error trying to fetch from database and return result in http response - ecto

Question

If I run Api.Category |> Api.Repo.all in iex I get this response in the terminal (basically I get two rows from the "categories" database table):
iex(1)> Api.Category |> Api.Repo.all
16:21:55.775 [debug] QUERY OK source="categories" db=5.2ms decode=6.3ms
SELECT c0."id", c0."name" FROM "categories" AS c0 []
[%Api.Category{__meta__: #Ecto.Schema.Metadata<:loaded, "categories">, id: 1,
name: "Grocery Products"},
%Api.Category{__meta__: #Ecto.Schema.Metadata<:loaded, "categories">, id: 2,
name: "Meals"}]
I am trying to send that response back in an http response but am getting this error (I think it is just not retrieving anything from the database).
Poison.EncodeError at GET /categories unable to encode value: {nil, "categories"} lib/poison/encoder.ex
378 def encode(%{__struct__: _} = struct, options) do
379 Poison.Encoder.Map.encode(Map.from_struct(struct), options)
380 end
381
382 def encode(value, _options) do
383 raise Poison.EncodeError, value: value
384 end
385end
Here is my function that tries to get the entries in "categories" database table and return them in an http response:
def getCategories(conn) do
categories = Api.Category |> Api.Repo.all
conn
|> put_resp_content_type("application/json")
|> send_resp(200, Poison.encode!(%{categories: categories}))
end
What am I doing wrong?
I set up the connections:
application.ex:
defmodule Api.Application do
use Application
def start( _type, _args ) do
import Supervisor.Spec, warn: false
children = [
worker(__MODULE__, [], function: :run),
supervisor(Api.Repo, []),
]
opts = [strategy: :one_for_one, name: Api.Supervisor]
Supervisor.start_link(children, opts)
end
def run do
{ :ok, _ } = Plug.Adapters.Cowboy.http Api.Router, []
end
end
repo.ex has all my database queries (including getCategories which should be in a controller) which I have been told is weird but hey, iteration 1, i just wanna get it working first:
defmodule Api.Repo do
use Ecto.Repo, otp_app: :api
require Ecto.Query
import Plug.Conn
def insertCategories do
categories = [
%Api.Category{name: "Grocery Products"},
%Api.Category{name: "Meals"}
]
Enum.each(categories, fn (category) -> insert(category) end)
end
def insertSubcategories do
subcategories = [
%Api.Subcategory{name: "Meat"},
%Api.Subcategory{name: "Dairy"},
%Api.Subcategory{name: "Confectionary"},
%Api.Subcategory{name: "Dessert"},
%Api.Subcategory{name: "Baking"},
%Api.Subcategory{name: "Condiments"},
%Api.Subcategory{name: "Beverages"},
%Api.Subcategory{name: "African"},
%Api.Subcategory{name: "Argentine"},
%Api.Subcategory{name: "Asian"},
%Api.Subcategory{name: "Asian Fusion"},
%Api.Subcategory{name: "BBQ"},
%Api.Subcategory{name: "Bakery"},
%Api.Subcategory{name: "Beverages"},
%Api.Subcategory{name: "Brazilian"},
%Api.Subcategory{name: "Breakfast"},
%Api.Subcategory{name: "British"},
%Api.Subcategory{name: "Cafe"},
%Api.Subcategory{name: "Cambodian"},
%Api.Subcategory{name: "Chinese"},
%Api.Subcategory{name: "Coffee and Tea"},
%Api.Subcategory{name: "Contemporary"},
%Api.Subcategory{name: "Continental"},
%Api.Subcategory{name: "Deli"},
%Api.Subcategory{name: "Desserts"},
%Api.Subcategory{name: "Drinks Only"},
%Api.Subcategory{name: "European"},
%Api.Subcategory{name: "Fijian"},
%Api.Subcategory{name: "Filipino"},
%Api.Subcategory{name: "Finger Food"},
%Api.Subcategory{name: "Fish and Chips"},
%Api.Subcategory{name: "French Fusion"},
%Api.Subcategory{name: "German"},
%Api.Subcategory{name: "Greek"},
%Api.Subcategory{name: "Grill"},
%Api.Subcategory{name: "Healthy Food"},
%Api.Subcategory{name: "Ice Cream"},
%Api.Subcategory{name: "Indian"},
%Api.Subcategory{name: "Indonesian"},
