can't seem to get rails association to limit columns with json

Question

I have a restaurant application I am building that allows restaurants to create menus online. Right now our page loads 6 items on initial page load, and then an infinite scroll interaction takes over as they continue to scroll down the page and dynamically loads 6 more items through AJAX.

Everything is working, but I'm having trouble limiting the fields of my database associations. For example, my items have their own sizes, which are established in the item model with

has_many :sizes

and then I access those sizes using

item.sizes

to organize my data for return through json, I use the following method (although I am quite welcoming to improvements in this method):

items = Item.where("blah blah")
items.each do |item|
    item[:sizes] = item.sizes
end

render :json => {:items => items}

All of this works and is simple enough, but when I try to limit the selected columns returned from the database, I start to get strange behavior. I'm not sure what is causing it. I have tried doing this:

item[:sizes] = item.sizes.select("id, name, price")

and I have tried doing this as well:

item[:sizes] = item.sizes.to_json(:only => [:id, :name, :price])

both of these methods work to return only the requested fields ON THE SERVER SIDE, but something seems to happen to it between the request and the return to the web browser (my guess is that it is caused by the render method) that seems to be undoing my unique field selectors and instead sending back all of the database data (so instead of only id, name, price, I am receiving about 20 unnecessary fields of data).

I'm looking for any suggestions that might help make this work. Also, if anybody has any other information on ways to improve query times, that would be much appreciated as well. I have already covered all of the basic tutorials, and ii am already using eager loading before accessing the items (I wonder if that has anything to do with my problems?), but I am trying to find good solutions to prevent the bottleneck being caused by these to_json and render :json methods.

Currently a series of queries that takes about 400 ms on the database is taking an additional 1000 ms in render :json.

Answer 1

You should limit the elements by passing your parameters to the render method like this:

render :json => items.to_json(:include => { :sizes => { :only => [:id, :name, :price] }}

this should do it for you.