Rails undefined local variable or method `line_item' - ruby-on-rails

Question

Hello I have ran into a bit of trouble when create this conditional for a cart I built in my website to display a total when a part is on sale. In my schema I have parts, line_items which has an id of parts and carts, and carts. Parts have a attribute of discount. If a part has a discount it will display the discount and the new price of the part. My line_items has a method called line_item_discount which will create a new sum of the parts if one part includes a discount. Although it displays the part, the discount, and the new price, the cart total is not updating it.
I created a method called total_price_with_discount here
class Cart < ActiveRecord::Base
has_many :order_items
belongs_to :user
has_many :line_items, dependent: :destroy
def add_part(part_id)
current_part = line_items.find_by(part_id: part_id)
if current_part
current_part.quantity += 1
else
current_part = line_items.build(part_id: part_id)
end
current_part
end
def total_price
line_items.to_a.sum { |item| item.total_price}
end
def total_price_with_discount
line_items.to_a.sum { |item| item.total_price.line_item_discount}
end
Now where I am getting stuck is inside the _cart partial I tried to create a conditional where if a part has a discount it will use the total_price_with_discount method but if a part does not have a discount it will use the total_price. I have tried quite a few ways to create the conditional but i keep getting messages like this
for some reason carts has no instance of line_items or parts it appears.
Here are my tables for carts, parts, and line_items
create_table "carts", force: :cascade do |t|
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.integer "user_id"
t.integer "quantity"
t.decimal "subtotal"
end
create_table "parts", force: :cascade do |t|
t.string "name"
t.text "description"
t.integer "category_id"
t.integer "price"
t.boolean "active"
t.integer "discount"
t.string "image"
t.integer "quantity"
end
create_table "line_items", force: :cascade do |t|
t.integer "part_id"
t.integer "cart_id"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.integer "quantity", default: 1
end
my parts model
class Part < ActiveRecord::Base
has_many :order_items
has_many :line_items
before_destroy :ensure_not_referenced_by_any_line_item
def ensure_not_referenced_by_any_line_item
if line_items.empty?
return true
else
errors.add(:base, 'Line Items present')
return false
end
end
def subtotal
parts.collect { |part| part.valid? ? (part.quantity * part.unit_price) : 0}.sum
end
def apply_discount
price - (discount.to_f/100 * price)
end
end
my line_items model
class LineItem < ActiveRecord::Base
belongs_to :part
belongs_to :cart
def total_price
part.price * quantity
end
def line_item_discount
part.price - (part.discount.to_f/100 * part.price) * quantity
end
end
and here is the partial view thats throwing the error
<h2>Your Cart</h2> <table>
<%= render(cart.line_items) %>
<tr class="total_line">
<td colspan="2">Total</td>
<%unless cart.line_items.part.discount?%>
<td class="total_cell"><%= number_to_currency(cart.total_price) %></td>
<%end%>
<%if cart.line_items.part.discount?%>
<td class="total_cell"><%= number_to_currency(cart.total_price_with_discount) %></td>
<%end%>
</tr>
</table>
<%= button_to 'Empty cart', cart, method: :delete, data: { confirm: 'Are you sure?' } %>
Thanks for any help and advice with this one

Answer 1

To the Cart model I would add:
has_many :parts, through: :line_items
Add a scope to the Part model:
scope :with_discounts, -> { where.not(discount: nil) }
Then change the view to:
<td class="total_cell">
<%if cart.parts.with_discount.any?%>
<%= number_to_currency(cart.total_price_with_discount) %>
<%else%>
<%= number_to_currency(cart.total_price) %>
<%end%>
</td>
Update: Instead of the above, I believe the code below is more efficient, but I'll present both options and let you pick.
Below we'll always use line_item_discount in the total_price method within the LineItem model:
def total_price
(part.price * quantity) - line_item_discount
end
def line_item_discount
return 0 if part.discount.blank?
(part.discount.to_f/100 * part.price) * quantity
end
Then you don't even need the if statement within the view, total_price will work either way:
<td class="total_cell"><%= number_to_currency(cart.total_price) %></td>
You can then remove the total_price_with_discount method from the Cart model.
We can also tweak the total_price method within the Cart model:
(Works with either code choice)
def total_price
line_items.sum(:total_price)
end

Answer 2

In your view you're calling cart.line_items.part but line_items is a collection of multiple LineItem objects, .part is not a valid method on that collection object.
As you can see in your error, the part method is missing for ActiveRecord::Associations::CollectionProxy.
You should create a scope on LineItem like:
scope :with_discounted_part, { joins(:part).where.not(part: { discount: nil }) }
And then in your view you can do:
<% if cart.line_items.with_discounted_part %>