Allow MR rule to store its source project rule

This is to enable MR override rule's `approvals_required` is never
less than the project rule's.

db/schema.rb

@@ -219,6 +219,13 @@ ActiveRecord::Schema.define(version: 20190103140724) do
     t.index ["usage_stats_set_by_user_id"], name: "index_application_settings_on_usage_stats_set_by_user_id", using: :btree
   end
 
+  create_table "approval_merge_request_rule_sources", id: :bigserial, force: :cascade do |t|
+    t.bigint "approval_merge_request_rule_id", null: false
+    t.bigint "approval_project_rule_id", null: false
+    t.index ["approval_merge_request_rule_id"], name: "index_approval_merge_request_rule_sources_1", unique: true, using: :btree
+    t.index ["approval_project_rule_id"], name: "index_approval_merge_request_rule_sources_2", using: :btree
+  end
+
   create_table "approval_merge_request_rules", id: :bigserial, force: :cascade do |t|
     t.datetime_with_timezone "created_at", null: false
     t.datetime_with_timezone "updated_at", null: false
@@ -3217,6 +3224,8 @@ ActiveRecord::Schema.define(version: 20190103140724) do
   add_foreign_key "application_settings", "namespaces", column: "custom_project_templates_group_id", on_delete: :nullify
   add_foreign_key "application_settings", "projects", column: "file_template_project_id", name: "fk_ec757bd087", on_delete: :nullify
   add_foreign_key "application_settings", "users", column: "usage_stats_set_by_user_id", name: "fk_964370041d", on_delete: :nullify
+  add_foreign_key "approval_merge_request_rule_sources", "approval_merge_request_rules", on_delete: :cascade
+  add_foreign_key "approval_merge_request_rule_sources", "approval_project_rules", on_delete: :cascade
   add_foreign_key "approval_merge_request_rules", "merge_requests", on_delete: :cascade
   add_foreign_key "approval_merge_request_rules_approvals", "approval_merge_request_rules", on_delete: :cascade
   add_foreign_key "approval_merge_request_rules_approvals", "approvals", on_delete: :cascade

ee/app/models/approval_merge_request_rule.rb

@@ -10,6 +10,8 @@ class ApprovalMergeRequestRule < ApplicationRecord
 
   has_and_belongs_to_many :approvals # This is only populated after merge request is merged
   has_many :approved_approvers, through: :approvals, source: :user
+  has_one :approval_merge_request_rule_source
+  has_one :approval_project_rule, through: :approval_merge_request_rule_source
 
   def project
     merge_request.target_project

ee/app/models/approval_merge_request_rule_source.rb

+# frozen_string_literal: true
+
+# Allow MR rule to lookup its project rule source
+class ApprovalMergeRequestRuleSource < ApplicationRecord
+  belongs_to :approval_merge_request_rule
+  belongs_to :approval_project_rule
+end

ee/db/migrate/20181204031331_create_approval_merge_request_rules_approval_project_rules.rb

+# frozen_string_literal: true
+
+class CreateApprovalMergeRequestRulesApprovalProjectRules < ActiveRecord::Migration[5.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def change
+    create_table(:approval_merge_request_rule_sources, id: :bigserial) do |t|
+      t.references(
+        :approval_merge_request_rule,
+        type: :bigint,
+        null: false,
+        foreign_key: { on_delete: :cascade },
+        index: { name: 'index_approval_merge_request_rule_sources_1', unique: true }
+      )
+      t.references(
+        :approval_project_rule,
+        type: :bigint,
+        null: false,
+        foreign_key: { on_delete: :cascade },
+        index: { name: 'index_approval_merge_request_rule_sources_2' }
+      )
+    end
+  end
+end

ee/spec/models/merge_request_spec.rb

@@ -49,15 +49,14 @@ describe MergeRequest do
     let!(:approval2) { create(:approval, merge_request: subject, user: member3) }
     let!(:approval3) { create(:approval, merge_request: subject, user: group1_member) }
     let!(:approval4) { create(:approval, merge_request: subject, user: group2_member) }
+    let!(:project_rule) { create(:approval_project_rule, project: project, name: 'foo', approvals_required: 12) }
 
     before do
       group1.add_guest(group1_member)
       group2.add_guest(group2_member)
 
-      rule = create(:approval_project_rule, project: project, name: 'foo', approvals_required: 12)
-
-      rule.users = [member1, member2]
-      rule.groups << group1
+      project_rule.users = [member1, member2]
+      project_rule.groups << group1
     end
 
     shared_examples 'skipping when unmerged' do