Rails/ActiveRecord has_many through: association on unsaved objects

Question

Let\'s work with these classes:

class User < ActiveRecord::Base
    has_many :project_participations
    has_many :projects, through: :project_participations,         


        

Answer 1

This is more of a rails structure thing at the level of the ruby data structures. To simplify it lets put it this way. First of all imagine User as a data structure contains:

1. project_participations Array
2. projects Array

And Project

1. users Array
2. project_participations Array

Now when you mark a relation to be :through another (user.projects through user.project_participations)

Rails implies that when you add a record to that first relation (user.projects) it will have to create another one in the second realation (user.project_participations) that is all the effect of the 'through' hook

So in this case,

user.projects << project
#will proc the 'through'
#user.project_participations << new_entry

Keep in mind that the project.users is still not updated because its a completely different data structure and you have no reference to it.

So lets take a look what will happen with the second example

u.project_participations << pp
#this has nothing hooked to it so it operates like a normal array

So In conclusion, this acts like a one way binding on a ruby data structure level and whenever you save and refresh your objects, this will behave the way you wanted.