Rails .to_json nested includes and methods

Defining assocations to be included in the rendered Json in Rails is pretty easy. You just use the options hash to define an include in to_json and by voodoo magic to_json will also traverse the ActiveRecord assocation and render the associated entity as a nested element in the Json. The code in question is quite simple:

@object.to_json({:include => :assocation_a})

By default Rails will only include attributes in to_json so if you want to also serialize the return value from a method (for example if you want to concatenate some attributes or compute something) you can do so through the options hash:

@object.to_json({:methods => :my_method})

You can also combine them easily if needed:

@object.to_json({:include => :assocation_a, :methods => :my_method})

But this calls the my_method on @object. How do you tell to_json to invoke my_method on @object.assocation_a?

Not trivial, but it works:

@object.to_json({:include => { :assocation_a => { :methods => :my_method }})

It gets spicy when you already had multiple includes in an array like this:

@object.to_json {:include => [ :assocation_a, :assocation_b ]} 

And now you only want to include my_method in assocation_a and not assocation b. simply replacing :assocation_a by a hash like before isn't going to work. It took me some time but this is what I came up with:

@object.to_json {:include => { :assocation_a => { :methods => :my_method }, :assocation_b => {} }} 

Did I mention that I am amazed by how flexible the syntax is here?