Rails 4, No Method Erreur lors de la tentative d'accès aux attributs d'un objet dans le modèle jbuilder

Question

J'utilise jbuilder pour créer une réponse json. Voir ci-dessous, mon modèle de JBuilder:

json.array! @items do |item| 
    json.title       item.title 
    json.description     item.description 
    json.category      item.categories.first.title 
    json.price       item.price/100.0 
    json.status       item.status 

    if !item.banned? 
     json.published_date    item.published_date 
     json.seller_name    item.seller.name 
    end 

    json.seller_latitude  item.seller.latitude.to_f 
    json.seller_longtitude  item.seller.longtitude.to_f 
end 

Je suis en mesure d'accéder à un objet de la collection Item.Categories comme si, sans problème:

item.categories.first

Lorsque je tente de tirer le titre attribut de l'objet de la catégorie de retour en haut que je reçois cette erreur dans le navigateur:

undefined method 'title' for nil:NilClass

Je mets une déclaration byebug juste avant cette ligne dans mon modèle de JBuilder:

item.categories.first.title

Et dans la console byebug je peux accéder à l'attribut title aucun problème. Voir la console byebug imprimer ci-dessous:

Started GET "/items.json" for ::1 at 2016-07-16 13:59:45 -0400 
Processing by ItemsController#index as JSON 
    Item Load (0.5ms) SELECT "items".* FROM "items" 

[1, 10] in /Users/Macbook/projects/VSC/app/views/items/index.json.jbuilder 
    1: 
    2: json.array! @items do |item| 
    3: json.title       item.title 
    4: json.description    item.description 
    5: byebug 
=> 6: json.category      item.categories.first.title 
    7: json.price       item.price/100.0 
    8: json.status       item.status 
    9: 
    10: if !item.banned? 
(byebug) item.categories.first.title 
    Category Load (0.3ms) SELECT "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1 ORDER BY "categories"."id" ASC LIMIT 1 [["item_id", 1]] 
"movies" 
(byebug) 

Je suis perplexe, qu'est-ce que je fais mal?

EDIT:

Voici le sql qui est déclenché lorsque JBuilder rend la vue:

Started GET "/items.json" for ::1 at 2016-07-16 14:24:29 -0400 
Processing by ItemsController#index as JSON 
    Item Load (0.8ms) SELECT "items".* FROM "items" 
    Category Load (0.5ms) SELECT "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1 ORDER BY "categories"."id" ASC LIMIT 1 [["item_id", 1]] 
    User Load (0.4ms) SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1 [["id", 1]] 
    Category Load (0.5ms) SELECT "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1 ORDER BY "categories"."id" ASC LIMIT 1 [["item_id", 2]] 
    CACHE (0.1ms) SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1 [["id", 1]] 
    Category Load (0.4ms) SELECT "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1 ORDER BY "categories"."id" ASC LIMIT 1 [["item_id", 3]] 
    CACHE (0.2ms) SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1 [["id", 1]] 
    Category Load (0.5ms) SELECT "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1 ORDER BY "categories"."id" ASC LIMIT 1 [["item_id", 4]] 
    CACHE (3.8ms) SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1 [["id", 1]] 
    Category Load (0.5ms) SELECT "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1 ORDER BY "categories"."id" ASC LIMIT 1 [["item_id", 5]] 
    CACHE (0.2ms) SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1 [["id", 1]] 
    Category Load (0.4ms) SELECT "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1 ORDER BY "categories"."id" ASC LIMIT 1 [["item_id", 6]] 
    CACHE (0.2ms) SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1 [["id", 1]] 
    Category Load (0.5ms) SELECT "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1 ORDER BY "categories"."id" ASC LIMIT 1 [["item_id", 7]] 
    CACHE (0.2ms) SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1 [["id", 1]] 
    Category Load (0.4ms) SELECT "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1 ORDER BY "categories"."id" ASC LIMIT 1 [["item_id", 8]] 
    CACHE (0.2ms) SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1 [["id", 1]] 
    Category Load (0.5ms) SELECT "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1 ORDER BY "categories"."id" ASC LIMIT 1 [["item_id", 9]] 
    CACHE (0.2ms) SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1 [["id", 1]] 
    Category Load (0.4ms) SELECT "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1 ORDER BY "categories"."id" ASC LIMIT 1 [["item_id", 10]] 
    CACHE (0.2ms) SELECT "users".* FROM "users" WHERE "users"."id" = $1 LIMIT 1 [["id", 1]] 
    Category Load (0.4ms) SELECT "categories".* FROM "categories" INNER JOIN "categorizations" ON "categories"."id" = "categorizations"."category_id" WHERE "categorizations"."item_id" = $1 ORDER BY "categories"."id" ASC LIMIT 1 [["item_id", 11]] 
    Rendered items/index.json.jbuilder (317.5ms) 
Completed 500 Internal Server Error in 329ms (ActiveRecord: 11.6ms) 

ActionView::Template::Error (undefined method `title' for nil:NilClass): 
    2: json.array! @items do |item| 
    3: json.title       item.title 
    4: json.description    item.description 
    5: json.category      item.categories.first.title 
    6: json.price       item.price/100.0 
    7: json.status       item.status 
    8: 
    app/views/items/index.json.jbuilder:5:in `block in _app_views_items_index_json_jbuilder___331360250519835275_70269069934300' 
    app/views/items/index.json.jbuilder:2:in `_app_views_items_index_json_jbuilder___331360250519835275_70269069934300' 

Answer 1

Vous voyez item.categories.first existe, cela signifie que another_item.categories.first existe toujours avec another_item est un item dans la boucle :

json.array! @items do |item| 
    json.category item.categories.first.title 
end 

Veuillez vérifier dans la console pour trouver les éléments qui n'ont pas de c ategory: Item.all.find_all{ |item| item.categories.empty? }