Add top-level GraphQL query for single board list

- Add specs
- Update docs
- Fixes N+1 issue in List.title

Changelog: added

app/graphql/resolvers/board_list_resolver.rb

+# frozen_string_literal: true
+
+module Resolvers
+  class BoardListResolver < BaseResolver.single
+    include Gitlab::Graphql::Authorize::AuthorizeResource
+    include BoardItemFilterable
+
+    type Types::BoardListType, null: true
+    description 'Find an issue board list.'
+
+    authorize :read_issue_board_list
+
+    argument :id, Types::GlobalIDType[List],
+             required: true,
+             description: 'Global ID of the list.'
+
+    argument :issue_filters, Types::Boards::BoardIssueInputType,
+             required: false,
+             description: 'Filters applied when getting issue metadata in the board list.'
+
+    def resolve(id: nil, issue_filters: {})
+      context.scoped_set!(:issue_filters, item_filters(issue_filters))
+
+      Gitlab::Graphql::Lazy.with_value(find_list(id: id)) do |list|
+        list if authorized_resource?(list)
+      end
+    end
+
+    private
+
+    def find_list(id:)
+      GitlabSchema.object_from_id(id, expected_type: ::List)
+    end
+  end
+end

app/graphql/types/board_list_type.rb

@@ -47,6 +47,19 @@ module Types
           .metadata
       end
     end
+
+    # board lists have a data dependency on label - so we batch load them here
+    def title
+      if object.association(:label).loaded? && object.label_id.present?
+        object.title
+      else
+        loader = Gitlab::Graphql::Loaders::BatchModelLoader.new(Label, object.label_id)
+        Gitlab::Graphql::Lazy.with_value(loader.find) do |label|
+          object.label = label
+          object.title
+        end
+      end
+    end
   end
   # rubocop: enable Graphql/AuthorizeTypes
 end

app/graphql/types/query_type.rb

@@ -136,6 +136,10 @@ module Types
           complexity: 5,
           resolver: ::Resolvers::TimelogResolver
 
+    field :board_list, ::Types::BoardListType,
+          null: true,
+          resolver: Resolvers::BoardListResolver
+
     def design_management
       DesignManagementObject.new(nil)
     end

app/policies/list_policy.rb

+# frozen_string_literal: true
+
+class ListPolicy < BasePolicy # rubocop:disable Gitlab/NamespacedClass
+  delegate { @subject.board.resource_parent }
+end

doc/api/graphql/reference/index.md

@@ -36,6 +36,19 @@ in [Removed Items](../removed_items.md).
 
 The `Query` type contains the API's top-level entry points for all executable queries.
 
+### `Query.boardList`
+
+Find an issue board list.
+
+Returns [`BoardList`](#boardlist).
+
+#### Arguments
+
+| Name | Type | Description |
+| ---- | ---- | ----------- |
+| <a id="queryboardlistid"></a>`id` | [`ListID!`](#listid) | Global ID of the list. |
+| <a id="queryboardlistissuefilters"></a>`issueFilters` | [`BoardIssueInput`](#boardissueinput) | Filters applied when getting issue metadata in the board list. |
+
 ### `Query.ciApplicationSettings`
 
 CI related settings that apply to the entire instance.

spec/graphql/resolvers/board_list_resolver_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Resolvers::BoardListResolver do
+  include GraphqlHelpers
+  include Gitlab::Graphql::Laziness
+
+  let_it_be(:guest)         { create(:user) }
+  let_it_be(:unauth_user)   { create(:user) }
+  let_it_be(:group)         { create(:group, :private) }
+  let_it_be(:group_label)   { create(:group_label, group: group, name: 'Development') }
+  let_it_be(:board)         { create(:board, resource_parent: group) }
+  let_it_be(:label_list)    { create(:list, board: board, label: group_label) }
+
+  describe '#resolve' do
+    subject { resolve_board_list(args: { id: global_id_of(label_list) }, current_user: current_user) }
+
+    context 'with unauthorized user' do
+      let(:current_user) { unauth_user }
+
+      it { is_expected.to be_nil }
+    end
+
+    context 'when authorized' do
+      let(:current_user) { guest }
+
+      before do
+        group.add_guest(guest)
+      end
+
+      it { is_expected.to eq label_list }
+    end
+  end
+
+  def resolve_board_list(args: {}, current_user: user)
+    force(resolve(described_class, obj: nil, args: args, ctx: { current_user: current_user }))
+  end
+end

