Question

I have three models: User, Product, Offer and a problem with the relationship between these models.

Scenario:

User 1 posts a product

User 2 can send User 1 an offer with an price e.g $ 10

User 1 can accept or reject the offer

My questions are now:

What is the right relationship between User, Product and Offer?

How can I handle those “accept or reject” actions?

Is there maybe a better solution?

User model:

class User < ActiveRecord::Base
    attr_accessible :name, :email, :password, :password_confirmation, :remember_me, :avatar, :screen_name
    has_many :products
    has_many :offers,:through => :products
end

Product model:

class Product < ActiveRecord::Base
    attr_accessible :content, :price, :title, :tag_list, :productimage, :user_id
    belongs_to :user
    has_many :offers, :through => :users
end

Offer model:

class Offer < ActiveRecord::Base
    attr_accessible :offer_price, :status, :user_id, :product_id
    has_many :products
    has_many :users, through: :products
end

Thanks in advance 🙂

EDIT:

I am using Rails 3.2.8

Answer 1

Warning: here comes a small novel.

Part 1: setting up the associations

I’d recommend reading the Rails guide on associations thoroughly, bookmark it, and read it again, because this is a key thing to understand properly, and can be a bit tricky – there are lots of options once you go beyond basic associations.

One thing to notice about your app is that your users have two roles, buyers and sellers. You’re going to need to be careful with the names of your associations – Does @user.offers return the offers the user has made, or the offers the user has received? You might want to be able to put lists of both these things in the user’s profile.

The basic relationships you’re describing are fairly simple:

• A user can sell many products, so User has_many :products and Product belongs_to :user

• A user can make many offers, so User has_many :offers and Offer belongs_to :user

• A product may receive many offers so Product has_many :offers and Offer belongs_to :product

That’s all well and good, and you could certainly get by just doing this – in which case you can skip down to Part 2 🙂

However, as soon as you start trying to add the through relationships the waters are going to get muddy. After all,

• Offer belongs_to :user (the buyer), but it also has a user through product (the seller)

• User has_many :products (that they are selling), but they also have many products through offers (that they are buying – well, trying to buy).

Aargh, confusing!

This is the point when you need the :class_name option, which lets you name an association differently to the class it refers to, and the :source option, which lets you name associations on the ‘from’ model differently to the ‘through’ model.

So you might then form your associations like this:

# User
has_many :products_selling, class_name: 'Product'
has_many :offers_received, class_name: 'Offer',
         through: :products_selling, source: :offers

has_many :offers_made, class_name: 'Offer'
has_many :products_buying, class_name: 'Product',
         through: :offers_made, source: :product


# Product
belongs_to :seller, class_name: 'User', foreign_key: :user_id
has_many :offers
has_many :buyers, class_name: 'User', through: :offers

# Offer
belongs_to :product
belongs_to :buyer, class_name: 'User', foreign_key: :user_id
has_one :seller, class_name: 'User', through: :product

Although if you renamed your user_id columns to seller_id in the products table, and buyer_id in the offers table, you wouldn’t need those :foreign_key options.

Part 2: accepting/rejecting offers

There’s a number of ways to tackle this. I would put a boolean field accepted on Offer and then you could have something like

# Offer
def accept
  self.accepted = true
  save
end

def reject
  self.accepted = false
  save
end

and you could find the outstanding offers (where accepted is null)

scope :outstanding, where(accepted: nil)

To get the accept/reject logic happening in the controller, you might consider adding new RESTful actions (the linked guide is another one worth reading thoroughly!). You should find a line like

resources :offers

in config/routes.rb, which provides the standard actions index, show, edit, etc. You can change it to

resources :offers do
  member do
    post :accept
    post :reject
  end
end

and put something like this in your OffersController

def accept
  offer = current_user.offers_received.find(params[:id])
  offer.accept
end

# similarly for reject

Then you can issue a POST request to offers/3/accept and it will cause the offer with id 3 to be accepted. Something like this in a view should do it:

link_to "Accept this offer", accept_offer_path(@offer), method: :post 

Note that I didn’t just write Offer.find(params[:id]) because then a crafty user could accept offers on the behalf of the seller. See Rails Best Practices.