Add ProcessAssignees class

Let's extract assignee processing code into a new service class,
and then add tests for that class.

app/services/issuable/process_assignees.rb

+# frozen_string_literal: true
+
+# This follows the rules specified in the specs.
+# See spec/requests/api/graphql/mutations/merge_requests/set_assignees_spec.rb
+
+module Issuable
+  class ProcessAssignees
+    def initialize(assignee_ids:, add_assignee_ids:, remove_assignee_ids:, existing_assignee_ids: nil, extra_assignee_ids: nil)
+      @assignee_ids = assignee_ids
+      @add_assignee_ids = add_assignee_ids
+      @remove_assignee_ids = remove_assignee_ids
+      @existing_assignee_ids = existing_assignee_ids || []
+      @extra_assignee_ids = extra_assignee_ids || []
+    end
+
+    def execute
+      if assignee_ids.blank?
+        updated_new_assignees = new_assignee_ids
+        updated_new_assignees |= add_assignee_ids if add_assignee_ids
+        updated_new_assignees -= remove_assignee_ids if remove_assignee_ids
+      else
+        updated_new_assignees = assignee_ids
+      end
+
+      updated_new_assignees.uniq
+    end
+
+    private
+
+    attr_accessor :assignee_ids, :add_assignee_ids, :remove_assignee_ids, :existing_assignee_ids, :extra_assignee_ids
+
+    def new_assignee_ids
+      existing_assignee_ids | extra_assignee_ids
+    end
+  end
+end

spec/services/issuable/process_assignees_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Issuable::ProcessAssignees do
+  describe '#execute' do
+    it 'returns assignee_ids when assignee_ids are specified' do
+      process = Issuable::ProcessAssignees.new(assignee_ids: %w(5 7 9),
+                                               add_assignee_ids: %w(2 4 6),
+                                               remove_assignee_ids: %w(4 7 11),
+                                               existing_assignee_ids: %w(1 3 9),
+                                               extra_assignee_ids: %w(2 5 12))
+      result = process.execute
+
+      expect(result.sort).to eq(%w(5 7 9).sort)
+    end
+
+    it 'combines other ids when assignee_ids is empty' do
+      process = Issuable::ProcessAssignees.new(assignee_ids: [],
+                                               add_assignee_ids: %w(2 4 6),
+                                               remove_assignee_ids: %w(4 7 11),
+                                               existing_assignee_ids: %w(1 3 11),
+                                               extra_assignee_ids: %w(2 5 12))
+      result = process.execute
+
+      expect(result.sort).to eq(%w(1 2 3 5 6 12).sort)
+    end
+
+    it 'combines other ids when assignee_ids is nil' do
+      process = Issuable::ProcessAssignees.new(assignee_ids: nil,
+                                               add_assignee_ids: %w(2 4 6),
+                                               remove_assignee_ids: %w(4 7 11),
+                                               existing_assignee_ids: %w(1 3 11),
+                                               extra_assignee_ids: %w(2 5 12))
+      result = process.execute
+
+      expect(result.sort).to eq(%w(1 2 3 5 6 12).sort)
+    end
+
+    it 'combines other ids when assignee_ids and add_assignee_ids are nil' do
+      process = Issuable::ProcessAssignees.new(assignee_ids: nil,
+                                               add_assignee_ids: nil,
+                                               remove_assignee_ids: %w(4 7 11),
+                                               existing_assignee_ids: %w(1 3 11),
+                                               extra_assignee_ids: %w(2 5 12))
+      result = process.execute
+
+      expect(result.sort).to eq(%w(1 2 3 5 12).sort)
+    end
+
+    it 'combines other ids when assignee_ids and remove_assignee_ids are nil' do
+      process = Issuable::ProcessAssignees.new(assignee_ids: nil,
+                                               add_assignee_ids: %w(2 4 6),
+                                               remove_assignee_ids: nil,
+                                               existing_assignee_ids: %w(1 3 11),
+                                               extra_assignee_ids: %w(2 5 12))
+      result = process.execute
+
+      expect(result.sort).to eq(%w(1 2 4 3 5 6 11 12).sort)
+    end
+
+    it 'combines ids when only add_assignee_ids and remove_assignee_ids are passed' do
+      process = Issuable::ProcessAssignees.new(assignee_ids: nil,
+                                               add_assignee_ids: %w(2 4 6),
+                                               remove_assignee_ids: %w(4 7 11))
+      result = process.execute
+
+      expect(result.sort).to eq(%w(2 6).sort)
+    end
+  end
+end