Proper Quantity into Cart - ruby-on-rails

Question

Just added a text field for quantity in my products listing. Regardless of the number I enter in the text field, the cart will show just an increment of one for that click. Of course, I haven't told it to do anything otherwise. But, how do I? I can't find the solution anywhere.
Here's my cart add method :
def add_product(product_id)
line_items.find_or_initialize_by(product_id: product_id).increment(:quantity)
end
and in LineItems #create :
def create
product = Product.find(params[:product_id])
#line_item = #cart.add_product(product.id)
quantity = params[:quantity]
Let me know if there is any other relevant code I can attach. Thanks.
View Code
<%= image_tag(product.image.url, class: 'prodli-img') %>
<h3><%= product.name %></h3>
<p><%= product.description %></p>
<span class="price"><%= number_to_currency(product.price) %></span>
<!--<p> <%= product.colors %> </p>-->
<div id= "text_field"><%= text_field_tag 'quantity' %> </div>
<%= button_to 'Add to Cart', line_items_path(:product_id => product) %>
<% end %>
LI create
def create
product = Product.find(params[:product_id])
#line_item = #cart.add_product(product.id)
#line_item.quantity = params[:quantity]
respond_to do |format|
if #line_item.save
format.html { redirect_to "/#products", notice: "Product added to cart!" }
format.xml { render :xml => #line_item,
:status => :created, :location => #line_item }
else
format.html { render :action => "new" }
format.xml { render :xml => #line_item.errors,
:status => :unprocessable_entity }
end
end
end

Answer 1

You can't easily pass additional parameters with a "button_to"
This should work better
<h3><%= product.name %></h3>
<p><%= product.description %></p>
<span class="price"><%= number_to_currency(product.price) %></span>
<%= form_for :line_item, url: product_line_items_path(product) do |f| %>
<%= f.text_field 'quantity' %>
<%= f.submit 'Add to Cart' %>
<% end %>
Your params in the create method will then look like...
=> {"utf8"=>"V",
"authenticity_token"=>"blah blah"
"line_item"=>{"quantity"=>"12"},
"commit"=>"Add to Cart",
"action"=>"create",
"controller"=>"line_items",
"product_id"=>"1"}
So you can retrieve the quantity with...
#line_item.quantity = params[:line_item][:quantity]

Answer 2

Quantity is an attribute of the #line_item object.
So you should be doing
#line_item.quantity = params[:quantity]
And when you save #line_item it will then have the new value stored.