Show count of other users signing in with an IP

Helps to identify possible misusers.

app/models/user/sign_in.rb

@@ -38,6 +38,12 @@ def self.retention_days
     AlaveteliConfiguration.user_sign_in_activity_retention_days
   end
 
+  def other_users
+    User.distinct.joins(:sign_ins).
+      where(user_sign_ins: { ip: ip }).
+      where.not(id: user_id)
+  end
+
   private
 
   def create?

app/views/admin/users/_sign_in_table.html.erb

@@ -22,6 +22,11 @@
           </td>
 
           <td><%= admin_date(sign_in.created_at, ago_only: true) %></td>
+
+          <td>
+            <strong><%= sign_in.other_users.size %></strong> others using this
+            IP
+          </td>
         </tr>
       <% end %>
     </table>

spec/models/user/sign_in_spec.rb

@@ -113,6 +113,38 @@
     end
   end
 
+  describe '#other_users' do
+    subject { sign_in.other_users }
+
+    before do
+      allow(AlaveteliConfiguration).
+        to receive(:user_sign_in_activity_retention_days).and_return(1)
+    end
+
+    let(:user_1) { FactoryBot.create(:user) }
+    let(:user_2) { FactoryBot.create(:user) }
+
+    let!(:sign_in_1) do
+      FactoryBot.create(:user_sign_in, user: user_1, ip: '1.1.1.1')
+    end
+
+    let!(:sign_in_2) do
+      FactoryBot.create(:user_sign_in, user: user_2, ip: '1.1.1.1')
+    end
+
+    let!(:sign_in_3) { FactoryBot.create(:user_sign_in, ip: '2.2.2.2') }
+
+    context 'when there are other users using the same IP' do
+      let(:sign_in) { sign_in_1 }
+      it { is_expected.to match_array([user_2]) }
+    end
+
+    context 'when there are no other users using the same IP' do
+      let(:sign_in) { sign_in_3 }
+      it { is_expected.to be_empty }
+    end
+  end
+
   describe '#save' do
     subject { sign_in.save }