CODEWITHSUNDEEP
ruby-on-railsactiverecordassociationshas-many
Blind Fish
Blind Fish

Reputation: 998

has_many / belongs_to associations in RoR

I've read the guide on associations but I feel like I'm still not completely comprehending so I want to ask a couple of questions just to be sure. Let's say I am making an app that will, among other things, list large cities all over the world. I would plan on having a view that starts at continent level and can be filtered down. So I would start with a Continent model. And then a Country model. Now, within the Continent model I would define an association as has_many :countries. And in the Country model I would use belongs_to :continents. That much I grasp. So my next model would be a model for states / province. Let's just call it Province since that is more common throughout the world. So now I have my Province model, and I would use belongs_to :country. And likewise Countries would have has_many :provinces. My first question is, how do I describe the association between Province and Continent? Has_many through describes associations where both models have many. A Province only has one Continent. Has_one through describes a relationship between objects that have a one to one relationship via a third object. Again, this isn't the case because a Continent will have many Provinces. So that is my primary question.. how to describe relationships that exist in a one to many through context. My second question would be just asking for tips on writing the migrations for this in a situation where I add another layer, say County, later on. But the main problem is just understand how to express the relationships I described. Or if they even need to be expressed.

ETA: If I were to use the has_many_through association, do I necessarily need to create a join table ( continent_province ), or can I simply use the countries table ie has_many :provinces -> through :countries?

Upvotes: 0

Views: 408

Answers (1)

railsdog
railsdog

Reputation: 1501

Don't get too wound up around a couple of small examples in some doc somewhere. The relationship support is wonderfully flexible. In the end, just give it a try -- I have a Tester application that has all sorts of proof of concepts in it -- that's its purpose.


class Project
  # one-to-many 
  has_many :scenarios
  # linking through scenarios
  has_many :unittests, :through => :scenarios
  # polymorphic relationship, everything can be relation to one or more appls
  has_many :appllinks, :as => :applinkable, :dependent => :destroy
  has_many :appls, :through => :appllinks, :order => 'name'
  blah blah blah
end

class Scenario
  # many-to-one 
  belongs_to :project
  # many-to-many
  has_many :scenariotests 
  has_many :unittests, :through => :scenariotests
  # polymorphic relationship, everything can be relation to one or more appls
  has_many :appllinks, :as => :applinkable, :dependent => :destroy
  has_many :appls, :through => :appllinks, :order => 'name'
  blah blah blah
end

class Unittest
  # many-to-many
  has_many :scenariotests
  has_many :scenarios, :through => :scenariotests
  # polymorphic relationship, everything can be relation to one or more appls
  has_many :appllinks, :as => :applinkable, :dependent => :destroy
  has_many :appls, :through => :appllinks, :order => 'name'
  blah blah blah
end

Upvotes: 1

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