Validating query parameters

Sometimes, you may want to validate the query parameters passed to your operation. For instance, let's suppose your articles/index operation accepts a by_user filter which is supposed to be a user ID, and you want to verify this value is an integer, if present.

You can accomplish this quite easily by embedding a dry-validation schema in your operation. You can write your operation as follows:

# app/resources/api/v1/article/operation/index.rb
module API
  module V1
    module Article
      module Operation
        class Index < Pragma::Operation::Index
          Schema = Dry::Validation.Schema do
            optional(:user_id).maybe(:int?)
          end

          step :validate_params!, before: 'retrieve', fail_fast: true

          def validate_params!(options, params:, **)
            result = Schema.(params)
            return true if result.success?

            options['result.response'] = Pragma::Operation::Response::UnprocessableEntity.new(
              errors: result.errors,
            ).decorate_with(Pragma::Decorator::Error)

            false
          end
        end
      end
    end
  end
end

As you can see, we are basically specifying a validation schema just as we do in contracts, except that the schema will not be synced with a model and is just used to ensure the parameters passed to the operation are correct.

Note that, while the example uses an Index operation, you can do this for any operation, including Create, Update and Destroy (even though passing query parameters to non-GET operations is highly discouraged).