Returning count of associations instead of all associations themselves

Question

So, an Object has many properties and a property belongs to an Object. That's currently the association. Now when I render all the objects, using Object.all and include the :properties associations, I get all the attributes of the property model in the rendered JSON. But I really don't need that. All I need is the number of associations there are. I.e., just the number of properties there are associated with any one object. Here is my code:

@objects = Object.all
respond_to do |format|
  format.json { 
    render json: @objects.to_json(:include => [:properties]) 
  }
end

This yields something like this. There is currently only 1 property associated with the 1 existing object, and as you can see in the JSON, I'm getting a lot of information about the associated properties, (or rather, property) that I don't need nor want.

"id": 2,
"attrs_go_here": "asdfsdaf"
"created_at": "2018-07-14T23:51:55.161Z",
"updated_at": "2018-07-14T23:51:55.161Z",
"properties": [
    {
        "id": 5,
        "more_attributes": "asdfasdfasdf"
        "created_at": "2018-07-14T23:53:14.917Z",
        "updated_at": "2018-07-14T23:53:14.917Z"
     }
]

But instead, I want something like this, where I get the number of associations:

"id": 2,
"attrs_go_here": "asdfsdaf"
"created_at": "2018-07-14T23:51:55.161Z",
"updated_at": "2018-07-14T23:51:55.161Z",
"properties": 1

My reasoning for only wanting the number is because there could potentially be hundreds of thousands of properties associated with any one object. And I feel like it would be really slow to render ALL the attributes of ALL the properties associated with an object when all I need is the number of associations there are between an object and its properties in the first place.

Answer 1

Add a method to you model Object

def properties_count
  self.properties.count
end

And on you controller do

render json: @objects.to_json(methods: [:properties_count]) 

Although I am not sure about a possible n+1. In case of that you should use

@objects = Object.includes(:properties).all

Edit Technically its not intended to give arguments to_json only takes getters. But you could do something along those lines:

class Object
  attr_accessor :attribute

  def method_with_params(attribute = nil)
    attribute ||= attribute
    # rest of your code
  end
end


@object.attribute = 'foodbar'
render json: @object.to_json(methods: [:method_with_params]

)