[Issue multiple assignee] Support multips assignee in slash commands

app/services/slash_commands/interpret_service.rb

@@ -86,15 +86,18 @@ module SlashCommands
       current_user.can?(:"admin_#{issuable.to_ability_name}", project)
     end
     command :assign do |assignee_param|
-      user = extract_references(assignee_param, :user).first
-      user ||= User.find_by(username: assignee_param)
+      users = extract_references(assignee_param, :user)
 
-      next unless user
+      if users.empty?
+        users = User.find_by(username: assignee_param.split(' ').map(&:strip))
+      end
+
+      next if users.empty?
 
       if issuable.is_a?(Issue)
-        @updates[:assignee_ids] = [user.id]
+        @updates[:assignee_ids] = users.map(&:id)
       else
-        @updates[:assignee_id] = user.id
+        @updates[:assignee_id] = users.last.id
       end
     end
 

spec/services/slash_commands/interpret_service_spec.rb

@@ -3,6 +3,7 @@ require 'spec_helper'
 describe SlashCommands::InterpretService, services: true do
   let(:project) { create(:project, :public) }
   let(:developer) { create(:user) }
+  let(:developer2) { create(:user) }
   let(:issue) { create(:issue, project: project) }
   let(:milestone) { create(:milestone, project: project, title: '9.10') }
   let(:inprogress) { create(:label, project: project, title: 'In Progress') }
@@ -388,6 +389,28 @@ describe SlashCommands::InterpretService, services: true do
       end
     end
 
+    context 'assign command with multiple assignees' do
+      let(:content) { "/assign @#{developer.username} @#{developer2.username}" }
+
+      before{ project.team << [developer2, :developer] }
+
+      context 'Issue' do
+        it 'fetches assignee and populates assignee_id if content contains /assign' do
+          _, updates = service.execute(content, issue)
+
+          expect(updates[:assignee_ids]).to match_array([developer.id, developer2.id])
+        end
+      end
+
+      context 'Merge Request' do
+        it 'fetches assignee and populates assignee_id if content contains /assign' do
+          _, updates = service.execute(content, merge_request)
+
+          expect(updates).to eq(assignee_id: developer.id)
+        end
+      end
+    end
+
     it_behaves_like 'empty command' do
       let(:content) { '/assign @abcd1234' }
       let(:issuable) { issue }