JSON request returns two responses - ruby-on-rails

Question

In my rails app I have an Item model which has many Versions. My controller has a versions action.
def versions
#item = Item.find(params[:id])
respond_to do |format|
format.json { render json: #item.versions }
end
end
Whenever I request versions, some items return them successfully and some don't. But when I go into the console, I can get the versions of all items with no problem.
Successful (returns JSON array of versions):
Started GET "/api/versions/4.json?"
Processing by ItemsController#versions as JSON
Parameters: {"id"=>"4"}
Completed 200 OK in 106ms (Views: 0.1ms | ActiveRecord: 19.8ms)
app[web.1]: cache: [GET /api/versions/4.json?] miss
heroku[router]: GET x.x.com/api/versions/4.json? dyno=web.1 queue=0 wait=0ms service=113ms status=200 bytes=831
Not successful (returns two responses, both status 200, no JSON data):
Started GET "/api/versions/1.json?"
Processing by ItemsController#versions as JSON
Parameters: {"id"=>"1"}
Completed 200 OK in 249ms (Views: 0.1ms | ActiveRecord: 162.9ms)
app[web.1]: cache: [GET /api/versions/1.json?] miss
heroku[router]: GET x.x.com/api/versions/1.json? dyno=web.1 queue=0 wait=0ms service=257ms status=200 bytes=3540
Any idea what could cause this? I don't know how to log SQL on Heroku so I'm out of ideas.

Answer 1

Re your SQL logging: Enable statement logging by issuing the SQL SET log_statement='all'; when you open each connection. Then examine the Heroku logs for Pg to see what's happening.
heroku logs --ps postgres
See:
Heroku's documentation on their PostgreSQL support
Heroku's logging documentation
a Google search for: how to log SQL on Heroku
This won't log query results, only query text, query parameters, and error/success.
To handle result logging you'll need to do it in your application; just print messages to stderr and Heroku will add them to the application logs so you can get them via heroku logs.
Your current code looks dodgy anyway. Looping over a result set and emitting json feels wrong:
It isn't explicitly clear that there can only be one result and thus only emit one json block; and
If there are no results, no json is emitted
Instead, save the result of the find - if any - to a variable. Then handle missing results properly: in a RESTful API you'd probably report HTTP status 404 Not Found; in a more RPC-style API you'd return an empty JSON document or a JSON document with a "not found" flag of some kind and report HTTP status 200 OK.

Answer 2

This was actually an error on the client side. On iOS I was handling the returned data in connection:didReceiveData:, which (when returning large chunks of data) is called numerous times with partial results. To fix it, I instead concatenated all the partial results in that callback and handled the data in connectionDidFinishLoading:. Stupid mistake, not related to Heroku or Rails.
Thanks for the help Craig.