added exactly_one_of validation

CHANGELOG.md

@@ -3,6 +3,7 @@ Next Release
 
 #### Features
 
+* [#637](https://github.com/intridea/grape/pull/637): Added 'exactly_one_of' validation - [@Morred](https://github.com/Morred).
 * [#626](https://github.com/intridea/grape/pull/626): Mutually exclusive params - [@oliverbarnes](https://github.com/oliverbarnes).
 * [#617](https://github.com/intridea/grape/pull/617): Running tests on Ruby 2.1.1, Rubinius 2.1 and 2.2, Ruby and JRuby HEAD - [@dblock](https://github.com/dblock).
 * [#397](https://github.com/intridea/grape/pull/397): Adds `Grape::Endpoint.before_each` to allow easy helper stubbing - [@mbleigh](https://github.com/mbleigh).

README.md

@@ -500,6 +500,16 @@ end
 
 **Warning**: Never define mutually exclusive sets with any required params. Two mutually exclusive required params will mean params are never valid, thus making the endpoint useless. One required param mutually exclusive with an optional param will mean the latter is never valid.
 
+Parameters can be defined as 'exactly_one_of', ensuring that exactly one parameter gets selected.
+
+```ruby
+params do
+  optional :beer
+  optional :wine
+  exactly_one_of :beer, :wine
+end
+```
+
 ### Namespace Validation and Coercion
 
 Namespaces allow parameter definitions and apply to every method within the namespace.

lib/grape/locale/en.yml

@@ -29,3 +29,4 @@ en:
         unknown_options: 'unknown options: %{options}'
         incompatible_option_values: '%{option1}: %{value1} is incompatible with %{option2}: %{value2}'
         mutual_exclusion: 'are mutually exclusive'
+        exactly_one: "- exactly one parameter must be provided"

lib/grape/validations.rb

@@ -134,6 +134,10 @@ def mutually_exclusive(*attrs)
         validates(attrs, mutual_exclusion: true)
       end
 
+      def exactly_one_of(*attrs)
+        validates(attrs, exactly_one_of: true)
+      end
+
       def group(*attrs, &block)
         requires(*attrs, &block)
       end

lib/grape/validations/exactly_one_of.rb

@@ -0,0 +1,26 @@
+module Grape
+  module Validations
+    require 'grape/validations/mutual_exclusion'
+    class ExactlyOneOfValidator < MutualExclusionValidator
+      attr_reader :params
+
+      def validate!(params)
+        super
+        if none_of_restricted_params_is_present
+          raise Grape::Exceptions::Validation, param: "#{all_keys}", message_key: :exactly_one
+        end
+        params
+      end
+
+      private
+
+      def none_of_restricted_params_is_present
+        keys_in_common.length < 1
+      end
+
+      def all_keys
+        attrs.map(&:to_sym)
+      end
+    end
+  end
+end

spec/grape/validations/exactly_one_of_spec.rb

@@ -0,0 +1,71 @@
+require 'spec_helper'
+
+describe Grape::Validations::ExactlyOneOfValidator do
+  describe '#validate!' do
+    let(:scope) do
+      Struct.new(:opts) do
+        def params(arg); end
+      end
+    end
+    let(:exactly_one_of_params) { [:beer, :wine, :grapefruit] }
+    let(:validator) { described_class.new(exactly_one_of_params, {}, false, scope.new) }
+
+    context 'when all restricted params are present' do
+      let(:params) { { beer: true, wine: true, grapefruit: true } }
+
+      it 'raises a validation exception' do
+        expect {
+          validator.validate! params
+        }.to raise_error(Grape::Exceptions::Validation)
+      end
+
+      context 'mixed with other params' do
+        let(:mixed_params) { params.merge!(other: true, andanother: true) }
+
+        it 'still raises a validation exception' do
+          expect {
+            validator.validate! mixed_params
+          }.to raise_error(Grape::Exceptions::Validation)
+        end
+      end
+    end
+
+    context 'when a subset of restricted params are present' do
+      let(:params) { { beer: true, grapefruit: true } }
+
+      it 'raises a validation exception' do
+        expect {
+          validator.validate! params
+        }.to raise_error(Grape::Exceptions::Validation)
+      end
+    end
+
+    context 'when params keys come as strings' do
+      let(:params) { { 'beer' => true, 'grapefruit' => true } }
+
+      it 'raises a validation exception' do
+        expect {
+          validator.validate! params
+        }.to raise_error(Grape::Exceptions::Validation)
+      end
+    end
+
+    context 'when none of the restricted params is selected' do
+      let(:params) { { somethingelse: true } }
+
+      it 'raises a validation exception' do
+        expect {
+          validator.validate! params
+        }.to raise_error(Grape::Exceptions::Validation)
+      end
+    end
+
+    context 'when exactly one of the restricted params is selected' do
+      let(:params) { { beer: true, somethingelse: true } }
+
+      it 'params' do
+        expect(validator.validate!(params)).to eql params
+      end
+    end
+  end
+end

spec/grape/validations_spec.rb

@@ -878,5 +878,41 @@ module SharedParams
         end
       end
     end
+
+    context 'exactly one of' do
+      context 'params' do
+        it 'errors when two or more are present' do
+          subject.params do
+            optional :beer
+            optional :wine
+            optional :juice
+            exactly_one_of :beer, :wine, :juice
+          end
+          subject.get '/exactly_one_of' do
+            'exactly_one_of works!'
+          end
+
+          get '/exactly_one_of', beer: 'string', wine: 'anotherstring'
+          expect(last_response.status).to eq(400)
+          expect(last_response.body).to eq("[:beer, :wine] are mutually exclusive")
+        end
+
+        it 'errors when none is selected' do
+          subject.params do
+            optional :beer
+            optional :wine
+            optional :juice
+            exactly_one_of :beer, :wine, :juice
+          end
+          subject.get '/exactly_one_of' do
+            'exactly_one_of works!'
+          end
+
+          get '/exactly_one_of'
+          expect(last_response.status).to eq(400)
+          expect(last_response.body).to eq("[:beer, :wine, :juice] - exactly one parameter must be provided")
+        end
+      end
+    end
   end
 end