Question

我有3张桌子：

类别
子类
categorySubcategories

1个类别有0到多个子类别。

我有这个函数从类别表中获取所有条目，只获取属于id为1的类别的子类别表的条目：

  def getCategories(conn) do
    categories = all Api.Category
    groceryItemSubcategories = from s in Api.Subcategory,
    join: cs in Api.CategorySubcategory, on: cs.c_id == 1,
    select: %{name: s.name, foo: cs.c_id}
    conn
    |> put_resp_content_type("application/json")
    |> send_resp(200, Poison.encode!(%{categories: categories, groceryItemSubcategories: groceryItemSubcategories}))
  end

发出此错误：

23:10:27.169 [error] #PID<0.339.0> running Api.Router terminated
Server: localhost:4000 (http)
Request: GET /categories
** (exit) an exception was raised:
    ** (Poison.EncodeError) unable to encode value: {"subcategories", Api.Subcategory}
        (poison) lib/poison/encoder.ex:383: Poison.Encoder.Any.encode/2
        (poison) lib/poison/encoder.ex:227: anonymous fn/4 in Poison.Encoder.Map.encode/3
        (poison) lib/poison/encoder.ex:228: Poison.Encoder.Map."-encode/3-lists^foldl/2-0-"/3

        (poison) lib/poison/encoder.ex:228: Poison.Encoder.Map.encode/3
        (poison) lib/poison/encoder.ex:227: anonymous fn/4 in Poison.Encoder.Map.encode/3
        (poison) lib/poison/encoder.ex:228: Poison.Encoder.Map."-encode/3-lists^foldl/2-0-"/3

        (poison) lib/poison/encoder.ex:228: Poison.Encoder.Map.encode/3
        (poison) lib/poison.ex:41: Poison.encode!/2

最后的答案是：

  def getCategories(conn) do
    categories = all Api.Category
    groceryItemSubcategories = Api.Repo.all(from s in Api.Subcategory,
    join: cs in Api.CategorySubcategory, on: cs.s_id == s.id,
    join: c in Api.Category, on: c.id == cs.c_id,
    where: c.id == 1,
    select: %{name: s.name, foo: cs.c_id}
    )
    conn
    |> put_resp_content_type("application/json")
    |> send_resp(200, Poison.encode!(%{categories: categories, groceryItemSubcategories: groceryItemSubcategories}))
  end

使错误消失的部分将语句包装在Api.Repo.All()中。 Dogbert真的是那个回答它的人，所以我不想回答这个问题。

Answer 1

原始代码存在两个主要问题：

您忘记在第二个查询中致电Repo.all。

您在查询中选择了一个元组，然后将其编码为JSON。 Poison确实将编码元组处理为JSON。您可以改为选择列表或地图，具体取决于您想要的数据结构。以下是如何选择地图：

groceryItemSubcategories = Api.Repo.all(from s in Api.Subcategory,
  join: cs in Api.CategorySubcategory, on: cs.s_id == s.id,
  join: c in Api.Category, on: c.id == cs.c_id,
  where: c.id == 1,
  select: %{name: s.name, c_id: cs.c_id})

Ecto连接查询导致（Poison.EncodeError）无法编码值

1 个答案: