Getting value from associated model - ruby-on-rails

Question

My setup: Rails 2.3.10, Ruby 1.8.7
Here are my models
class User
has_many :user_projects
end
class Project
has_many :user_projects
#has a column named "project_type"
end
class UserProject
belongs_to :user
belongs_to :project
#fields: project_id, user_id
end
When I return a JSON string of a user and his related user_projects records, I also want to include in the user_project record the project.project_type column. Note: I don't want to also include the entire project record in the results. A possible solution is dup the project_type field in user_projects but I prefer not to do that if possible, is there another way to accomplish this during the find/read action?
Just to be clear, here's the JSON output I'm looking for
{
"user": {
"username": "bob",
"id": 1,
"email": "bob#blah.com"
"user_projects": [
{
"id": 15,
"user_id": 1,
"project_id": 10,
"project_type": "marketing"
}
{
"id": 22,
"user_id": 1,
"project_id": 11,
"project_type": "sales"
}
]
}

Answer 1

class UserProject
belongs_to :user
belongs_to :project
#fields: project_id, user_id
attr_reader :type
def type
self.project.type
end
end
class MyController < AC
def action
#model = whatever
respond_to do |format|
format.json { render :json => #model.to_json(:methods => :type)}
end
end
end
Hope this helps.

Answer 2

You could try using the :only key in a nested include:
user.to_json(:include => {:user_projects => {:include => {:project => {:only => :type}}}})
But I would add has_many :projects, :through => :user_projects to User so you can do the simpler:
user.to_json(:include => {:projects => {:only => [:id, :type]}})
Also, an off-topic cautionary note: never use 'type' as a column name in Rails unless you are using STI (ie. the project types are ruby subclasses of Project).
-
Edit
Here's a way to add project_type to UserProject like you want
class UserProject
belongs_to :user
belongs_to :project
delegate :type, :to => :project, :prefix => true
end
user.to_json(:include => {:user_projects => {:methods => :project_type}})