assigning and reusing variables in Rails - design pattern - ruby-on-rails

Question

I've been using the same pattern for returning json code (see example below). I'm getting a collection of photos and storing it in the variable. If tag param is present, I'm getting a more specific collection and reassigning it to the same variable. Then returning it as json. What would be a better design pattern to achieve the same thing?
photos = collection_of_photos
if params[:tag]
photos = photos.find_all {|photo| some condition}
end
render json: photos

Answer 1

If the photos are ActiveRecord objects you should use a scope to generate the appropriate query for the exact data you need. Otherwise, Rails will load all of the photos instead of the subset you need. Assuming your photos records have a single tag attribute, you can do something like the following:
class Photo < ApplicationRecord
scope :with_tag, (tag) -> { where(tag: tag) }
end
In your controller you'd only need the call that scope since ActiveRecord scopes return an #all scope if no parameters are provided:
render json: Photo.with_tag(params[:tag])
# Equivalent when params[:tag] is nil
render json: Photo.all
Now say you're not dealing with ActiveRecord objects. Say you have an array of hashes:
photos = [{ name: '1.jpg', tag: 'flower'}, ... ]
You can create a helper method to perform the filtering:
class PhotosController < ApplicationController
def index
photos = [{ name: '1.jpg', tag: 'flower'}, ... ]
render json: select_photos_by_tag(photos, params[:tag])
end
private
def select_photos_by_tag(photos, tag)
if tag
photos.select { |photo| photo[:tag] == tag }
else
photos
end
end
end