Implement singular iid for IssuesResolver and ProjectType

app/graphql/resolvers/issues_resolver.rb

@@ -2,7 +2,9 @@
 
 module Resolvers
   class IssuesResolver < BaseResolver
-    extend ActiveSupport::Concern
+    argument :iid, GraphQL::ID_TYPE,
+              required: false,
+              description: 'The IID of the issue, e.g., "1"'
 
     argument :iids, [GraphQL::ID_TYPE],
               required: false,
@@ -22,6 +24,7 @@ module Resolvers
       # Will need to be be made group & namespace aware with
       # https://gitlab.com/gitlab-org/gitlab-ce/issues/54520
       args[:project_id] = project.id
+      args[:iids] ||= [args[:iid]].compact
 
       IssuesFinder.new(context[:current_user], args).execute
     end

app/graphql/resolvers/merge_requests_resolver.rb

@@ -17,7 +17,7 @@ module Resolvers
     def resolve(**args)
       return unless project.present?
 
-      args[:iids] ||= [args[:iid]]
+      args[:iids] ||= [args[:iid]].compact
 
       args[:iids].map(&method(:batch_load))
         .select(&:itself) # .compact doesn't work on BatchLoader

app/graphql/types/project_type.rb

@@ -85,6 +85,11 @@ module Types
           null: true,
           resolver: Resolvers::IssuesResolver
 
+    field :issue,
+          Types::IssueType,
+          null: true,
+          resolver: Resolvers::IssuesResolver.single
+
     field :pipelines,
           Types::Ci::PipelineType.connection_type,
           null: false,

spec/graphql/resolvers/issues_resolver_spec.rb

@@ -33,6 +33,10 @@ describe Resolvers::IssuesResolver do
       expect(resolve_issues).to contain_exactly(issue, issue2)
     end
 
+    it 'finds a specific issue with iid' do
+      expect(resolve_issues(iid: issue.iid)).to contain_exactly(issue)
+    end
+
     it 'finds a specific issue with iids' do
       expect(resolve_issues(iids: issue.iid)).to contain_exactly(issue)
     end