Add top-level GraphQL query for single milestones

This makes it convenient because we won't need separate queries for
project milestones and group milestones.

app/graphql/types/query_type.rb

@@ -47,6 +47,15 @@ module Types
           null: false,
           description: 'Fields related to design management'
 
+    field :milestone, ::Types::MilestoneType,
+          null: true,
+          description: 'Find a milestone',
+          resolve: -> (_obj, args, _ctx) { GitlabSchema.find_by_gid(args[:id]) } do
+      argument :id, ::Types::GlobalIDType[Milestone],
+               required: true,
+               description: 'Find a milestone by its ID'
+    end
+
     field :user, Types::UserType,
           null: true,
           description: 'Find a user',

changelogs/unreleased/graphql-milestones-top-level.yml

+---
+title: Add GraphQL query for a single milestone
+merge_request: 38682
+author:
+type: added

doc/api/graphql/reference/gitlab_schema.graphql

@@ -8960,6 +8960,11 @@ type MilestoneEdge {
   node: Milestone
 }
 
+"""
+Identifier of Milestone
+"""
+scalar MilestoneID
+
 enum MilestoneStateEnum {
   active
   closed
@@ -11621,6 +11626,16 @@ type Query {
   """
   metadata: Metadata
 
+  """
+  Find a milestone
+  """
+  milestone(
+    """
+    Find a milestone by its ID
+    """
+    id: MilestoneID!
+  ): Milestone
+
   """
   Find a namespace
   """

doc/api/graphql/reference/gitlab_schema.json

@@ -25146,6 +25146,16 @@
           "enumValues": null,
           "possibleTypes": null
         },
+        {
+          "kind": "SCALAR",
+          "name": "MilestoneID",
+          "description": "Identifier of Milestone",
+          "fields": null,
+          "inputFields": null,
+          "interfaces": null,
+          "enumValues": null,
+          "possibleTypes": null
+        },
         {
           "kind": "ENUM",
           "name": "MilestoneStateEnum",
@@ -34301,6 +34311,33 @@
               "isDeprecated": false,
               "deprecationReason": null
             },
+            {
+              "name": "milestone",
+              "description": "Find a milestone",
+              "args": [
+                {
+                  "name": "id",
+                  "description": "Find a milestone by its ID",
+                  "type": {
+                    "kind": "NON_NULL",
+                    "name": null,
+                    "ofType": {
+                      "kind": "SCALAR",
+                      "name": "MilestoneID",
+                      "ofType": null
+                    }
+                  },
+                  "defaultValue": null
+                }
+              ],
+              "type": {
+                "kind": "OBJECT",
+                "name": "Milestone",
+                "ofType": null
+              },
+              "isDeprecated": false,
+              "deprecationReason": null
+            },
             {
               "name": "namespace",
               "description": "Find a namespace",

spec/graphql/types/query_type_spec.rb

@@ -17,6 +17,7 @@ RSpec.describe GitlabSchema.types['Query'] do
       current_user
       snippets
       design_management
+      milestone
       user
       users
     ]

spec/requests/api/graphql/milestone_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe 'Querying a Milestone' do
+  include GraphqlHelpers
+
+  let_it_be(:current_user) { create(:user) }
+  let_it_be(:project) { create(:project) }
+  let_it_be(:milestone) { create(:milestone, project: project) }
+
+  let(:query) do
+    graphql_query_for('milestone', { id: milestone.to_global_id.to_s }, 'title')
+  end
+
+  subject { graphql_data['milestone'] }
+
+  before do
+    post_graphql(query, current_user: current_user)
+  end
+
+  context 'when the user has access to the milestone' do
+    before_all do
+      project.add_guest(current_user)
+    end
+
+    it_behaves_like 'a working graphql query'
+
+    it { is_expected.to include('title' => milestone.name) }
+  end
+
+  context 'when the user does not have access to the milestone' do
+    it_behaves_like 'a working graphql query'
+
+    it { is_expected.to be_nil }
+  end
+
+  context 'when ID argument is missing' do
+    let(:query) do
+      graphql_query_for('milestone', {}, 'title')
+    end
+
+    it 'raises an exception' do
+      expect(graphql_errors).to include(a_hash_including('message' => "Field 'milestone' is missing required arguments: id"))
+    end
+  end
+end