has_and_belongs_to_many for the same model

rubyonrails-talk
1

Rails 3.1.3

I am wondering if has_and_belongs_to_many association can be used for a single model?

More specifically, say I have a model called, Person, another model called Meeting.

Person 1 and Person 2 share some kind of association through Meeting so that

person1 = Person.find(...)

then,

person1.meeting returns person2.

If my understanding is correct, has_and_belongs_to_many association is used for one model with another.

Somehow I want to find a model object from the same object. Do you think we can do that?

soichi

2

https://www.google.com/search?q=rails+self+referential

3

Circular associations are possible, I do it all the time for comment systems.

4

rails self referential - Google Search

Thanks. This is what I wanted.

But in my case, two model objects (in the RailsCasts, User) are equal. So, I tried

in person.rb

    has_many :give_meetinges, :class_name => "Meeting", :foreign_key => "give_id"     has_many :gives, :class_name => "Person", :through => :give_meetinges, :source => :person     has_many :take_meetinges, :class_name => "Meeting", :foreign_key => "take_id"     has_many :takes, :class_name => "Person", :through => :take_meetinges, :source => :person

in meeting.rb

    belongs_to :give_person, :class_name => "Person"     belongs_to :take_person, :class_name => "Person"

in the view, ('take' is person object)

        <%= take.give_meetings.give_person %>

raises an error,

undefined method `give_plan' for :ActiveRecord::Relation

Since it's 'Relation',

        <%= take.give_meetings.first.give_person %>

also raises an error

undefined method `give_plan' for nil:NilClass

Can see problems in my code?

soichi

5

>rails self referential - Google Search

Thanks. This is what I wanted.

But in my case, two model objects (in the RailsCasts, User) are equal. So, I tried

in person.rb

    has_many :give_meetinges, :class_name => "Meeting", :foreign_key => "give_id"     has_many :gives, :class_name => "Person", :through => :give_meetinges, :source => :person     has_many :take_meetinges, :class_name => "Meeting", :foreign_key => "take_id"     has_many :takes, :class_name => "Person", :through => :take_meetinges, :source => :person

I'm not sure if you need to specify the class_name for a has_many through association

in meeting.rb

    belongs_to :give_person, :class_name => "Person"     belongs_to :take_person, :class_name => "Person"

this means that the foreign key is set to give_person_id and take_person_id but in your person model, the foreign key is give_id and take_id

in the view, ('take' is person object)

        <%= take.give_meetings.give_person %>

raises an error,

undefined method `give_plan' for :ActiveRecord::Relation

Since it's 'Relation',

        <%= take.give_meetings.first.give_person %>

also raises an error

undefined method `give_plan' for nil:NilClass

this is an expected error if there are no give_meetings for take. if there is at least on give_meetings for take, this will not raise an error (assuming that your associations and foreign keys are setup correctly)

i don't see any give_plan on your code. paste the code that stacktrace points to the source of this error.

6

i don't see any give_plan on your code.

Sorry, I pasted a wrong portion of code error.

it's

7

in meeting.rb

    belongs_to :give_person, :class_name => "Person"     belongs_to :take_person, :class_name => "Person"

this means that the foreign key is set to give_person_id and take_person_id but in your person model, the foreign key is give_id and take_id

This was the problem. I changed them to give_person_id and take_person_id and now it works!

Thanks for your help!

soichi