spec/graphql/types/query_type_spec.rb

@@ -27,6 +27,7 @@ RSpec.describe GitlabSchema.types['Query'] do
       runner
       runners
       timelogs
+      board_list
     ]
 
     expect(described_class).to have_graphql_fields(*expected_fields).at_least
@@ -136,4 +137,14 @@ RSpec.describe GitlabSchema.types['Query'] do
       is_expected.to have_graphql_resolver(Resolvers::TimelogResolver)
     end
   end
+
+  describe 'boardList field' do
+    subject { described_class.fields['boardList'] }
+
+    it 'finds a board list by its gid' do
+      is_expected.to have_graphql_arguments(:id, :issue_filters)
+      is_expected.to have_graphql_type(Types::BoardListType)
+      is_expected.to have_graphql_resolver(Resolvers::BoardListResolver)
+    end
+  end
 end

spec/requests/api/graphql/boards/board_list_query_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe 'Querying a Board list' do
+  include GraphqlHelpers
+
+  let_it_be(:current_user) { create(:user) }
+  let_it_be(:project) { create(:project) }
+  let_it_be(:board) { create(:board, resource_parent: project) }
+  let_it_be(:label) { create(:label, project: project, name: 'foo') }
+  let_it_be(:list) { create(:list, board: board, label: label) }
+  let_it_be(:issue1) { create(:issue, project: project, labels: [label]) }
+  let_it_be(:issue2) { create(:issue, project: project, labels: [label], assignees: [current_user]) }
+
+  let(:filters) { {} }
+  let(:query) do
+    graphql_query_for(
+      :board_list,
+      { id: list.to_global_id.to_s, issueFilters: filters },
+      %w[title issuesCount]
+    )
+  end
+
+  subject { graphql_data['boardList'] }
+
+  before do
+    post_graphql(query, current_user: current_user)
+  end
+
+  context 'when the user has access to the list' do
+    before_all do
+      project.add_guest(current_user)
+    end
+
+    it_behaves_like 'a working graphql query'
+
+    it { is_expected.to include({ 'issuesCount' => 2, 'title' => list.title }) }
+
+    context 'with matching issue filters' do
+      let(:filters) { { assigneeUsername: current_user.username } }
+
+      it 'filters issues metadata' do
+        is_expected.to include({ 'issuesCount' => 1, 'title' => list.title })
+      end
+    end
+
+    context 'with unmatching issue filters' do
+      let(:filters) { { assigneeUsername: 'foo' } }
+
+      it 'filters issues metadata' do
+        is_expected.to include({ 'issuesCount' => 0, 'title' => list.title })
+      end
+    end
+  end
+
+  context 'when the user does not have access to the list' do
+    it { is_expected.to be_nil }
+  end
+
+  context 'when ID argument is missing' do
+    let(:query) do
+      graphql_query_for('boardList', {}, 'title')
+    end
+
+    it 'raises an exception' do
+      expect(graphql_errors).to include(a_hash_including('message' => "Field 'boardList' is missing required arguments: id"))
+    end
+  end
+
+  context 'when list ID is not found' do
+    let(:query) do
+      graphql_query_for('boardList', { id: "gid://gitlab/List/#{non_existing_record_id}" }, 'title')
+    end
+
+    it { is_expected.to be_nil }
+  end
+
+  it 'does not have an N+1 performance issue' do
+    a, b = create_list(:list, 2, board: board)
+    ctx = { current_user: current_user }
+    project.add_guest(current_user)
+
+    baseline = graphql_query_for(:board_list, { id: global_id_of(a) }, 'title')
+    query = <<~GQL
+      query {
+        a: #{query_graphql_field(:board_list, { id: global_id_of(a) }, 'title')}
+        b: #{query_graphql_field(:board_list, { id: global_id_of(b) }, 'title')}
+      }
+    GQL
+
+    control = ActiveRecord::QueryRecorder.new do
+      run_with_clean_state(baseline, context: ctx)
+    end
+
+    expect { run_with_clean_state(query, context: ctx) }.not_to exceed_query_limit(control)
+  end
+end