include 2nd level in to_json

I'm using this code to convert a model to json. If i try to use an include 2nd level like this:

p = Product.includes({ :variants => { :stocks => :size } }).where(:id => params[:id]).first
render :json => p.variants.to_json(:include => { :stocks => { :include => :size } })

I receive this error:

undefined method `macro' for nil:NilClass

How I can solve that?开发者_如何学运维

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Try this:

render :json => p.variants.map { |v| v.as_json(:include => {:stocks => {:include => :size}}) }

Info about Object#as_json/to_json here.