render :json => 'string here' expected result

I have done this a thousand times but I'm still not comfortable with how the render :json handles str开发者_JAVA百科ings.

To set a scope let's talk about Rails 3

This is how it behaves right now:

...
render :json => 'This is the string'
...

Will be returning to the browser:

This is the string

That's actually not a valid JSON response :S

Ideally it should be rendering something like this:

"This is the string"

Even the rails guides say:

You don’t need to call to_json on the object that you want to render. If you use the :json option, render will automatically call to_json for you.

And calling "This is the string".to_json is actually returning "\"This is the string\"" as expected.

"This is the string".to_json #=> "\"This is the string\""

Am I so wrong ?

I agree that this is unexpected behavior at first, but it actually makes some good sense.

Consider, for example, what you would expect this to do:

output = {'foo' => 'bar'}.to_json
render :json => output

Even though the to_json is kinda redundant, you expect the result to be {foo: "bar"}. However, note that the result of {'foo' => 'bar'}.to_json is actually a string. So, the above code block is equivalent to:

render :json => '{foo: "bar"}'

If render were to JSON-encode strings passed to :json, you would get "{foo: \"bar\"}", which is definitely not expected behavior.

So here's the deal: render checks to see if the :json argument is a string. If so, it assumes that it's a JSON string and you already ran to_json, and passes the string along. If not, it runs to_json on the object.

I think the documentation should probably clarify that, but there you have it. Though it's not exactly intuitive at first glance, I would be surprised if it worked any other way.