Add Issue.relatedMergeRequests to GraphQL API

As related MRs are currently parsed every time
from the issue description and its notes, the
computation is expensive.

To prevent performance problems with queries
that return up to 100 issues, the resolver is
limited to 1 execution per query.

Changelog: added

app/graphql/types/issue_type.rb

@@ -67,6 +67,13 @@ class IssueType < BaseObject
     field :merge_requests_count, GraphQL::Types::Int, null: false,
                                                       description: 'Number of merge requests that close the issue on merge.',
                                                       resolver: Resolvers::MergeRequestsCountResolver
+
+    field :related_merge_requests, Types::MergeRequestType.connection_type,
+                                   null: true,
+                                   description: 'Merge requests related to the issue. This field can only be resolved for one issue in any single request.' do
+      extension ::Gitlab::Graphql::Limit::FieldCallCount, limit: 1
+    end
+
     field :relative_position, GraphQL::Types::Int, null: true,
                                                    description: 'Relative position of the issue (used for positioning in epic tree and issue boards).'
     field :upvotes, GraphQL::Types::Int,
@@ -182,6 +189,17 @@ def closed_as_duplicate_of
       Gitlab::Graphql::Loaders::BatchModelLoader.new(Issue, object.duplicated_to_id).find
     end
+    def related_merge_requests
+      # rubocop: disable CodeReuse/ActiveRecord
+      MergeRequest.where(
+        id: ::Issues::ReferencedMergeRequestsService.new(project: object.project, current_user: current_user)
+        .execute(object)
+        .first
+        .map(&:id)
+      )
+      # rubocop: enable CodeReuse/ActiveRecord
+    end
+
     def discussion_locked
       !!object.discussion_locked
     end

doc/api/graphql/reference/index.md

@@ -13217,6 +13217,7 @@ Relationship between an epic and an issue.
 | <a id="epicissuenotes"></a>`notes` | [`NoteConnection!`](#noteconnection) | All notes on this noteable. (see [Connections](#connections)) |
 | <a id="epicissueparticipants"></a>`participants` | [`UserCoreConnection`](#usercoreconnection) | List of participants in the issue. (see [Connections](#connections)) |
 | <a id="epicissueprojectid"></a>`projectId` | [`Int!`](#int) | ID of the issue project. |
+| <a id="epicissuerelatedmergerequests"></a>`relatedMergeRequests` | [`MergeRequestConnection`](#mergerequestconnection) | Merge requests related to the issue. This field can only be resolved for one issue in any single request. (see [Connections](#connections)) |
 | <a id="epicissuerelatedvulnerabilities"></a>`relatedVulnerabilities` | [`VulnerabilityConnection`](#vulnerabilityconnection) | Related vulnerabilities of the issue. (see [Connections](#connections)) |
 | <a id="epicissuerelationpath"></a>`relationPath` | [`String`](#string) | URI path of the epic-issue relation. |
 | <a id="epicissuerelativeposition"></a>`relativePosition` | [`Int`](#int) | Relative position of the issue (used for positioning in epic tree and issue boards). |
@@ -14963,6 +14964,7 @@ Describes an issuable resource link for incident issues.
 | <a id="issuenotes"></a>`notes` | [`NoteConnection!`](#noteconnection) | All notes on this noteable. (see [Connections](#connections)) |
 | <a id="issueparticipants"></a>`participants` | [`UserCoreConnection`](#usercoreconnection) | List of participants in the issue. (see [Connections](#connections)) |
 | <a id="issueprojectid"></a>`projectId` | [`Int!`](#int) | ID of the issue project. |
+| <a id="issuerelatedmergerequests"></a>`relatedMergeRequests` | [`MergeRequestConnection`](#mergerequestconnection) | Merge requests related to the issue. This field can only be resolved for one issue in any single request. (see [Connections](#connections)) |
 | <a id="issuerelatedvulnerabilities"></a>`relatedVulnerabilities` | [`VulnerabilityConnection`](#vulnerabilityconnection) | Related vulnerabilities of the issue. (see [Connections](#connections)) |
 | <a id="issuerelativeposition"></a>`relativePosition` | [`Int`](#int) | Relative position of the issue (used for positioning in epic tree and issue boards). |
 | <a id="issueseverity"></a>`severity` | [`IssuableSeverity`](#issuableseverity) | Severity level of the incident. |

spec/requests/api/graphql/issue/issue_spec.rb

@@ -154,6 +154,47 @@ def query(fields)
     end
   end
+  context 'when selecting `related_merge_requests`' do
+    let(:issue_fields) { ['relatedMergeRequests { nodes { id } }'] }
+    let_it_be(:user) { create(:user) }
+    let_it_be(:mr_project) { project }
+    let!(:merge_request) do
+      attributes = {
+        author: user,
+        source_project: mr_project,
+        target_project: mr_project,
+        source_branch: 'master',
+        target_branch: 'test',
+        description: "See #{issue.to_reference}"
+      }
+
+      create(:merge_request, attributes).tap do |merge_request|
+        create(:note, :system, project: issue.project, noteable: issue,
+          author: user, note: merge_request.to_reference(full: true))
+      end
+    end
+
+    before do
+      project.add_developer(current_user)
+
+      post_graphql(query, current_user: current_user)
+    end
+
+    it 'returns the related merge request' do
+      expect(issue_data['relatedMergeRequests']['nodes']).to include a_hash_including({
+        'id' => merge_request.to_global_id.to_s
+      })
+    end
+
+    context 'no permission to related merge request' do
+      let_it_be(:mr_project) { create(:project, :private) }
+
+      it 'does not return the related merge request' do
+        expect(issue_data['relatedMergeRequests']['nodes']).to be_empty
+      end
+    end
+  end
+
   context 'when there is a confidential issue' do
     let!(:confidential_issue) do
       create(:issue, :confidential, project: project)

spec/requests/api/graphql/project/alert_management/alerts_spec.rb

@@ -16,7 +16,7 @@
   let(:fields) do
     <<~QUERY
       nodes {
-        #{all_graphql_fields_for('AlertManagementAlert', excluded: ['assignees'])}
+        #{all_graphql_fields_for('AlertManagementAlert', excluded: %w[assignees relatedMergeRequests])}
       }
     QUERY
   end

spec/support/shared_examples/requests/api/graphql/issue_list_shared_examples.rb

@@ -5,7 +5,7 @@
   let(:fields) do
     <<~QUERY
     nodes {
-      #{all_graphql_fields_for('issues'.classify)}
+      #{all_graphql_fields_for('issues'.classify, excluded: ['relatedMergeRequests'])}
     }
     QUERY
   end
@@ -683,6 +683,28 @@ def assignees_as_global_ids(issues)
     end
   end
+  context 'when selecting `related_merge_requests`' do
+    let(:fields) do
+      <<~QUERY
+      nodes {
+        relatedMergeRequests {
+          nodes {
+            id
+          }
+        }
+      }
+      QUERY
+    end
+
+    it 'limits the field to 1 execution' do
+      post_query
+
+      expect_graphql_errors_to_include(
+        '"relatedMergeRequests" field can be requested only for 1 Issue(s) at a time.'
+      )
+    end
+  end
+
   it 'includes a web_url' do
     post_query