Add projectId field to Issues GraphQL API

app/graphql/types/issue_type.rb

@@ -127,6 +127,9 @@ module Types
     field :timelogs, Types::TimelogType.connection_type, null: false,
           description: 'Timelogs on the issue.'
 
+    field :project_id, GraphQL::INT_TYPE, null: false, method: :project_id,
+          description: 'ID of the issue project.'
+
     def author
       Gitlab::Graphql::Loaders::BatchModelLoader.new(User, object.author_id).find
     end

doc/api/graphql/reference/index.md

@@ -8805,6 +8805,7 @@ Relationship between an epic and an issue.
 | <a id="epicissuemovedto"></a>`movedTo` | [`Issue`](#issue) | Updated Issue after it got moved to another project. |
 | <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="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). |
 | <a id="epicissueseverity"></a>`severity` | [`IssuableSeverity`](#issuableseverity) | Severity level of the incident. |
@@ -9853,6 +9854,7 @@ Returns [`VulnerabilitySeveritiesCount`](#vulnerabilityseveritiescount).
 | <a id="issuemovedto"></a>`movedTo` | [`Issue`](#issue) | Updated Issue after it got moved to another project. |
 | <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="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. |
 | <a id="issuesladueat"></a>`slaDueAt` | [`Time`](#time) | Timestamp of when the issue SLA expires. |

spec/graphql/types/issue_type_spec.rb

@@ -18,7 +18,7 @@ RSpec.describe GitlabSchema.types['Issue'] do
                 confidential discussion_locked upvotes downvotes user_notes_count user_discussions_count web_path web_url relative_position
                 emails_disabled subscribed time_estimate total_time_spent human_time_estimate human_total_time_spent closed_at created_at updated_at task_completion_status
                 design_collection alert_management_alert severity current_user_todos moved moved_to
-                create_note_email timelogs]
+                create_note_email timelogs project_id]
 
     fields.each do |field_name|
       expect(described_class).to have_graphql_field(field_name)