Rails 4 - Strong Parameters - Nested Objects

Question

I've got a pretty simple question. But haven't found a solution so far.

So here's the JSON string I send to the server:

{
  "name" : "abc",
  "groundtruth" : {
    "type" : "Point",
    "coordinates" : [ 2.4, 6 ]
  }
}

Using the new permit method, I've got:

params.require(:measurement).permit(:name, :groundtruth)

This throws no errors, but the created database entry contains null instead of the groundtruth value.

If I just set:

params.require(:measurement).permit!

Everything get's saved as expected, but of course, this kills the security provided by strong parameters.

I've found solutions, how to permit arrays, but not a single example using nested objects. This must be possible somehow, since it should be a pretty common use case. So, how does it work?

Answer 1

As odd as it sound when you want to permit nested attributes you do specify the attributes of nested object within an array. In your case it would be

Update as suggested by @RafaelOliveira

params.require(:measurement)
      .permit(:name, :groundtruth => [:type, :coordinates => []])

On the other hand if you want nested of multiple objects then you wrap it inside a hash… like this

params.require(:foo).permit(:bar, {:baz => [:x, :y]})

Rails actually have pretty good documentation on this: http://api.rubyonrails.org/classes/ActionController/Parameters.html#method-i-permit

For further clarification, you could look at the implementation of permit and strong_parameters itself: https://github.com/rails/rails/blob/master/actionpack/lib/action_controller/metal/strong_parameters.rb#L246-L247