Eenvoudige filters in Rails API

Filters

You have probably seen this before:

# app/controllers/api/v1/things_controller.rb

module API
  module V1
    class ThingsController < BaseController
      def index
        @things = Thing.all
        @things = @things.where(size: params[:size]) if params[:size]
        @things = @things.where('name ILIKE ?', "%#{params[:name_contains]}%") if params[:name_contains]

        render json: @things
      end
    end
  end
end

Why do I consider it to be a bad code? Because it simply makes our controller fat.
IMHO we should extract as many logic as we can from controllers and use a purpose-related
utils or services. In this case we will implement a generic filter that we will be able
to use across many controllers.

But wait, first let’s analyse the current code. It can be bad but works, though.
We have some initial scope (Thing.all) and then are limiting it if user has passed
related parameter. For each filter we actually check if the param was passed and if so,
we apply a filter. The second thing is that we don’t need to use the ivar, we can use
plain old local variables.

Ok, then. Couldn’t we use some service object to mutate the initial scope?
The execution can look like this:

# app/controllers/api/v1/things_controller.rb

module API
  module V1
    class ThingsController < BaseController
      def index
        scope = Thing.all
        things = Things::IndexFilter.new.call(scope, params)

        render json: things
      end
    end
  end
end

It looks much better now, but of course we have to implement the filter yet.
Note that call’s signature will be the same for all resources, so we can have
some generic class for this task.

# app/services/generic/index_filter.rb

module Generic
  class IndexFilter
    EMPTY_HASH = {}.freeze

    def self.filters
      EMPTY_HASH
    end

    def call(scope, params)
      apply_filters!(self.class.filters.keys, scope, params)
    end

    private

    def apply_filters!(filter_keys, scope, params)
      filter_keys.inject(scope.dup) do |current_scope, filter_key|
        apply_filter!(filter_key, current_scope, params)
      end
    end

    def apply_filter!(filter_key, scope, params)
      filter = fetch_filter(filter_key)
      return scope unless apply_filter?(filter, params)

      filter[:apply].call(scope, params)
    end

    def apply_filter?(filter, params)
      filter[:apply?].call(params)
    end

    def fetch_filter(filter_key)
      self.class.filters.fetch(filter_key) { raise ArgumentError, 'unknown filter' }
    end
  end
end

Seems complicated? Not really – all the magic happens in #apply_filters!.
We take the duplicate of the initial scope and apply each filter to it.

When we apply the scope it means we mutate the duplicate of our initial scope.
And we expect filters to be implemented as a hash in the self.filters methode
of a child class. Let’s do it.

# app/services/things/index_filter.rb

module Things
  class IndexFilter < Generic::IndexFilter
    FILTERS = {
      size_filter: {
        apply?: ->(params) {
          params[:size].is_a?(String)
        },
        apply: ->(scope, params) {
          scope.where(size: params[:size])
        }
      }.freeze,
      name_contains_filter: {
        apply?: ->(params) {
          params[:name_contains].is_a?(String)
        },
        apply: ->(scope, params) {
          scope.where('name ILIKE ?', "%#{params[:name_contains]}%")
        }
      }.freeze
    }.freeze

    def self.filters
      FILTERS
    end
  end
end

That’s it! We have written more code, but the simple filters will look the same
way for all the resources. We have cleaned controller from the code responsible
of filtering and provided ‘specialised’ class for this purpose that follows very
clear convention.

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