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]
  }
}

By default you'll only get the JSON that represents modelb in your example above. But, you can tell Rails to include the other related objects as well:

def export
  @export_data = ModelA.find(params[:id])
  respond_to do |format|
    format.html
    format.json { render :json => @export_data.to_json(:include => :modelb) }
  end
end

You can even tell it to exclude certain fields if you don't want to see them in the export:

render :json => @export_data.to_json(:include => { :modelb => { :except => [:created_at, updated_at]}})

Or, include only certain fields:

render :json => @export_data.to_json(:include => { :modelb => { :only => :name }})

And you can nest those as deeply as you need (let's say that ModelB also has_many ModelC):

render :json => @export_data.to_json(:include => { :modelb => { :include => :modelc }})

If you want to include multiple child model associations, you can do the following:

render :json => @export_data.to_json(include: [:modelA, :modelB, :modelN...])

If you want a more flexible approach to rendering json, you can consider using the gem jbuilder: https://github.com/rails/jbuilder

It allows you to render custom attributes, instance variables, associations, reuse json partials in a convenient way.