Cannot use optional StrongParameters with :new

[jamesarosen]

I can't come up with a good way to use StrongParameters for optional parameters for a :new action.

In Rails 3, I wrote something like

class FooController < ApplicationController

  def new
    @foo = Foo.new(foo_params)
  end

  private

  def foo_params
    params[:foo] || {}
  end

end

In Rails 4, I first tried

def foo_params
  params.require(:foo).permit(:bar, :baz)
end

That causes an ActionController::ParameterMissing: param not found: feature if people go to /foos/new without providing any arguments (like ?foo[bar]=varValue).

The next thing I tried was

def foo_params
  params[:foo] ||= {} if params[:action] == 'new'
  params.require(:foo).permit(:bar, :baz)
end

This has the same problem (likely because :foo is neither required nor permitted by the time ||= is evaluated).

Lastly, I tried

def foo_params
  params.permit(:foo).permit(:bar, :baz)
end

That, however, always returns an empty Hash.

[jamesarosen]

The solution is the rather obtuse

params.permit(:foo => [ :bar, :baz ])[:foo] ||= ActionController::Params.new({})

[senny]

What happens when you simply do:

params.permit(foo: [:bar, :baz])

[jamesarosen]

If I do that, I get back { foo: { bar: "bar value", baz: "baz value" } }. The issue is that if I use require(:foo).permit(:bar, :baz), I get back { bar: "bar value", baz: "baz value" } -- no nesting.

[jamesarosen]

It may be that there is no good code solution to this in Rails and it has to be treated as a documentation issue. In that case, I recommend expressing that the following are similar

params.require(:foo).permit(:bar, :baz)
params.permit(foo: [ :bar, :baz ])[:foo]

[senny]

@fxn should I update the documentation in the guide?

[senny]

Another thing you could try is:

params.fetch(foo:, {}).permit(:bar, :baz)

[steveklabnik]

It's also important to remember that Strong Parameters are not intended to cover every case, just make simple patterns easy.

[fxn]

The solution with fetch is the one I like the most, it captures the fact that :foo is optional (that is the point of fetch isn't it), and returns an empty strong hash otherwise.

Maybe we could have a little section with a cookbook to solve problems like this and others that have been reported in the guide, this is new and it would be useful to explain with examples how to use the API to solve use cases that deviate from the normal one.

[senny]

@fxn I'm on it. Will open a PR to discuss the examples.

[senny]

I'm closing this for now as there is no bug or unexpected behaviour.

[jamesarosen]

Thanks, folks :)