Question

I can’t seem to wrap my head around this, so I thought I’d post and see if anyone could help me out (please pardon the question if it’s insultingly simple: it’s complicated to me right now!)

I have these models:

order
service
customer

I think they speak for themselves: a service is what the customer buys when they place an order.

Ok.

So, naturally, I setup these relationships:

# a customer can have many orders
class Customer
   has_many :orders
end

# an order belongs to a single customer and can have many services
class Order
   belongs_to :customer
   has_many :services
end

… but here’s where I trip up:

# a service can belong to many orders
class Service
   # belongs_to :order ???
end

Because my understanding of belongs_to is that–if I put it there–a service could only belong to one order (it would have only one value in the order_id key field–currently not present–tying it to only one order, where it needs to be able to belong to many orders).

What am I missing here?

Answer 1

There are two ways to handle this. The first is a rails-managed many-to-many relationship. In this case, you use a “has_and_belongs_to_many” relationship in both the Order and Service models. Rails will automatically create a join table which manages the relationships. The relationships look like this:

class Order
  has_and_belongs_to_many :services
end

class Service
  has_and_belongs_to_many :orders
end

The second way is to manage the join table yourself through an intermediate model. In this case, you might have another model called “LineItem” that represents a Service in the context of an Order. The relationships look like this:

class LineItem
  belongs_to :order
  belongs_to :service
end

class Order
  has_many :line_items
end

class Service
  has_many :line_items
end

I prefer the second myself. It’s probably just me, but I don’t get as confused about what’s going on when it’s explicit. Plus if I ever want to add some attributes to the relationship itself (like perhaps a quantity in your case) I’m already prepared to do that.