Rails Object Relationships and JSON Rendering

Disclaimer, I know very little about Rails. I'll try to be succinct. Given the following model relations in Rails:

class ModelA < ActiveRecord::Base
  belongs_to :ModelB

...

class ModelB < ActiveRecord::Base
    has_many :ModelA

When calling the show action of the ModelA controller the returned JSON should show all ObjectAs that are children of the ObjectB of which the ObjectA in question is a child of.

So if I have an ObjectB that contains ObjectA's of ID 1, 2 and 3 and then access: /modela/1.json

I should see:

{
  "modelb": {
    "id": "1",
    "modela": [insert the ModelA JSON for ID's 1, 2 and 3]
  }
}