%Api.Subcategory{name: "International"},
%Api.Subcategory{name: "Irish"},
%Api.Subcategory{name: "Italian"},
%Api.Subcategory{name: "Japanese"},
%Api.Subcategory{name: "Jewish"},
%Api.Subcategory{name: "Juices"},
%Api.Subcategory{name: "Kiwi"},
%Api.Subcategory{name: "Korean"},
%Api.Subcategory{name: "Latin"},
%Api.Subcategory{name: "American"},
%Api.Subcategory{name: "Lebanese"},
%Api.Subcategory{name: "Malaysian"},
%Api.Subcategory{name: "Mediterranean"},
%Api.Subcategory{name: "Mexican"},
%Api.Subcategory{name: "Middle Eastern"},
%Api.Subcategory{name: "Mongolian"},
%Api.Subcategory{name: "Moroccan"},
%Api.Subcategory{name: "Nepalese"},
%Api.Subcategory{name: "North Indian"},
%Api.Subcategory{name: "Pacific"},
%Api.Subcategory{name: "Persian"},
%Api.Subcategory{name: "Pizza"},
%Api.Subcategory{name: "Portuguese"},
%Api.Subcategory{name: "Pub Food"},
%Api.Subcategory{name: "Seafood"},
%Api.Subcategory{name: "Singaporean"},
%Api.Subcategory{name: "South Indian"},
%Api.Subcategory{name: "Spanish"},
%Api.Subcategory{name: "Sri Lankan"},
%Api.Subcategory{name: "Steakhouse"},
%Api.Subcategory{name: "Street Food"},
%Api.Subcategory{name: "Sushi"},
%Api.Subcategory{name: "Taiwanese"},
%Api.Subcategory{name: "Thai"},
%Api.Subcategory{name: "Turkish"},
%Api.Subcategory{name: "Vietnamese"},
]
Enum.each(subcategories, fn (subcategory) -> insert(subcategory) end)
end
def insertCategorySubcategories do
categorySubcategories = [
%Api.CategorySubcategory{c_id: 1, s_id: 1},
%Api.CategorySubcategory{c_id: 1, s_id: 2},
%Api.CategorySubcategory{c_id: 1, s_id: 3},
%Api.CategorySubcategory{c_id: 1, s_id: 4},
%Api.CategorySubcategory{c_id: 1, s_id: 5},
%Api.CategorySubcategory{c_id: 1, s_id: 6},
%Api.CategorySubcategory{c_id: 1, s_id: 7},
%Api.CategorySubcategory{c_id: 2, s_id: 8},
%Api.CategorySubcategory{c_id: 2, s_id: 9},
%Api.CategorySubcategory{c_id: 2, s_id: 10},
%Api.CategorySubcategory{c_id: 2, s_id: 11},
%Api.CategorySubcategory{c_id: 2, s_id: 12},
%Api.CategorySubcategory{c_id: 2, s_id: 13},
%Api.CategorySubcategory{c_id: 2, s_id: 14},
%Api.CategorySubcategory{c_id: 2, s_id: 15},
%Api.CategorySubcategory{c_id: 2, s_id: 16},
%Api.CategorySubcategory{c_id: 2, s_id: 17},
%Api.CategorySubcategory{c_id: 2, s_id: 18},
%Api.CategorySubcategory{c_id: 2, s_id: 19},
%Api.CategorySubcategory{c_id: 2, s_id: 20},
%Api.CategorySubcategory{c_id: 2, s_id: 21},
%Api.CategorySubcategory{c_id: 2, s_id: 23},
%Api.CategorySubcategory{c_id: 2, s_id: 24},
%Api.CategorySubcategory{c_id: 2, s_id: 25},
%Api.CategorySubcategory{c_id: 2, s_id: 26},
%Api.CategorySubcategory{c_id: 2, s_id: 27},
%Api.CategorySubcategory{c_id: 2, s_id: 28},
%Api.CategorySubcategory{c_id: 2, s_id: 29},
%Api.CategorySubcategory{c_id: 2, s_id: 30},
%Api.CategorySubcategory{c_id: 2, s_id: 31},
%Api.CategorySubcategory{c_id: 2, s_id: 32},
%Api.CategorySubcategory{c_id: 2, s_id: 33},
%Api.CategorySubcategory{c_id: 2, s_id: 34},
%Api.CategorySubcategory{c_id: 2, s_id: 35},
%Api.CategorySubcategory{c_id: 2, s_id: 36},
%Api.CategorySubcategory{c_id: 2, s_id: 37},
%Api.CategorySubcategory{c_id: 2, s_id: 38},
%Api.CategorySubcategory{c_id: 2, s_id: 39},
%Api.CategorySubcategory{c_id: 2, s_id: 40},
%Api.CategorySubcategory{c_id: 2, s_id: 41},
%Api.CategorySubcategory{c_id: 2, s_id: 42},
%Api.CategorySubcategory{c_id: 2, s_id: 43},
%Api.CategorySubcategory{c_id: 2, s_id: 44},
%Api.CategorySubcategory{c_id: 2, s_id: 45},
%Api.CategorySubcategory{c_id: 2, s_id: 46},
%Api.CategorySubcategory{c_id: 2, s_id: 47},
%Api.CategorySubcategory{c_id: 2, s_id: 49},
%Api.CategorySubcategory{c_id: 2, s_id: 50},
%Api.CategorySubcategory{c_id: 2, s_id: 51},
%Api.CategorySubcategory{c_id: 2, s_id: 52},
