问来源于stack exchange，提问作者martins