Add tests for pagination module extracted from API

spec/lib/api/helpers/pagination_spec.rb

+require 'spec_helper'
+
+describe API::Helpers::Pagination do
+  let(:resource) { Project.all }
+
+  subject do
+    Class.new.include(described_class).new
+  end
+
+  describe '#paginate' do
+    let(:value) { spy('return value') }
+
+    before do
+      allow(value).to receive(:to_query).and_return(value)
+
+      allow(subject).to receive(:header).and_return(value)
+      allow(subject).to receive(:params).and_return(value)
+      allow(subject).to receive(:request).and_return(value)
+    end
+
+    describe 'required instance methods' do
+      let(:return_spy) { spy }
+
+      it 'requires some instance methods' do
+        expect_message(:header)
+        expect_message(:params)
+        expect_message(:request)
+
+        subject.paginate(resource)
+      end
+    end
+
+    context 'when resource can be paginated' do
+      before do
+        create_list(:empty_project, 3)
+      end
+
+      describe 'first page' do
+        before do
+          allow(subject).to receive(:params)
+            .and_return({ page: 1, per_page: 2 })
+        end
+
+        it 'returns appropriate amount of resources' do
+          expect(subject.paginate(resource).count).to eq 2
+        end
+
+        it 'adds appropriate headers' do
+          expect_header('X-Total', '3')
+          expect_header('X-Total-Pages', '2')
+          expect_header('X-Per-Page', '2')
+          expect_header('X-Page', '1')
+          expect_header('X-Next-Page', '2')
+          expect_header('X-Prev-Page', '')
+          expect_header('Link', any_args)
+
+          subject.paginate(resource)
+        end
+      end
+
+      describe 'second page' do
+        before do
+          allow(subject).to receive(:params)
+            .and_return({ page: 2, per_page: 2 })
+        end
+
+        it 'returns appropriate amount of resources' do
+          expect(subject.paginate(resource).count).to eq 1
+        end
+
+        it 'adds appropriate headers' do
+          expect_header('X-Total', '3')
+          expect_header('X-Total-Pages', '2')
+          expect_header('X-Per-Page', '2')
+          expect_header('X-Page', '2')
+          expect_header('X-Next-Page', '')
+          expect_header('X-Prev-Page', '1')
+          expect_header('Link', any_args)
+
+          subject.paginate(resource)
+        end
+      end
+    end
+
+    def expect_header(name, value)
+      expect(subject).to receive(:header).with(name, value)
+    end
+
+    def expect_message(method)
+      expect(subject).to receive(method)
+        .at_least(:once).and_return(value)
+    end
+  end
+end