%Api.CategorySubcategory{c_id: 2, s_id: 53},
%Api.CategorySubcategory{c_id: 2, s_id: 54},
%Api.CategorySubcategory{c_id: 2, s_id: 55},
%Api.CategorySubcategory{c_id: 2, s_id: 56},
%Api.CategorySubcategory{c_id: 2, s_id: 57},
%Api.CategorySubcategory{c_id: 2, s_id: 58},
%Api.CategorySubcategory{c_id: 2, s_id: 59},
%Api.CategorySubcategory{c_id: 2, s_id: 60},
%Api.CategorySubcategory{c_id: 2, s_id: 61},
%Api.CategorySubcategory{c_id: 2, s_id: 62},
%Api.CategorySubcategory{c_id: 2, s_id: 63},
%Api.CategorySubcategory{c_id: 2, s_id: 64},
%Api.CategorySubcategory{c_id: 2, s_id: 65},
%Api.CategorySubcategory{c_id: 2, s_id: 66},
%Api.CategorySubcategory{c_id: 2, s_id: 67},
%Api.CategorySubcategory{c_id: 2, s_id: 68},
%Api.CategorySubcategory{c_id: 2, s_id: 69},
%Api.CategorySubcategory{c_id: 2, s_id: 70},
%Api.CategorySubcategory{c_id: 2, s_id: 71},
%Api.CategorySubcategory{c_id: 2, s_id: 72},
%Api.CategorySubcategory{c_id: 2, s_id: 73},
%Api.CategorySubcategory{c_id: 2, s_id: 74},
]
Enum.each(categorySubcategories, fn (categorySubcategory) -> insert(categorySubcategory) end)
end
def getCategories(conn) do
insertCategories
categories = all Api.Category
conn
|> put_resp_content_type("application/json")
|> send_resp(200, Poison.encode!(%{categories: categories}))
end
def getSubcategories do
Api.Subcategory |> all
end
end
relevant part of router.ex
get "/categories/" do
[controller] = ["repo"]
Api.Repo.getCategories(conn)
end
I browse to localhost:4000/categories and get the error at the top.
output of IO.puts(categories) - it has a lot of duplicated rows but that is another issue lol.
16:59:54.922 [debug] QUERY OK source="categories" db=5.2ms decode=3.7ms
SELECT c0."id", c0."name" FROM "categories" AS c0 []
[%Api.Category{__meta__: #Ecto.Schema.Metadata<:loaded, "categories">, id: 1,
name: "Grocery Products"},
%Api.Category{__meta__: #Ecto.Schema.Metadata<:loaded, "categories">, id: 2,
name: "Meals"},
%Api.Category{__meta__: #Ecto.Schema.Metadata<:loaded, "categories">, id: 3,
name: "Grocery Products"},
%Api.Category{__meta__: #Ecto.Schema.Metadata<:loaded, "categories">, id: 4,
name: "Meals"},
%Api.Category{__meta__: #Ecto.Schema.Metadata<:loaded, "categories">, id: 5,
name: "Grocery Products"},
%Api.Category{__meta__: #Ecto.Schema.Metadata<:loaded, "categories">, id: 6,
name: "Meals"},
%Api.Category{__meta__: #Ecto.Schema.Metadata<:loaded, "categories">, id: 7,
name: "Grocery Products"},
%Api.Category{__meta__: #Ecto.Schema.Metadata<:loaded, "categories">, id: 8,
name: "Meals"},
%Api.Category{__meta__: #Ecto.Schema.Metadata<:loaded, "categories">, id: 9,
name: "Grocery Products"},
%Api.Category{__meta__: #Ecto.Schema.Metadata<:loaded, "categories">, id: 10,
name: "Meals"},
%Api.Category{__meta__: #Ecto.Schema.Metadata<:loaded, "categories">, id: 11,
name: "Grocery Products"},
%Api.Category{__meta__: #Ecto.Schema.Metadata<:loaded, "categories">, id: 12,
name: "Meals"}]
16:59:55.038 [error] #PID<0.340.0> running Api.Router terminated
Server: localhost:4000 (http)
Request: GET /categories
** (exit) an exception was raised:
** (Poison.EncodeError) unable to encode value: {nil, "categories"}
(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/encoder.ex:259: anonymous fn/3 in Poison.Encoder.List.encode/3

Answer 1

You need to make a custom implementation of Poison.Encoder for your struct so that it doesn't try to encode the __meta__ field of the struct (which contains a tuple in it which Poison does not handle). The easiest way to do this is to add a #derive to the struct with the names of the fields you want to encode:
defmodule Api.Category do
...
#derive {Poison.Encoder, only: [:id, :name]} # <- add this
schema "categories" do
...
end
...
end