Rails to_json issue

Question

I have a strange issue with to_json method in my Rails 3 app. (well at least I think it is to do with to_json)

In my controller, I am getting the list of all the libraries stored in the DB

@libraries = Library.where( "latitude IS NOT NULL AND longitude IS NOT NULL" )

And then I create a json file that contains the library information above.

my_file = File.new("public/javascripts/libraries.json", "w")
my_file.write "var libs = {'libraries' : "
my_file.write @libraries.to_json( :only => [ :id, :name, :address, :latitude, :longitude ])
my_file.write "};"

Then in my view, I display each library object on Google Map. In the view file, I am reading the json file by loading the libraries.json file as a javascript file.

Now the problem is that the library objects are displayed on Google map SOMETIMES, but not all the time, and through Firebug, I was able to determine that sometimes the "libs" variable, that is contained in the JSON file is "undefined".

This makes me think that the file has not been completely being written, or the data in the file has not been completely been loaded. But I am not too sure what it is.

Does anyone have an idea what could be causing this?

Jatin Ganhotra · Accepted Answer · 2011-04-26 13:46:40Z

I am not quite sure why the problem is there, but I think it is related to the file that you are creating.

I would suggest you to simply return the json data to the view and then you can process it at the client side using javascript or jquery or any other framework.

This way you can also ensure that the data is correct by looking at it in Firebug.

OR

Instead of making it a file, just hold the json data in a variable and you can then access the variable in the view.
Why go for the trouble of creating a file, when you can do it without it.

Reasons:

Creating a file is an IO process, and that would add to the cost.
Each time, you are fetching the data from the view, you will make a fetch call, again an IO call. You can do the same thing by just putting it in a variable, that way it would all be there in the memory.

I am saying so because each time, you are creating a new file, and the files that you created before the current http call are gone and of no use.

DanneManne · Accepted Answer · 2011-04-26 13:52:17Z

Yes, it is probably caused by the fact that you write it to a file instead of just including it in a tag. Because what probably happens (sometimes) is that the server gets the request, writes the json file, then renders a view and sending that view to the client. The client is then told to load the libraries.json file resulting in another request to the server to retrieve that file. But what if that file in that exact moment is being rewritten because another client has sent a request at the same time? It will fail.

There are a few other ways you could go about this. First and probably the quickest to get up and running is to include this in your controller and view instead of writing to a file:

#controller
@libraries = Library.where( "latitude IS NOT NULL AND longitude IS NOT NULL" ).to_json( :only => [ :id, :name, :address, :latitude, :longitude ])

#view
<script type="text/javascript">
var libs = {'libraries': <%= @libraries.html_safe %>}
</script>

But a better solution would probably be to schedule some job or task to load the library from the database at schedule intervals and write to a json file perhaps once a day because I guess it is not really necessary to load these from the database all the time unless they change from one minute to the next.

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