Expose method that returns GitLab API paths

lib/api/api.rb

@@ -10,6 +10,14 @@ module API
     NAMESPACE_OR_PROJECT_REQUIREMENTS = { id: NO_SLASH_URL_PART_REGEX }.freeze
     COMMIT_ENDPOINT_REQUIREMENTS = NAMESPACE_OR_PROJECT_REQUIREMENTS.merge(sha: NO_SLASH_URL_PART_REGEX).freeze
 
+    def self.root_path(version: 'v4')
+      unless versions.include?(version)
+        raise ArgumentError, 'Unknown API version!'
+      end
+
+      File.join('/', prefix.to_s, version.to_s)
+    end
+
     insert_before Grape::Middleware::Error,
                   GrapeLogging::Middleware::RequestLogger,
                   logger: Logger.new(LOG_FILENAME),

spec/lib/api/api_spec.rb

+require 'spec_helper'
+
+describe API::API do
+  describe '.prefix' do
+    it 'has a prefix defined' do
+      expect(described_class.prefix).to eq :api
+    end
+  end
+
+  describe '.version' do
+    it 'uses most recent version of the API' do
+      expect(described_class.version).to eq 'v4'
+    end
+  end
+
+  describe '.versions' do
+    it 'returns all available versions' do
+      expect(described_class.versions).to eq ['v3', 'v4']
+    end
+  end
+
+  describe '.root_path' do
+    it 'returns predefined API version path' do
+      expect(described_class.root_path).to eq '/api/v4'
+    end
+
+    it 'returns a version provided as keyword argument' do
+      expect(described_class.root_path(version: 'v3')).to eq '/api/v3'
+    end
+
+    it 'raises an error if version is not known' do
+      expect { described_class.root_path(version: 'v10') }
+        .to raise_error ArgumentError
+    end
+  end
+end